lwc-300/301 operator manual · lwc-300/301 operation manual doc-0361 rev c 9© 2018 droplet...

LWC-300/301

Liquid Water Content Sensor

Operator’s Manual

DOC-0361, Revision C

2400 Trade Centre Avenue

Longmont, CO 80503 USA

A L L R I G H T S R E S E R V E D

DOC-0361 Rev C © 2018 DROPLET MEASUREMENT TECHNOLOGIES LLC

i i

C O N T E N T S

1.0 General Introduction ......................................................................................... 4

1.1 Software License ................................................................................................................... 4

1.2 Warranty ............................................................................................................................... 4

2.0 Product Introduction ......................................................................................... 4

2.1 General Specifications ........................................................................................................... 6

2.2 Electrical Specifications ......................................................................................................... 6

2.3 Sensing Element Specifications ............................................................................................. 6

2.4 Physical Specifications ........................................................................................................... 6

2.5 Operating Limits .................................................................................................................... 7

3.0 Theory of Operation .......................................................................................... 7

4.0 Installation ........................................................................................................ 8

4.1 Sensor Strut ........................................................................................................................... 8

4.2 Electronic Control Box ........................................................................................................... 8

4.3 System Power ........................................................................................................................ 8

4.4 Anti-Ice Power ....................................................................................................................... 9

5.0 Operation Instructions ...................................................................................... 9

5.1 Ground Testing before use .................................................................................................... 9

5.2 Flight Operations ................................................................................................................... 9

6.0 Servicing or Replacing the LWC Sensor ............................................................. 10

6.1 Testing the LWC sensor card ............................................................................................... 11

7.0 LWC Calculations and Formulas ........................................................................ 13

8.0 Setting Wire Temperature Estimation............................................................... 14

9.0 Data Interpretation .......................................................................................... 14

Appendix A: References ................................................................................................. 15

Appendix B: Wiring Diagrams ........................................................................................ 16

Appendix C: Connections for NI-DAQ Option .................................................................. 19

Appendix D: Mounting Guides ....................................................................................... 20

Appendix E: Min/Max Resistance @ Temp ..................................................................... 21

LWC-300/301 Operation Manual

DOC-0235 Rev C i i i © 2018 DROPLET MEASUREMENT TECHNOLOGIES LLC

Appendix F: Revisions to Manual .................................................................................. 22

F i g u r e s

Figure 1: Liquid Water Content Sensor Strut (Left) and Control Box (Right) ............................. 5

Figure 2: LWC-300 Circuitry Used to Control Master Coil Temperature .................................... 7

Figure 3: Sensor Strut, Sensor Cable, and Control Box ............................................................. 8

Figure 4: Removing the LWC-301 cover for access to the LWC Sensor card ............................ 10

Figure 5:Removing the LWC Sensor Card .............................................................................. 10

Figure 6: Testing details of the sensor card ........................................................................... 11

Figure 7:Reinserting the sensor card after service ................................................................. 11

Figure 8: Check alignment and seal with O-ring grease ......................................................... 12

Figure 9: Re-assembly of the strut after service .................................................................... 12


4

1.0 General Introduction

In no event will Droplet Measurement Technologies, LLC (DMT) be liable for direct, indirect, special,

incidental or consequential damages resulting from any defect or omissions in this manual.

DMT reserves the right to make changes to this manual and the products it describes at any time,

without notice or obligation. Revised editions are found on the manufacturer’s website.

All DMT product names and the Droplet Measurement Technologies Logo are trademarks of Droplet

Measurement Technologies, LLC.

All other brand and product names are trademarks, or registered trademarks, of their respective owners.

1.1 Software License

DMT licenses its software only upon the condition that you accept all of the terms contained in this

license agreement.

This software is provided by DMT “as is” and any express or implied warranties, including, but not limited

to, the implied warranties of merchantability and fitness for a particular purpose are disclaimed. Under

no circumstances and under no legal theory, whether in tort, contract, or otherwise, shall DMT or its

developers be liable for any direct, indirect, incidental, special, exemplary, or consequential damages

(including damages for work stoppage; computer failure or malfunction; loss of goodwill; loss of use, data

or profits; or for any and all other damages and losses). Some states do not allow the limitation or

exclusion of implied warranties and you may be entitled to additional rights in those states.

1.2 Warranty

The seller warrants that the equipment supplied will be free from defects in material and workmanship

for a period of eighteen months from date of shipment or 12 months from the date of either installation

or first use whichever comes first. When returning the equipment to DMT for warranty or service

procedures, the equipment owner will pay for shipping to DMT, while DMT will pay the return shipping

expense. Consumable components, such as tubing, filters, pump diaphragms, and Nafion humidifiers and

dehumidifiers are not covered by this warranty.

2.0 Product Introduction

The LWC-300/301 is the DMT implementation of an instrument originally developed by Warren

King at the Australian CSIRO (see Appendix A: References). It is used for measurement of cloud

liquid water content. This sensor, oftentimes referred to as the "King Probe", is used primarily for

the study of cloud micro-physical processes, aircraft icing, aircraft icing certifications, and cloud

seeding.

The LWC-300 operates under the principle that liquid water content can be calculated from

measuring the heat released when water droplets are vaporized. A heated cylinder is exposed to

the airstream and intercepts oncoming droplets. The electronics maintain this sensor at a constant


DOC-0361 Rev C 5 © 2018 DROPLET MEASUREMENT TECHNOLOGIES LLC

temperature (approximately 150o C) and monitors the power required to regulate the temperature

as droplets vaporize. This power is directly related to the amount of heat taken away by convection

plus the heat of vaporization. The convective heat losses are known empirically and vary with

airspeed, ambient temperature and ambient pressure. The liquid water content is calculated from

total power requirements minus convective power losses.

The system consists of a sensing coil held in the external air stream by a heated strut (see Error!

Not a valid bookmark self-reference., left) and an electronic control box with digital signal

processor (Error! Not a valid bookmark self-reference., right). Customers who have purchased the

optional Particle Analysis and Display System (PADS) also receive a computer with PADS software

installed. The electronic control box uses a serial RS-422 protocol to deliver Analog, Digital, or a

simultaneous delivery of both Analog and Digital via a 10-pin connector to communicate with the

PADS software, which displays the amount of power consumed in keeping the wire at the constant

temperature, the calculated LWC, and several housekeeping parameters.

Figure 1: Liquid Water Content Sensor Strut (Left) and Control Box (Right)


6

2.1 General Specifications

Measured Parameters: Liquid Water Content

Measured LWC Rangei 0 – 3 g/m3

Output Type Serial, RS-422, 115,200 baud rate, 8-N-1

Air Speed Range 0 – 200 m/sec

Calibration: Not required

Special Features • Interchangeable circuit card sensor

• DSP control

Serial Output Rate 1hz or 10hz

Control Frequency Response >25 Hz

Recorded Parameters See DOC-0290 Rev-A, PADS LWC-300 Manual.pdf

2.2 Electrical Specifications

Operating Power: 28 VDC, 7.5 A maximum

Anti-ice Power: 28 VDC, 8 A

2.3 Sensing Element Specifications

Master Coil length: 2.0 cm

Master Coil diameter: 0.18 cm

Effective wire temperature: 150°C

NOTE: The PADS system is preconfigured with these constant values in place.

2.4 Physical Specifications

Weight:

• Control Box: .5 kg / 1 lb

• Sensor Strut: .6 kg / 1.25 lb

Dimensions:

• Sensor strut: 11.5 cm W x 6.4cm L x 19.5 cm H (4.5” W x 2.5” L x 7.625” H)

• Electronic box: 12.1 cm W x 12.1 cm L x 6.7 cm H (4.75” W x 4.75” L x 2.625” H)

• Electronics box mounting plate: 9.7 cm W x 15.3 cm L x 0.3 cm H (3.8” W x 6.0“ L x 0.1” H)

i Measured range depends on airspeed, temperature, and pressure. 0 – 3 g/m3 is typical.



2.5 Operating Limits

Altitude: 0 to 12,200 m / 0 to 40,000 ft

Temperature: -40 to +40 °C / -40 to +104 °F

Humidity: 0 – 100%

3.0 Theory of Operation

The hot-wire LWC sensor coil is maintained at approximately 150o C, and acts as a variable resistor

in one arm of a Wheatstone Bridge circuit. The resistance of the sensor wire decreases as the wire

temperature decreases. Temperature decreases can be caused by vaporization of water droplets,

or convective heat losses due to the air density that flows past the sensor. The resistance of the

Master Coil is directly proportional to its temperature. The DSP control circuit maintains the sensor

at constant temperature by maintaining it at a constant resistance. A Wheatstone Bridge is formed

of four resistances, of which the Master Coil LWC sensor is one. The LWC 300 has three fixed

resistors that form the other legs (see Figure 2). To determine current flow through the LWC

sensor, voltage drop across a 0.5Ω resistor is measured and multiplied x2. The voltage at the top of

the Wheatstone Bridge times the current, gives the power required to maintain a constant

resistance (temperature) of the Master Sensing Coil.

The 1K potentiometer and the 8.45K resistor supplies a “Set Point” voltage relative to changes in

the Switching Power Supply’s DC level. This Set Point voltage is used in the control algorithm and

generates an “Error” voltage. When the bridge circuit is in balance, an Error of 1.46V will be logged.

If not in balance, the pulse-width modulated control signal will increase or decrease the duty cycle

as needed.

Figure 2: LWC-300 Circuitry Used to Control Master Coil Temperature


8

4.0 Installation

4.1 Sensor Strut

The sensor strut should be mounted on the aircraft with the sensing coil facing forward into the air

stream. The preferable location is on the underside of the wing, away from any obstruction

upstream and an area where there is no prop wash or the possibility of water shedding from

propellers or other surfaces. Mounting on the fuselage of large aircraft should be avoided unless an

extension strut is used to insure the sensing wire is out of the boundary layer, far away from areas

where there may be “shadowing” or disturbed airflow that would bias the measurements. Refer to

Appendix E “Mounting Guides” for dimensions of the strut and control box mount holes.

Consider the aircraft’s research speed and angle-of-attack when selecting a location and

orientation. The sensing strut may require a "skin-doubler” to insure a stable mount. Consult an

aircraft structural engineer before the final location is selected.

CAUTION! Keep the sensor strut and control box away from any transmitting antennas and cables.

These can cause RFI noise problems in electronic control circuits or aircraft communication radios.

Figure 3: Sensor Strut, Sensor Cable, and Control Box

4.2 Electronic Control Box

The control box should be mounted within 1 meter of the Sensor Strut. The factory-supplied cable

should be used to inter-connect the Sensor Strut and the Control Box. See Appendix-C for cable and

wiring details.

4.3 System Power

The 28 VDC system power should be protected with a 10 Amp circuit breaker. The LWC-300/301

can be operated on the ground and not burn out or damage the sensing coil.

DATA POWER



4.4 Anti-Ice Power

NOTE: The strut will become hot and can be damaged if left activated for extended periods.

The 28 VDC Anti-Ice power should be protected with a 10 Amp circuit breaker. The Anti-Ice power

should be applied just after takeoff and deactivated before landing. If possible, direct the anti-ice

power through a 10A relay which is activated by the aircraft weight-on-wheels switch. If not, pull

the anti-ice circuit breaker when the aircraft is on the ground. The anti-ice system can be tested on

the ground for periods of 15 seconds or less, one can feel the strut warming within that time.

5.0 Operation Instructions

5.1 Ground Testing before use

The LWC system should be tested before flight (1) after initial installation, (2) if a sensor has been

replaced, or (3) if a problem with the system is suspected. See Section 5 for instructions on how to

access the LWC Card. One way to achieve operational validation, direct the output air of a Shop-Vac

onto the sensor coil. This will give sufficient airflow to operate the sensor. At system power-up,

“Start Sampling,” and “Enable Hotwire”. PADS will display a power spike as the sensor reaches

operating temperature. A hand water-sprayer can then be used to add mist to the air flow. PADS

will display the power required to maintain the sensor at operating temperature as the spray cools

the sensor or if air velocity is increased. The “Fixed LWC” display will be accurate if you have

matched the TAS, ambient pressure/temperature with the Shop-Vac and surrounding conditions.

5.2 Flight Operations

System power is typically applied after engine start, Anti-Ice power should be applied after takeoff.

Always deactivate Anti-ice power before landing to preserve your unit.

A “Slave Coil Status” indicator light is displayed at the top of the LWC-300 software tab. A green

light indicates operational Slave Coils. A red light indicates that the Slave Coils have failed. Without

functioning slave coils, the master coil element will have heat loss transversely out of the ends of

the Master Coil. This will negatively influence measurement quality. If Slave Coils have failed, the

sensor PCB should be replaced at first opportunity. View this monitor at the end of each flight to

ensure that the Slave Coil is operational throughout the flight.

Failure of the Master Coil would be indicated by a near-zero power and negative LWC estimation. If

the Master Coil or Slave Coils have failed, replace the sensor with a fresh sensor card before the

next flight (See Section 5).

NOTE: When using Dual mode (Analog +Digital) cable assembly (ASSY-1899), the LWC 300/301

requires serial communication with PADS to begin operating and provide analog output.


1 0

6.0 Servicing or Replacing the LWC Sensor

The LWC sensor card is consumable component and will eventually wear out. The sensor life can be

shortened by debris on the runway, bug or bird strikes, hail impacts, or the abrasive nature of flying

through ice crystals. The LWC-300/301 comes with one sensor card installed and 2 spare cards.

1. Using a static dissipative wrist-strap and a 3/32” hex key, unscrew the seven screws that

hold the LWC strut side-plate in place (Figure 4 left).

Figure 4: Removing the LWC-301 cover for access to the LWC Sensor card

2. Open the two sides of the LWC strut. Be careful to not pull the two halves far apart, or the

connecting heater wires (Figure 4 right) could be damaged.

3. Remove the LWC sensor by pulling it out of its edge-card socket (Figure 5). Be careful not to

damage or displace the LWC Strut sealing gaskets that are located under the card.

Figure 5: Removing the LWC Sensor Card

Heater wires

SEALING GASKET

3/32”

Hex key


DOC-0361 Rev C 1 1 © 2018 DROPLET MEASUREMENT TECHNOLOGIES LLC

6.1 Testing the LWC sensor card

Figure 6: Testing details of the sensor card

4. After removing the card, test using an Ohm meter across the specified contacts (Figure 6).

Replace the sensor card if any of the values are below or above specified numbers.

5. Reinstall the card. The card is symmetrical, so it does not matter which side faces up. The

sensor card must be inserted into the socket so that the hole in the card aligns with the

strut screw hole as shown in (Figure 7).

Figure 7: Reinserting the sensor card after service

Slave Coil 1Ω Master Coil 2Ω Slave Coil 1Ω

Socket


1 2

6. Apply a thin layer of O-ring grease every time the unit is re-assembled to both O-rings

(arrows) to help prevent water ingestion. (Figure 8).

Figure 8: Check alignment and seal with O-ring grease

7. Make sure the heater wires don’t get pinched when assembling the two halves of the unit.

Carefully screw down the seven screws that secure the LWC strut (Figure 9).

Figure 9: Re-assembly of the strut after service



7.0 LWC Calculations and Formulas

The power dissipated by the sensing wire is the total of the convective, radiative and latent heat of

vaporization losses, i.e.

Pt = Pd + Pr + Pw

The convective heat losses, Pd, result from the air that flows past the heated sensor. The radiative

heat losses are negligible compared to the convective losses and are not included in the overall

heat balance equation. The latent heat losses are a result of water droplets that strike or pass near

the sensor and are vaporized. The convective heat loss term has been empirically derived

(Zukauskas and Ziugzda, 1985) and is related to the Reynolds number and Prandtl number by

Pd = A0πk(Ts - Ta)RexPRy

where k is the thermal conductivity of the air, Ts is the sensor temperature, Ta is the air

temperature, Re is the Reynold's number, Pr is the Prandtl number and A0, x and y are constants for

a heated cylinder at high Reynold's number. The Reynold’s number is expressed as

Re = v

VP

where ρ, V, P and are the air density, velocity, pressure and viscosity, respectively, and d is the

diameter of the sensor. The Prandtl number is the thermal conductivity divided by the viscosity.

The density, viscosity, and thermal conductivity are all functions of temperature. The temperature

used to calculate these quantities is referred to as the film temperature and is normally computed

as the arithmetic average between the sensor temperature and the environmental air temperature.

The heat loss that results from vaporizing droplets is computed as follows:

Pw = ldvw [LV+c(Tb - Ta)]

where l is the sensor length, w is the liquid water content, Lv is the latent heat of vaporization, c is

the specific heat of water, and Tb is the boiling point of water. When the total power is measured,

the liquid water content is calculated as

w = ( ) abv

d

TTclldv −+

−


1 4

8.0 Setting Wire Temperature Estimation

The temperature of the sensor wire is difficult to measure directly. The temperature of the sensor

is set at the factory to be approximately 150 °C by using a 60m/s wind tunnel. Each sensor has a

slightly different resistance causing a slightly different effective temperature. The sensor wire

temperature is more accurately determined from actual flight data. Assuming that air speed,

pressure altitude, and ambient temperature are accurately known, and being used by PADS, the

only remaining unknown is the sensor wire temperature. Sensor wire temperature affects the

Reynold’s number, the Prandtl number, and thermal conductivity in the dry air power (DAP) and

LWC calculation. In principle, if the sensor wire temperature is correctly selected in the software,

then the derived LWC value will be equal to zero when in clear dry air. Under the main LWC-300

tab, go to the Tools tab, and you will see an Effective Wire Temperature parameter. Adjust this

value until the LWC, Using Calculated DAP (LWC tab) reads close to zero.

The software setting for the effective sensor wire temperature is very important to the LWC

calculation. If the temperature is too low, the calculated dry air power loss will be too low and the

calculated LWC will be greater than zero in clear air. Likewise, assuming too hot of a temperature

will cause an overestimation of the dry air power term, leading to negative LWC values in clear air.

The best technique is to fly the sensor in clear air over the range of air speeds and temperatures

that the sensor is expected to encounter during normal operation. During these flights, try a

number of different settings for the wire temperature parameter. Select the setting that causes the

smallest possible zero offset under all conditions. Once the optimum sensor wire temperature is

determined, save the configuration file to preserve the new setting for future flights.

9.0 Data Interpretation

The LWC-300/301 sensor is limited by collection efficiency considerations on the small droplet end

of the spectrum and by vaporization time on the large end. The sensor has a diameter of

approximately 1.8 mm and small water droplets, less than 10 µm, will not impact with 100%

efficiency as they follow the airflow around the sensor. These losses are typically about 5% for 10

µm droplets but increase to greater than 20% for diameters less than 5 µm. Since the largest

fraction of the water mass is carried in droplets greater than 10 µm it is not usually necessary to

compensate for the smaller droplet size. In developing clouds, however, near cloud base where

droplets are still quite small, or in cloud edges where entrainment and evaporation is occurring, the

underestimation of liquid water content can be significant.

On the large droplet side, the LWC-300/301 begins to underestimate the liquid water contained in

drops larger than 50µm as a result of incomplete evaporation as these larger droplets impact,

shatter, and water is carried away by the airstream before sufficient heat has been transferred to

vaporize them.



Appendix A: References

• King, W.D., D.A. Parkin, and R.J. Handsworth, 1978: A hot-wire liquid water device having fully

calculable response characteristics, J. Appl. Met., 17, 1809-1813.

• Zukauskas, A. And J. Ziugzda, 1985: Heat Transfer of a Cylinder in a Crossflow, G.F. Hewitt,

Editor, Hemisphere Publishing Corp., Washington D.C., 208 pp.

• DOC-0476 - DB9F PIN Removal and Re-install

• DOC-0290 – PADS Hotwire LWC Module

• DOC – 0342 PADS Appendix C Module


1 6

Appendix B: Wiring Diagrams


1 8



Appendix C: Connections for NI-DAQ Option


2 0

Appendix D: Mounting Guides



Appendix E: Min/Max Resistance @ Temp


2 2

Appendix F: Revisions to Manual

Rev. Date Rev. No. Summary Section

8/21/13 A Updated DOC-0354 for LWC-300 Throughout

10/1/14 A-2 Updated block diagram, Theory of Operation, and

wiring diagram

Throughout

2/20/18 B Updated Wiring diagrams, Reference DOC-0476 Appendices

3/27/18 C “LWC-300/301” New Photos, with and without NIDaq Throughout

lwc-300/301 operator manual · lwc-300/301 operation manual doc-0361 rev c 9© 2018 droplet...

Documents