recommended verification practice for rosemount 3144p · pdf fileprocess control, critical...

Temp_Verification_RevAD.doc

Recommended Verification Practice for

Rosemount 3144P HART Temperature Transmitters

And

Resistance Temperature Detectors

March 2010


Table of Contents Section 1: Introduction and Temperature Measurement Overview Section 2: Model 3144P Temperature Transmitter Overview Section 3: Transmitter Verification Guidelines Section 4: Transmitter Verification Frequency Section 5: Temperature Sensor Overview Section 6: Temperature Sensor Verification Guidelines and Procedure Appendix A: Glossary of Terms Appendix B: Sample Methodology To Determine Verification Frequency Appendix C: Sensor Verification Background Appendix D: Transmitter Troubleshooting Appendix E: Notes to Transmitter and Sensor Verification


Section 1: Introduction and Temperature Measurement Overview

This guideline will provide users of Rosemount Model 3144P HART Temperature Transmitters and Temperature Sensors to develop specific verification (and re-verification) procedures for their particular installation. This guideline is not intended to address the verification guidelines for Rosemount’s Fieldbus version of the 3144P family of Temperature transmitters. This document provides guidelines and standards as recommended by Rosemount. It is the end-user’s responsibility to develop their own individualized procedures incorporating the policies, procedures and standards common to their own company and industry sector for all instrumentation. It is assumed that the transmitters and sensors used are correctly specified, sized, and installed by the end-user. This document will focus solely on verification practices.

The measurement of temperature is extremely common in today’s industrial environment. Industries such as Oil & Gas, Chemicals, Biopharmaceuticals, Power Generation, Metals & Mining, Automotive, HVAC, Plastics and Aerospace all depend on the measurement and/or control of temperature to some degree. Moreover, many modern industrial processes in these industries are extremely sensitive to changes in temperature, for process success, yields, and safety. As a result, temperature measurement technology has also developed to keep pace with demand. Modern electronics, software, materials, and manufacturing processes have combined to provide a broad array of temperature measurement solutions available to end-users of all types. In modern industrial process industries (e.g. Chemical, Oil & Gas, Biopharmaceuticals, etc..) the preferred choice of Temperature measurement equipment is usually composed of a sensing unit, either an RTD (Resistance Temperature Detector) or a thermocouple, and oftentimes a temperature transmitter to read, interpret, and relay the temperature signal back to where it can be utilized to full advantage. Additional equipment such as thermowells, extension hardware, indicating meters can also be specified as required. In order to ascertain the effectiveness of any temperature measurement systems, one must take into account the performance criteria for each relevant component. In most cases, this will require analysis of both the temperature


sensor and temperature transmitter. To focus on one, at the expense of the other could result in substantial misinterpretation of a system’s performance or suitability. Detailed information on Model 3144P HART Temperature transmitter operation, installation and maintenance can be referenced in Rosemount document 00809-0100-4021, “Model 3144P Smart Temperature Transmitter, Product Manual”. For additional questions, please contact Rosemount technical support personnel on www.rosemount.com.


Section 2: Model 3144P Temperature Transmitter Overview

The scope of this guideline from the Temperature Transmitter perspective includes the HART versions of the Model 3144P transmitter. The Model 3144P is a single and dual-sensor input transmitter. This dual-input capability allows for the measurement of differential temperature, average temperature, or to provide redundant temperature measurements. These transmitters utilize a dual-compartment housing design that enables high reliability and stability in harsh environments. This configuration isolates the sensitive electronic components from environmental effects (such as humidity) in the terminal compartment. Additionally, these transmitters are capable of being matched to a calibrated sensor, thereby increasing measurement system accuracy. In order to lessen the need for future transmitter verification and limit operating costs, the Model 3144P transmitter is outfitted with stability specifications of up to 5 years in duration.

In the Model 3144P, a microprocessor-based sensor board is used to complete the analog-to-digital conversion from the temperature sensor (RTD or thermocouple) to the transmitter. One hundred percent of production units are fully temperature characterized to compensate for the temperature-dependency of electronic components and increase accuracy and stability. The sensor output is fed into the Output Board that carries the transmitter specific data and completes the HART/4-20mA output conversion. The Model 375 and 475 Universal HART communicators are used to communicate locally with the Models 3144P.


3144P Functional Block Diagram

Analog – Digital Conversion

Microprocessor - Sensor Linearization - Rerange -Damping - Diagnostics - Eng Units - Communications - Temperature Correction - Averaging - Hot Backup - Differential Temperature

Digital- Analog

Conversion

Digital HART Communications

Reference Input

Sensor 1

Sensor 2

Galvanic Isolation

Cold Junction Compensation

4-20 mA

signal


Diagnostic ALERTS and ALARMS: Model 3144P transmitters are designed with Diagnostics that initiate an ALERT or ALARM when the appropriate condition exists. ALERTS – cover diagnostics that are determined to not affect the devices’ ability to output the correct signal and thus will not disrupt the 4-20mA output. For example, “Process Variable out of Range”. To read an “ALERT”, one must read the indicating meter, read on a 375/475 Communicator, or an Asset Management System that can read HART information. ALARMS – cover diagnostics that are determined to affect the device ability to output the correct signal. Detected alarms will drive the transmitter output high or low as specified by the user. Alarms will be read on the indicating meter, Model 375/475 Communicator or any asset management system that can read HART information. In the event of a transmitter failure or malfunction, the Model 3144P is designed with various diagnostics. If the device detects any failure that affects the transmitter output, the device will drive the Hardware Alarm (high or low as selected by the user). The alarm output levels are < 3.75 mA low and > 21.75 mA high. Option for NUMUR levels is also available (< 3.6 mA and > 22.75 mA). Alarms will also be indicated on the transmitter meter and via the Model 275 communicator.

3.9

Normal Operating Range

mA

Hardware Alarm

Process Variable Out of Range

> 21.75 < 3.75

20 20.8

4


HART Field Communications Protocol Communications Signal

Physical Layer The HART protocol uses a communication physical layer called Bell 202 Frequency Shift Keying (FSK). A very small sinewave signal with an amplitude of around 1.2 volts peak-to-peak is superimposed on the 4-20 mA analog signal. Since the average value of the HART digital signal is zero, the value of the analog signal is basically unchanged. This allows for simultaneous analog and digital communication on the same set of loop wires.

Ones and Zeros The HART protocol like most digital communication uses a string of zeros (0) and ones (1) to transfer a message between the host device and a slave device. A frequency of 1200 Hertz (Hz) will be equal to a digital value of “one”, while a frequency of 2200 Hz will equal a “zero” digital value.

Process Variable

Communications

20 mA

4 mA

+0.5 mA

–0.5 mA

“1” = 1200 Hz

“0” = 2200 Hz

Frequency Shift Keying Based on Bell


Communications A HART 4-20 mA loop is similar to a 4-20 MA analog loop. One of the most important items is to ensure the correct amount of loop resistance is in place, to generate the HART digital signal. A HART loop needs a minimum of 250 ohms and a maximum of 1100 ohms of loop resistance. This loop resistance is required to generate a digital signal with enough voltage and current to be read by both the HART master (375/475) and the HART slave (3144P transmitter).

Voltage

Current E = I R

PowerSupplyMinimum 250


Section 3: Transmitter Verification Guidelines The Need For Verification While it is true that most of a temperature point’s drift with time can be associated to the temperature sensor, the temperature transmitter may also drift. The electrical components that comprise a temperature transmitter are designed and specified for their accuracy and stability, but it is a known fact that all electrical components drift. Even though Rosemount-specified resistors, capacitors, inductors and other components undergo rigorous testing, they all experience some amount of performance drift over time due to temperature-related stresses and other affects. Individual component drift over time may be small, but the combined effects of these drifts may amalgamate to eventually affect the performance of various transmitter subsystems, and the transmitter as a whole. Due to this drift, temperature transmitters will periodically be “calibrated”. This is accomplished through the use of an INPUT Trim (or Sensor Trim) as described next. Transmitter Verification Overview – Transmitter Trimming

To calibrate a 3144P transmitter the user may use one or more “trim” functions. All 3144P transmitters are “factory-characterized”, which means they are shipped with an industry-standard sensor (resistance versus temperature in the case of an RTD) curve in the transmitter’s memory. In actual operation, the transmitter uses this stored information to continuously interpret the sensor input and output the process variable in the appropriate engineering units. Verification of a 3144P transmitter entails attaching the transmitter to known resistance (or millivolt) sources at one or more points, and adjusting or “trimming” the transmitter to the correct output reading. This adjustment will change the transmitter’s interpretation of the sensor input across the entire temperature range, resulting in a calibrated transmitter. Two types of Trimming functions are available for transmitter verification. A Single-Point Trim, as its name implies, entails calibrating the transmitter at a single temperature (i.e. resistance) point and shifting the entire sensor curve appropriately. A Two-Point Trim entails calibrating the transmitter at two different temperatures (i.e. resistances) and than adjusting the sensor curve by both shifting and changing the slope. The Two-Point Trim is recommended in most applications. See Appendix C for additional information.


Section 4: Transmitter Verification Frequency

In general, the verification frequency for a transmitter is very application

dependent. The accuracy and relative importance of the measurement as determined by the user. For Temperature measurement points intended for process control, critical systems (i.e. safety shutdown, etc..), or extremely high value processes, more frequent verification checks, on the order of 1 per month initially may be required. This time duration may be gradually increased to 3 or 6 months if the Temperature transmitter readings are holding true. Please reference Appendix B for a sample methodology for determination of Verification frequency.

The 3144P transmitter has a 1 year and a 5 year stability specification as follows. 1) +/- 0.1% of reading or 0.1°C, whichever is greater, for 24 months for RTDs 2) +/- 0.1% of reading or 0.1°C, whichever is greater, for 12 months for thermocouples 3) +/- 0.25% of reading or 0.25°C, whichever is greater, for 5 years for RTDs 4) +/- 0.5% of reading or 0.5°C, whichever is greater, for 5 years for thermocouples

This information is generated from accelerated life testing involving temperature cycles and gives a very good indication of transmitter drift in actual operation.

In general, the verification/verification interval for a given application is determined by two criteria:

1) Installed performance and stability of the transmitter 2) Accuracy requirements of the application.

By calculating the installed performance of the transmitter and comparing

this against the application performance requirements, the verification interval can be approximated. 4.1 Calculating Transmitter Installed Performance To calculate the performance of a transmitter installed in a specific application, one must factor in all potential aspects that could have a detrimental impact. These typically include: Digital Accuracy (of electronics) Digital/Analog accuracy (of D/A conversion)


Ambient Temperature Effects (changes in ambient temperature affecting transmitter electronics). The total performance of the transmitter will be a RSS (Root Sum Squares) of these factors: Total Performance = (Digital2 + DA2 + TE2) ½ where DA is the Digital/Analog accuracy and TE is the Ambient Temperature Effects accuracy. Full explanations and listings of these accuracies can be found in the 3144/3244 Product Data Sheets (Rosemount document 00813-0100-4724 or 00813-0100-4021 for the 3144P) 4.2 Comparing Transmitter Performance and Application Requirements When the Application requirements are known, and the transmitter total performance and stability has been determined, one can set the required verification interval. For example, if the application requires + 0.5% Reading performance (for safety, product quality, etc.) and the Transmitter Performance is + 0.10% plus stability of +0.10% per year, the maximum interval could theoretically be set at 3+ years. Note: Verification intervals can be shortened for critical measurement points, and lengthened for non-critical measurement points as determined by user experience. 4.3 General Recommended Calibration Practices for Temperature Transmitters In lieu of enduser-determined application-specific verification intervals the following general guidelines may be used as “manufacturer recommended practices” for calibration of temperature transmitters. (Note: this may be used to assist users in their compliance with EPA regulations governing Greenhouse Gas measurement points.) Type of Installation Recommended Calibration Interval “Benign” installations Once every 3 years “Severe” service installations Once every year or end-user determined (whichever is less) “Benign” installations are defined as measurement points where the transmitter is not exposed to high levels of shock, vibration and extreme/repeated ambient temperature extremes.


Section 5: Temperature Sensor Overview Overview The two most common methods of industrial temperature measurement today are with thermocouples and resistance temperature detectors (RTD’s). While the installed base of industrial temperature sensors is composed mostly of thermocouples mainly due to lower costs and wider temperature ranges, new sensor installations are gradually migrating to RTD usage due to improved manufacturing technologies (and therefore lower costs), better accuracy/stability, and less susceptibility to electromagnetic interference. To keep with the scope of this document, this section will be limited to a discussion of basic RTD principles. Operating Principles (RTDs) RTD’s operate on the basic principle that the resistance of a metal increases with temperature, i.e. thermoresistivity. This electrical resistance property of any given type of metal is termed resistivity, and is denoted by “r”. The overall resistance R, of a quantity of metal (a piece of wire for example) will be proportional to its length L, and inversely proportional to its cross-sectional area A, i.e. R = (r X L) / A Note that the resistivity “r” will change with temperature for any given metal. A plot of a metal’s Resistance versus Temperature for any given metal shall be a curve that is “almost” linear in nature. Many types of RTD metals are in existence, including platinum, copper, nickel, and others. Metals are usually chosen on the basis of many factors including, a) The temperature coefficient of resistance (i.e. slope of Resistance vs. Temperature curve) and whether it is large enough to give resistance changes that can be easily measured as temperature changes. b) Whether the metal will react with materials used during the sensor fabrication process (and thereby negatively affecting the measurement readings). c) Whether the metal can be easily and economically fabricated into form factors that permit easy manufacture and use.


In today’s industrial environment, platinum is the most commonly utilized RTD metal. Platinum possesses the desirable properties of high accuracy, high repeatability, and a relatively high resistance change per degree of temperature change. Additionally, platinum RTD’s are fairly linear throughout their temperature range. A common abbreviation found in industry for platinum RTD’s is PRT for Platinum Resistance Thermometer. In order to practically use RTD’s in industrial applications, temperature resistance formulas for platinum have been devised. A common equation, the Callendar-Van Dusen equation compensates for the small deviation from linearity inherent in the platinum resistance versus temperature curve. The Callendar-Van Dusen equation is: where: t = Temperature in C Rt = Resistance of the sensor at temperature t. Ro = Resistance of the sensor at 0C Unique, sensor-specific constants The most typical nominal value of alpha for industrial PRTs is .00385 although other values are also available. The Callendar-Van Dusen equation can be used to easily convert from the RTD’s resistance value to Temperature and vice versa. RTD Drift As a general rule, all sensors, RTDs included will drift with time. RTD drift can be induced physically or chemically, but is always present. Most manufacturers will provide some quantitative measure of expected RTD drift with time, but these numbers should only be used as a rough guideline for users. Most sensor drift specifications are under lab or “simulated” use conditions, and will not be representative of actual installed conditions in real industrial processes. Physical effects on RTD stability include both mechanical and thermal factors. Mechanical effects such as vibration (whether process or environmentally-induced) and shock (due to improper handling) introduces defects in the crystalline structure of an RTD’s underlying Platinum sensing element. This in turn will affect the RTD’s resistance versus temperature relationship. Thermal effects are primarily caused by differing thermal coefficients of expansion between a sensor’s Platinum sensing element and the material

Rt = Ro + Ro[t-(0.01t-1)(0.01t)-(0.01t-1)(0.01t)3]


(typically ceramic) used to house or support the platinum. As temperature rises and falls, this imbalance (however minute) will introduce stresses in the platinum, thereby affecting performance. Chemical phenomena (i.e. contamination or poisoning) of an RTD’s Platinum sensing element will also have a negative effect on a sensor’s performance with time. This shift in resistance versus temperature performance will accelerate with elevated temperatures, i.e. above 400 or 500°C. RTD Verification The general goal of RTD verification is to either experimentally determine the actual, unique verification constants () for each individual RTD or to check the RTD’s resistance reading at one or more finely controlled temperatures.

The former is accomplished by finding the actual resistance versus temperature points for an RTD at 3 or 4 points. This information is then used to mathematically find the constants by solving for 3 (or 4) unknowns. The later is accomplished by simply immersing the sensor into baths, ovens, or calibrators operating at known (and controlled) temperatures, and measuring the RTD’s resistance reading. This reading can be compared with the ideal or factory reading. Callendar-Van Dusen Constants and Sensor Accuracy Standard RTD’s conform to industry-defined resistance versus temperature curves (such as those prescribed by ASTM or IEC). In general, RTD’s can be manufactured to conform to these industry-standard curves to some level of accuracy and industrial temperature transmitters (including the 3144P family) have these industry standard curves embedded in memory. Higher levels of accuracy may be accomplished by through the use of the Callendar-Van Dusen constants described above. These constants are unique for each individual RTD, and once known, can accurately reproduce the RTD’s unique Resistance versus Temperature relationship through the Callendar-Van Dusen equation. Practical implementation of this entails the “loading” of an RTD’s unique set of constants into a 3144P temperature transmitter, and thereafter having the transmitter interpret resistance and temperature values from these constants.


Section 6: Temperature Sensor Verification Guidelines and Procedure

Background Sensor verification practices may vary depending on the level of accuracy desired from the measurement, the criticality of the measurement, the equipment used, etc. For the purposes of this report, RTD verification shall be segmented by the criticality of the measurement. The following is a general guideline for determining the level of verification required for a given temperature sensor measurement, as well as some recommended procedures, notes on equipment, and other guidelines. Actual practices will be dependent on individual user needs and preferences, and corporate or industry best practices.

Tolerance Level Accuracy

Low +/- 1.0 to 2.0C Medium +/- .05 to 1.0C High +/- 0.5C or better

*Note: Accuracy levels are recommended minimums. Sensor Verification Practices and Measurement Criticality

Low-Level of Criticality While it is assumed that all measurements need to have some level of accuracy and reliability, there will be some applications where the failure or gross inaccuracy of the measurement point does not present an immediate or severe consequence. Points such as these can generally be classified as purely monitoring applications where it is either beneficial to know the periodically see the general status of the process in question. RTD verification for these types of points should comprise a single point verification check. The most common, but by no means exclusive, methodology is that of an ice-point check as described next.

An RTD’s ice point or also called R0 is the most fundamental measurement point, providing the user an opportunity to glean much information regarding the relative health of the sensor. The ice point is popular both because of its relative ease of achieving and maintaining, and because it is included in every major sensor/verification standard in the world. As an RTD drifts due to mechanical or thermal forces, it’s Resistance versus Temperature curve will shift or translate. Ice Point checks (over time) will allow the user to determine the severity of this drift. For instance, if the ice point for a given sensor is maintaining Class B (an IEC-specified tolerance level) on that sensor, then the user has a reasonable certainty that the sensor performs to Class B across its operating range. The ice point check is a basic indication that


the sensor is structurally and materially sound. A check of ice-point shift followed by an appropriate trim to the entire sensor curve (i.e. translating the curve by the amount of ice-point shift) is the best type of sensor re-verification for low-criticality applications.

Other temperatures may be used in place of an ice-point check provided the verification equipment setup is adequate for the accuracy level required.

Medium Level Criticality For a more critical process where accuracy is of a higher concern, it is recommended that the user conduct periodic two point verification checks. One of these points should be the ice point. The second point can be 100°C (i.e. the boiling point of water) or some other easily reproducible point. A two point check allows the determination of two pieces of information. First, the Ro or ice point will be known. Secondly, the user can calculate the RTD’s Alpha value, which is a general indication of the 1st order slope of the RTD’s Resistance versus Temperature curve. This gives another level of indication of the quality of the sensor. While mechanical or thermal degradation may be reflected in the ice point drifting, a chemical/material contamination of an RTD’s platinum sensing element is often accompanied by a shift in the RTD’s Alpha property. If a history is kept and Alpha is degrading, this is an indication that the sensor is degrading. NOTE: As a rule of thumb, typically as R0 drifts up, the Alpha property decreases. This may be an indication that the purity or quality of platinum is diminishing.

High Level Criticality For the most important processes, where measurement accuracy and stability is of the utmost concern, it is recommended that a “full” verification or recharacterization of the RTD is performed. Processes where this may be relevant are various applications involving very fine process control where yield may be negatively impacted if temperature is not tightly regulated. A full span verification or characterization is basically a redetermination of an RTD’s unique Resistance versus Temperature relationship. This is typically accomplished by comparison verification of the RTD in question at either 3 or 4 verification points (4 is required if the verification temperature range bridges 0°C). As a result, the most accurate picture of the RTD’s health and performance will be accomplished. Comparison of Verification Results (How to Handle Verification Results) For non-calibrated sensors (i.e. sensors without uniquely known constants such as IEC Class A or Class B sensors) the user should compare the verification results with the resistance versus temperature information found in industry standard specifications (e.g. IEC-751). These standards publish the


nominal resistance versus temperature information for a “standard” industrial RTD, as well as the tolerance information around this nominal. For sensors originally “calibrated” at the factory (i.e. those requiring very high accuracy), the new verification check data may be compared with the actual accuracy desired for the specific application or the original factory-provided information. For High Level Criticality applications, the sensor should be fully re-verified or re-characterized, and the new Resistance versus Temperature data should be used moving forward. For Low and Medium-Level Criticality applications, the sensors are simply reused with the standard factory assumed Resistance versus Temperature data. Significant deviation of the temperature instrumentation from the initial or theoretical values (as defined above) should flag the user to a potential failure or replacement condition. As a minimum step, the sensor should be replaced with a new unit. Gross deviations in readings should warrant an investigation of both the process and the instrumentation for possible fault conditions. Sensor Verification Equipment and Procedure Overview For the most part, the same basic procedures and equipment are employed regardless of the complexity of the verification scheme. Typically, the verification media will consist of either a fluid bath or a dry-block calibrator. If a free-standing fluid bath is utilized, then some type of accurate, standard thermometer will be required to monitor the actual verification bath temperature. Lastly, a Digital Multimeter (DMM) will be required to determine the resistance (temperature) of the sensor to be validated. As a rule of thumb, the equipment utilized in the verification setup should be at least 4 times better than the resolution of the phenomena being measured. Sensor Verification Equipment Guidelines DMM: A wide range and variation (in capability and cost) are available on the market. Ideally, a DMM accurate to at least 4 decimal places is recommended. The user should match the DMM’s capability to the level of verification to be completed. Entry-level DMMs may not have the EMF compensation required for high accuracy measurements. If long wires or multiple wire junctions are utilized, then EMF compensation may be a requirement. Otherwise, for lower-criticality measurements, such as just an ice –point check, an entry-level device may be adequate. Manufacturers such as Keithley, Hewlett Packard, and others offer a broad range of devices. Examples include (but are not limited to): HP3478A (4-decimal, entry to mid-level) Keithley 2001 (mid-level) Keithley 2002 (mid-to-high level) HP3458A (5 or 6 decimal, high-level).


Fluid Baths / Dry-Block Calibrator: Again, a wide range of fluid baths and dry-block calibrators are available. The level of accuracy of the fluid bath or dry-block calibrator will be determined by the verification to be performed. Typically, a good fluid bath will provide a level of uncertainty of approximately .01°C uncertainty. Typically Dry Block calibrators are 2x or 3x this level. In general, fluid baths are harder to maintain (special facilities, fluid storage and refilling, special training, etc) and more costly, but can more accurate. Fluid baths also require a separate Standard thermometer. Dry-block calibrators have internal thermometers, eliminating the need for a separate Standard. Regardless, both a fluid bath’s Standard thermometer and the Dry-Block calibrator will to be periodically re-calibrated. For field verification, the only practical alternatives are typically portable Dry-Block calibrators. NOTE: Some calibrators will provide a separate specification for Uniformity and/or other contributors to uncertainty. The user should take all effects into account when determining the overall uncertainty of the calibrator. Standards: Temperature Standards are available at varying levels of uncertainty depending requirements. Typically, Standards provide up to a .01C level of uncertainty and are traceable back to NIST or some other nationally or internationally recognized agency. Certified digital thermometers are also suitable for sensor verification provided that the accuracy is suitable, and the certification is current. Sensor Verification Procedure Overview Regardless of the level of verification to be performed, the procedure utilized will be a comparison verification. In other words, the fluid bath’s Standard or the Dry Block calibrator’s thermometer will define temperature and the user collects the resistance at that temperature. At the highest level of verification (i.e. full characterization of the resistance versus temperature relationship) the temperature and resistance values will be utilized to calculate the RTD’s unique Callendar-Van Dusen equation. Otherwise the resistance and temperature data will be compared with past readings. Sensor Verification Procedure: General Steps Step 1: Set up fluid bath or dry block calibrator per manufacturer’s instructions. Step 2: Set up DMM per manufacturer’s instructions. Enable EMF compensation if available. NOTE: Most DMM manufacturers do NOT set EMF compensation to “On” for a default setting. Step 3: For fluid bath, clean sensor before insertion into bath. This will minimize contamination of bath fluid. Step 4: Insert sensor(s) into bath or calibrator. If required, use an equilibrium block to lessen effects of bath or calibrator non-uniformities.


Step 5: Connect sensors to DMM. Recheck for good electrical connections. It is preferable to utilize a 4-wire measurement (or 3-wire with dual element sensors). This will allow for leadwire compensation. Step 6: Set fluid bath or dry block calibrator to desired temperature. Step 7: Monitor the output of the bath or calibrator and allow time for bath or calibrator to stabilize at temperature. Typically, once the medium has reached some level of stability, there will be some perturbation around the setpoint. A dry block will typically cycle on and off at the achieved temperature. NOTE 1: At higher temperatures of above 400°C, if the resistance value of the target sensor appears to continuously decrease after the bath or calibrator is stable, user may want to verify that sensor’s Insulation Resistance (IR) value is not compromised (versus RTD manufacturer’s specifications). IR will always degrade somewhat with temperature but sensors that are wet or structurally compromised will be excessively and negatively affected. In these instances, the addition of heat results in moisture redistribution. This moisture reduces the sensor’s effective resistance as current starts to shunt. NOTE: An IR of >10MegOhms is considered satisfactory). NOTE 2: For FIELD VERIFICATION, the general steps and guidelines outlined above are valid with the exception of the use of a portable dry block calibrator and a portable DMM. The user should pay special attention to the accuracy levels associated with this equipment in order to achieve a good verification. The user should also be vigilant of extremes in environmental conditions that could affect the verification process. NOTE 3: Many industrial Temperature Verification practices involve 3, 5 or another number of point checks around a setpoint or across a temperature range. Many of these practices may be offshoots of verification practices for capacitive pressure devices. In general, the platinum elements found in industrial RTDs are usually more predictable (when operating normally, and when failing) and fairly linear. In most instances, the user will receive satisfactory results for Temperature RTD’s without doing multiple point verifications, especially around a set point. If two or three points are “behaving” correctly, then temperatures in between these points are in all likelihood also “good” (although RTD readings can deviate at higher temperatures, i.e. > 600°C). SYSTEM VERIFICATIONS (TRANSMITTER AND SENSOR COMBINATION)

Many installations utilize “System Verifications” or the combined verification of a temperature transmitter and sensor assembly. This type of a verification is based on the sound principle that the transmitter/sensor measurement is a temperature SYSTEM measurement, and therefore should be treated as such. This methodology may also be more readily accomplished with less time and fewer resources than individual RTD and Transmitter verifications. Note: Investigations into deviations in measurement output of a system should


include determination of the specific cause of the inaccuracy (i.e. element, transmitter, wiring, terminations, etc).

Two general methodologies exist for system verifications. In the first, the transmitter effectively takes the place of the DMM in a standard RTD verification as described previously in this section. In this case, the DMM will still be used, but to measure transmitter 4-20ma output.

The second methodology involves the Trimming functions described in

Section 3. In this instance, the sensor and verification bath effectively take the place of the decade box as the resistance source. The transmitter is then “trimmed” per the procedure in Section 3.


Appendix A: Glossary of Terms Alpha: An RTD constant. Calculated from the RTD’s resistance at 0°C and 100°C. Analog Output Trim: Analog Output trim is the process used to adjust the analog output (transmitter’s current output at the 4 and 20 mA points) to match the plant standard or the control loop. ASTM: American Society of Testing and Materials. A consortium based in the United States that, among other things, defines guidelines for industrial RTD’s (ASTM E644 and ASTM E1137). Beta: An RTD constant. Callendar-Van Dusen: 1) The set of constants (Alpha, Delta, Beta and ice point) that fully characterize an RTD’s resistance versus temperature relationship. 2) The equation used to characterize an RTD’s resistance versus temperature relationship. Configuration: Process of setting parameters that determine how the transmitter functions. Delta: An RTD constant. IEC: International Electrotechnical Commission. An international consortium that, among other things, defines guidelines for industrial RTD’s (IEC-751). Class A and Class B: RTD tolerance grades as defined by IEC-751. Grade A and Grade B: RTD tolerance grades as defined by ASTM. Primary Standards: Laboratory grade temperature sensors that are calibrated at nationally or internationally recognized calibration agencies. These sensors have the highest accuracies available. R0 or Ice Point: An RTD’s resistance reading at 0°C. Secondary Standards: Laboratory grade temperature sensors that are calibrated and traceable to Primary Standards. Sensor Trim: Sensor trim is the process used to adjust the position of the factory characterization curve to optimize the transmitter performance over a specified temperature range or to adjust for mounting effects. Verification: A general term for the checking of a temperature sensor or transmitter performance in the field.

Rt = Ro + Ro[t-(0.01t-1)(0.01t)-(0.01t-1)(0.01t)3]


Appendix B: Sample Methodology To Determine Verification

Frequency

In order to maintain and verify the accuracy of any measurement system, it is necessary to conduct regular verifications and to perform a verification of a measurement system component if it is deemed to be outside of acceptable limits. Values for instrument stability (or lack thereof) are commonly provided for instruments of all types. While it is possible to calculate a recommended verification interval from these specifications (i.e. by factoring in how much drift is acceptable for a given application), the results may be misleading. Most instrument stability information is based upon either laboratory conditions (e.g. an RTD soaking in a temperature oven) or some arbitrary environmental conditions (i.e. nominal vibration and elevated temperature) meant to simulate a “typical” installation. Regardless, the most accurate determination of a verification interval would factor in actual, installed conditions. This is due to the fact that differing levels of process-induced shock and vibration, differing ambient temperature levels, shock due to use and handling, etc. will impact the performance of measurement instruments, and hence their associated verification intervals. Sensors in particular may have vastly different drift rates, and therefore verification intervals depending upon installation conditions. Simply “checking” a measurement instrument’s accuracy at an arbitrary interval (i.e. every week, month, 6 months, etc..) may result in a system that is operating out of specification for a period of time leading up to the verification check that revealed a need for verification, or it may lead to additional costs due to needless (or too frequent) verifications of subsystems that may not drift appreciably in the time interval used. Both of these eventualities may be avoided if an analysis is completed which results in a verification period, which is less than the earliest chance of a system or component drifting out of acceptable specifications. This maximum period of time for verification can be part of a preventative maintenance program. The following general method is a common way to determine verification intervals. Step 1: For a given instrument (an RTD for example) utilized in one “grouping” of installations (e.g. all pipe flow temperature measurements of one velocity and temperature) perform verification checks of as many systems as possible for a given period of time (e.g. monthly). For an installation grouping where the differences between actual and expected readings are relatively constant (i.e. small standard deviation of differences) a relatively small number of measurements (e.g. 3 to 5) is required. For installations with larger variations


from reading to reading (i.e. larger standard deviations of differences) a larger number of readings (>5) would be desirable. Step 2: Calculate the mean and standard deviation of all differences from the expected readings for the given grouping. Tabulate a graph of the standard deviations for the given calibration interval as in Figure 1. Repeat Step 1 for each grouping at the time interval chosen to complete Figure 1.

Error (Degrees or Ohms)

Verification Period (Months) 1 2 3 4 5 6 7 8 9 . . . . . . . . .

Figure 1

Graph of standard deviations of differences

between actual and expected instrument

readings.


Step 3: Generate a graph similar to Figure 2 where the plot of one, two, and three standard deviations is presented, as well as a line with the maximum acceptable error for the given installation. Assuming a normal distribution of the errors, the intersection of the 2 standard deviation curve with the line of maximum error would provide the recommended verification period for a 95% certainty that the system will remain within specification between verification periods (in this instance, 9 months). The intersection of the 3 standard deviation curve with the line of maximum error would provide a recommended verification period with a 99% certainty that the system will remain within specification between verification periods (in this instance, 4 months)

2 Standard Deviations

1 Standard Deviation

Error (Degrees or Ohms)

Verification Period (Months) 1 2 3 4 5 6 7 8 9 . . . . . . . . .

Figure 2

3 Standard Deviations

Line representing maximum

acceptable error


Note that if there is not clear trend in the standard deviation graphs then there may be an intrinsic problem with the system in question or with the procedure utilized. This can range from measurement error during the experiment, to extreme system fluctuations, to a process fluctuating outside of normal conditions. The use of this methodology will have to be balanced by the criticality of the system or reading provided by the instrument in question. If many different processes (with many different process conditions) exist in an installation, it may be cost prohibitive to perform the above methodology. For instance, if non-critical or benign processes exist, these may by checked every 6 to 12 months. If very critical or very severe processes (high flowrates, severe process shutdown/startup conditions, shocks to process surges, etc..) are present then above mentioned methodology is highly recommended for those particular points. The cost of performing the method for these applications is outweighed by the benefits of knowing when the system/subsystem is predicted to go “out-of-spec”. Another variation is to utilize different intersection points depending on process criticality, i.e. 2 sigma line for less critical systems, and the 3 sigma line for more critical systems, etc. It is also relevant to mention that once this methodology is completed, it does not need to be duplicated for a given installation unless a relevant variable changes (i.e. flow rate, temperature, vibration level, etc..). A library of experimentally determined verification intervals may be generated once and reused as required throughout the plant (or in other plants as applicable). Please note, that a change in sensor design or vendor source may require a new experimentally-determined verification interval. This necessity is dependent on how different the sensor designs are from each other. This overall methodology is very similar to other industrial (and governmental) methods used as part of Preventative Maintenance (PM) programs.


Appendix C Sensor Verification Background Sensor Trim: Verification Overview Sensor trims may be accomplished with either a single point trim or a 2 point trim. A single point trim is fairly typical and most cases adequate, but 2 point trims can provide better ultimate accuracy for critical measurement points. 1 or 2 Point Trim - A one point trim is sufficient if the transmitter and sensor will be operating for the majority of it’s uptime around a singular setpoint or a very small temperature range. - A one point trim shifts the transmitter’s resistance versus temperature curve up or down based on the actual reading of the singular point. This is equivalent to a vertical translation of the sensor curve. The shape and general slope of the sensor curve is not changed. - Note that the one point trim is less suitable if the accuracy across wide temperature ranges is important. Certain skewness may be introduced at the temperature extremes which could negatively impact measurement readings. - A two point trim is appropriate when the accuracy of a temperature range is important (e.g. 50-150°C). This type of a transmitter trim accepts the actual reading at two different points and appropriately adjusts the sensor curve to compensate. The resultant effect is the translation AND rotation of the sensor curve to account for drift.


1) The best method of calibrating a Temperature measurement point to verify the instruments as a complete system, (i.e. Transmitter + Sensor). This type of a verification setup ensures that the entire system is characterized as it will be used in practice. A possible methodology is to take the Temperature assembly and immerse the sensor (sans thermowell or protection tube) into a well-maintained verification bath or a suitable dry block verification cell. The user should then verify the 4-20mA points and trim the upper and lower points as required. 2) An alternate methodology is to calibrate the Temperature Transmitter as an individual unit. This method is also valid, and may be a more practical alternative to system verification. In this instance, the user applies a well-characterized input (such as a decade box or other resistance source) and again, trims the upper and lower points of the transmitter.


Appendix D: Transmitter Troubleshooting

SYMPTOM POTENTIAL SOURCE

CORRECTIVE ACTION

Transmitter Does Not Communicate with HART Communicator

Loop Wiring

High Output Sensor Input Failure or Connection

- Connect a HART communicator and enter the transmitter test mode to isolate a sensor failure. - Check for a sensor open circuit - Check the process variable to see if it is out of range.

Loop Wiring - Check for dirty or defective terminals, interconnecting pins, or receptacles

Power Supply - Check the output voltage of the power supply at the transmitter terminals. It should be 12.0 to 42.4 VDC (over entire 3.90 to 20.5 mA operating range).

Electronics Module - Connect a HART communicator and enter the transmitter test mode to isolate module failure. - Connect a HART communicator and check the sensor limits to ensure verification adjustments are within the sensor range.

Erratic Output Loop Wiring Electronics Module - Connect a HART communicator and

enter the transmitter test mode to isolate module failure.

Low Output or No Output

Sensor Element - Connect a HART communicator and enter the transmitter test mode to isolate a sensor failure. - Check the process variable to see if it is out of range.

Loop Wiring Electronics Module - Connect a HART communicator and

enter the transmitter test mode to isolate module failure. - Connect a HART communicator and check the sensor limits to ensure verification adjustments are within the sensor range.


Appendix E: Notes to Transmitter and Sensor Verification Potential Verification Issues - The use of portable “calibrators” is very widespread in today’s industrial environment. These devices are often able to provide a very accurate, simulated sensor input into a Temperature transmitter (whether resistance for RTD’s or millivolts for Thermocouples). For Thermocouples, the millivolts mode of many calibrators will also enable a built-in CJC (cold junction compensation) feature. It should be noted that while Rosemount Temperature transmitters operate under a “pulse current” environment, many commercially available Calibrators will only operate when fed with “constant current”. If this is the case with a particular verification setup, the operator may switch the Temperature transmitter into “Active Calibrator Mode” to engage the constant current feature of the transmitter. This may be accomplished with a Model 375/475 Communicator or through the AMS (Asset Management Solutions) system. Also note that even if constant current mode is enabled, your particular calibrator may still not operate correctly if there are discrepancies in current between the transmitter and the calibrator. Unstable transmitter readings may result. - For system verifications (i.e. sensor attached to transmitter), users must be cognizant of accuracy issues. When a sensor and transmitter combination is calibrated (for instance in a Calibrator) unwanted EMFs (i.e. voltages) may be introduced, which will affect the verification accuracy. Errors of approx 0.1C for a 0 to 100C span have been detected. While under normal operation the transmitter will compensate for these EMFs, but during the verification trim operation, the transmitter will automatically engage Active Calibrator Mode. This mode does not allow the transmitter to compensate for EMFs. General Note on Transmitter / Sensor Verification In general, the sensor portion of a temperature measurement system will display more drift than the transmitter. Therefore, while transmitter verification and reverification is important, special care and attention should also be focused on the sensor. While it may or may not be less time consuming to recalibrate a sensor and transmitter as a singular unit, more accurate results can oftentimes be obtained if the units are calibrated separately. Verification Equipment - General In general, the combined uncertainty of a verification setup should be at least 4x less than the expected values for the verification. This total verification measurement uncertainty can be calculated by RSS’ing (Root Sum of Squares)


the component uncertainties such as: verification bath, dry block calibrator, standard thermometers, meters, etc. Verification Equipment – Input Accuracy A common issue with single or 2 point trims, is the use of verification sensor inputs that are not accurate enough for the overall verification/measurement accuracy desired. Use of furnaces, industrial sensors, and active calibrators may introduce measurement errors (such as unwanted EMFs) or may just not be accurate enough for the intended application. High quality decade boxes or fixed resistors may be used to avoid these issues.

recommended verification practice for rosemount 3144p · pdf fileprocess control, critical...

Documents