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RESISTANCE TEMPRATURE DETECTOR(RTD)PRESENTED BYMITESH KUMARRall No-13/EI/26Applied Electronics & Instrumentation Engg.Haldia Institute Of Technology

INTRODUCTION CONSTRUCTION RESISTANCE MATERIALS FOR RTD CONFIGURATION APPLICATION ADVANTAGE DISADVANTAGE CONCLUSION REFERENCE

CONTENT

Resistance thermometers, also calledresistance temperature detectors(RTDs), aresensorsused to measure temperature by correlating the resistance of the RTD element with temperature. Most RTD elements consist of a length of fine coiled wire wrapped around a ceramic or glass core. The element is usually quite fragile, so it is often placed inside a sheathed probe to protect it. The RTD element is made from a pure material, typically platinum, nickel or copper. The material has a predictable change in resistance as the temperature changes and it is this predictable change that is used to determine temperature.WHAT IS RTD?????

The construction is typically such that the wire is wound on a form (in a coil) on notched mica cross frame to achieve small size, improving the thermal conductivity to decrease the response time and a high rate of heat transfer is obtained. In the industrial RTDs, the coil is protected by a stainless steel sheath or a protective tube.

So that, the physical strain is negligible as the wire expands and increase the length of wire with the temperature change. If the strain on the wire is increasing, then the tension increases. Due to that, the resistance of the wire will change which is undesirable. So, we dont want to change the resistance of wire by any other unwanted changes except the temperature changes.CONSTRUCTION

COMMON RESISTANCE MATERIALS FOR RTD:PLATINUM (most popular and accurate)

2. NICKEL

3. COPPER

4. BALCO(rare)

Platinum resistance thermometers (PRTs)offer excellent accuracy over a wide temperature range (from200 to +850 C). Standard sensors are available from many manufacturers with various accuracy specifications and numerous packaging options to suit most applications. Unlike thermocouples, it is not necessary to use special cables to connect to the sensor.

The principle of operation is to measure the resistance of a platinum element. The most common type (PT100) has a resistance of 100 ohms at 0 C and 138.4 ohms at 100 C. There are also PT1000 sensors that have a resistance of 1000 ohms at 0 C.PLATINUM RTD

RESISTANCE VS TEMPRATURE CHARACTERISTICS OF RTD.(Pt, Cu, Ni)

If we now consider a four-wire configuration with a 1mA precision current source,[9]we obtain the following relationship between temperature and measured voltage

TEMPRATURE AND RESISTANCE RELATIONSHIP

Here , R(t)is the resistance at temperatureT,R(o) is the resistance at 0C, and the constants (for an alpha=0.00385 platinum RTD) are

Since theBandCcoefficients are relatively small, the resistance changes almost linearly with the temperature.For positive temperature, if we resolve the quadratic equation we obtain the following relationship between temperature and resistance:

The relation between temperature and resistance is given by theCallendar-Van Dusen- equation.

2-WIRE RTD(CONFIGuRATION)The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required, as the resistance of the connecting wires is added to that of the sensor, leading to errors of measurement. This configuration allows use of 100meters of cable. This applies equally to balanced bridge and fixed bridge system.

For a balanced bridge the usual setting is with R2=R3 and R1 around the middle of the range of the RTD. So for example, if we are going to measure between 0 and 100C (32 and 212F), RTD resistance will range from 100 ohm to 138.4 ohm. We would choose R1=120 ohm. In that way we get a small measured voltage in the bridge.

3-WIRE RTD

If we know the value of Vs and Vo , we can find Rg then tempereture value using calibration equation.Assume R1=R2If R3=Rg ;then Vo=0 and the bridge is balanced . We can just slove the equation to get expression for RgThis expression assume , when the load resistance R(load)=0,if R(load) is present in a situation then Rg becomes,So, there is an error in the RTD resistance value because of the R(load) resistance.

Air conditioning and refrigeration servicing

Food Processing

Stoves and grills

Textile production

Plastics processing

Petrochemical processing

Micro electronics

Air, gas and liquid temperature measurement

APPLICATION

Very high accuracy

Excellent stability

Interchangeable sensors

No drift

Ability to measure narrow spans

Suitability for remote measurementAdvantages

Low sensitivity

Higher cost than thermocouples

No point sensing

Affected by shock and vibration

Requires three or four-wire operationDISADVANTAGES

CONCLUSIONRTD is most liner ,most accurate temperature sensors to measure temperature.Right from the stoves and grills to the gain time Petrochemical processing industry ,RTD sensors are used everywhere.The worthiness of RTD far outweigh its disadvantage and this in turn makes it a boon.

REFERENCEhttps://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&cad=rja&uact=8&ved=0ahUKEwjP2ZHsoqrMAhVIG44KHYcxC58QFgg2MAQ&url=http%3A%2F%2Fwww.omega.com%2Fprodinfo%2Frtd.html&usg=AFQjCNHejCbJQ2n-yU-2erDSSherIJ8F6w&sig2=idHtZePthjJGOxMK_GtvqAhttps://www.google.co.in/url?sa=t&rct=j&q=&esrc=s&source=web&cd=6&cad=rja&uact=8&ved=0ahUKEwjP2ZHsoqrMAhVIG44KHYcxC58QFgg9MAU&url=http%3A%2F%2Fwww.omega.com%2Ftemperature%2Fpdf%2Frtd_gen_specs_ref.pdf&usg=AFQjCNEu4PaDriKgKPbAxMWcpbLRkbQiZQ&sig2=B7o13K01t372PnLFKDZU5g