Overview of Temperature Measurement

7/29/2019 Overview of Temperature Measurement

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Overview of TemperatureMeasurement


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Outline

Thermocouples

overview, reference junction, proper connections, types, speciallimits of error wire, time constants, sheathing, potential problems,DAQ setup

RTDs overview, bridges, calibration, accuracy, response time, potentail

problems

Thermistors Infrared Thermometry

fundamentals, emissivity determination, field of view

Other Non-electronic measurement, thin-film heat flux gauge

Temperature Controllers How to Choose

Standards, cost, accuracy, stability, sensitivity, size, contact/non-contact, temperature range, fluid type


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Thermocouples

Seebeck effect If two wires of dissimilar metals are joined at both ends and

one end is heated, current will flow.

If the circuit is broken, there will be an open circuit voltage

across the wires.

Voltage is a function of temperature and metal types.

For small DTs, the relationship with temperature is linear

For largerDTs, non-linearities may occur.

V TD D


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Measuring the Thermocouple Voltage If you attach the thermocouple directly to a voltmeter, you will

have problems.

You have just created another junction! Your displayed voltagewill be proportional to the difference between J1 and J2 (andhence T

1

and T2

). Note that this is Type T thermocouple.


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External Reference Junction

A solution is to put J2 in an ice-bath; then you knowT2, and your output voltage will be proportional toT1-T2.


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Other types of thermocouples

Many thermocouples dont have one copper wire.Shown below is a Type J thermocouple.

If the two terminals arent at the same temperature,this also creates an error.


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Isothermal Block The block is an electrical insulator but good heat

conductor. This way the voltages for J3 and J4 cancelout. Thermocouple data acquisition set-ups includethese isothermal blocks.

If we eliminate the ice-bath, then the isothermal blocktemperature is our reference temperature

1 blockV T T


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Software Compensation

How can we find the temperature of the block? Use athermister or RTD.

Once the temperature is known, the voltage

associated with that temperature can be subtractedoff.

Then why use thermocouples at all? Thermocouples are cheaper, smaller, more flexible and

rugged, and operate over a wider temperature range.

Most data acquisition systems have softwarecompensation built in. To use Labview,youll need to

know if you have a thermister or RTD.


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Hardware Compensation

With hardware compensation, the temperature of theisothermal block again is measured, and then abattery is used to cancel out the voltage of the

reference junction.

This is also called an electronic ice point reference.

With this reference, you can use a normal voltmeterinstead of a thermocouple reader. You need a

separate ice-point reference for every type ofthermocouple.


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Making Thermocouple Beads

Soldering, silver-soldering, butt or spot or beaded gaswelding, crimping, and twisting are all OK.

The third metal introduced doesnt effect results as

long as the temperature everywhere in the bead isthe same.

Welding should be done carefully so as to notdegrade the metals.

If you consistently will need a lot of thermocouples,you can buy a thermocouple welder; you stick the twoends into a hole, hit a button, and the welding isdone.


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Time Constant vs. Wire Diameter


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Time Constant vs. Wire Diameter, cont.


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Thermocouple Types

If you do your owncalibration, you can

usually improve on the

listed uncertainties.


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Thermocouple Types, cont.

Type B very poor below 50C; reference junction temperaturenot important since voltage output is about the same from 0 to42 C

Type E good for low temperatures since dV/dT () is high forlow temperatures

Type J cheap because one wire is iron; high sensitivity butalso high uncertainty (iron impurities cause inaccuracy)

Type T good accuracy but low max temperature (400 C); onelead is copper, making connections easier; watch for heat being

conducted along the copper wire, changing your surface temp Type K popular type since it has decent accuracy and a wide

temperature range; some instability (drift) over time

Type N most stable over time when exposed to elevatedtemperatures for long periods


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Sheathing and SLE

Special Limits of Error wire can be used to improve accuracy.

Sheathing of wires protects them from the environment (fracture,oxidation, etc.) and shields them from electrical interference.

The sheath should extend completely through the medium ofinterest. Outside the medium of interest it can be reduced.

Sometimes the bead is exposed and only the wire is covered bythe sheath. In harsher environments, the bead is also covered.This will increase the time constant.

Platinum wires should be sheathed in non-metallic sheathssince they have a problem with metallic vapor diffusion at hightemperatures.


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Sheathing, cont.

From J. Nicholas & D. White, 2001, Traceable Temperatures: An Introduction to

Temperature Measurement and Calibration, 2nd

ed. John Wiley & Sons.


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Potential Problems

Poor bead construction Weld changed material characteristics because the weld

temp. was too high.

Large solder bead with temperature gradient across it Decalibration

If thermocouples are used for very high or coldtemperatures, wire properties can change due to diffusion ofinsulation or atmosphere particles into the wire, cold-

working, or annealing. Inhomogeneities in the wire; these are especially bad inareas with large temperature gradients; esp. common in iron.Metallic sleeving can help reduce their effect on the finaltemperature reading.


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Potential Problems, cont.

Shunt impedence As temperature goes up, the resistance of many insulation

types goes down. At high enough temperatures, this createsa virtual junction. This is especially problematic for small

diameter wires. Galvanic Action

The dyes in some insulations form an electrolyte in thewater. This creates a galvanic action with a resulting emfpotentially many times that of the thermocouple. Use anappropriate shield for a wet environment. T Type

thermocouples have less of a problem with this.


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Potential Problems, cont.

Thermal shunting It takes energy to heat the thermocouple, which results in a small

decrease in the surroundings temperature. For tiny spaces, thismay be a problem.

Use small wire (with a small thermal mass) to help alleviate thisproblem. Small-diameter wire is more susceptible to decalibrationand shunt impedence problems. Extension wire helps alleviate thisproblem. Have short leads on the thermocouple, and connect themto the same type of extension wire which is larger. Extension wirehas a smaller temperature range than normal wire.

Noise Several types of circuit set-ups help reduce line-related noise. You

can set your data acquisition system up with a filter, too.

Small-diameter wires have more of a problem with noise.


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Potential Problems

Conduction along the thermocouple wire In areas of large temperature gradient, heat can be

conducted along the thermocouple wire, changing the bead

temperature. Small diameter wires conduct less of this heat.

T-type thermocouples have more of a problem with this thanmost other types since one of the leads is made of copperwhich has a high thermal conductivity.

Inaccurate ice-point


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Data Acquisition Systems for

Thermocouples

Agilent, HP, and National Instruments are probablythe most popular DAQ systems

Example National Instruments DAQ setup forthermocouples and costs

item part number cost

16-bit temperature data acquisition card PCI 6232E 1495

analog input module for thermocouples SCXI-1112 695

chassis SCXI-1000 695terminal block for thermocouples SCXI-1303 275

shielded cable SH68-68-EP 95

Total cost: 3255


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Things to Note During System Assembly

Make sure materials are clean, esp. for high temperatures.

Check the temperature range of materials. Materials may degradesignificantly before the highest temperature listed.

Make sure you have a good isothermal junction.

Use enough wire that there are no temperature gradients where its

connected to your DAQ system. If youre using thermocouple connectors, use the right type for your

wire.

If youre using a DAQ system, use the right set-up for thermocouples.

Check the ice-point reference.

Provide proper insulation for harsh environments.

Pass a hair-dryer over the wire. The temperature reading should onlychange when you pass it over the bead.

Mount a thermocouple only on a surface that is not electrically live(watch for this when measuring temperatures of electronics).


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RTDs (Resistance Temperature

Detectors)

Resistivity of metals is a function of temperature.

Platinum often used since it can be used for a wide temperaturerange and has excellent stability. Nickel or nickel alloys are usedas well, but they arent as accurate.

In several common configurations, the platinum wire is exposeddirectly to air (called a bird-cage element), wound around abobbin and then sealed in molten glass, or threaded through aceramic cylinder.

Metal film RTDs are new. To make these, a platinum or metal-

glass slurry film is deposited onto a ceramic substrate. Thesubstrate is then etched with a laser. These RTDs are verysmall but arent as stable (and hence accurate).

RTDs are more accurate but also larger and more expensivethan thermocouples.


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RTD geometry

From Nicholas & White, Traceable Temperatures.

Sheathing: stainless steel or iconel, glass, alumina, quartz

Metal sheath can cause contamination at high temperatures andare best below 250C.

At very high temperatures, quartz and high-purity alumina are

best to prevent contamination.


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Resistance Measurement

Several different bridge circuits are used to determinethe resistance. Bridge circuits help improve theaccuracy of the measurements significantly. Bridge

output voltage is a function of the RTD resistance.


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Resistance/Temperature Conversion

Published equations relating bridge voltage totemperature can be used.

For very accurate results, do your own calibration. Several electronic calibrators are available.

The most accurate calibration that you can do easily yourselfis to use a constant temperature bath and NIST-traceablethermometers. You then can make your own calibrationcurve correlating temperature and voltage.


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Accuracy and Response Time

Response time is longer than thermocouples; for a sheath, response time can easily be 10 s.


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Potential Problems

RTDs are more fragile than thermocouples.

An external current must be supplied to the RTD. This currentcan heat the RTD, altering the results. For situations with high

heat transfer coefficients, this error is small since the heat isdissipated to air. For small diameter thermocouples and still airthis error is the largest. Use the largest RTD possible andsmallest external current possible to minimize this error.

Be careful about the way you set up your measurement device.

Attaching it can change the voltage. When the platinum is connected to copper connectors, a voltage

difference will occur (as in thermocouples). This voltage must besubtracted off.


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Thermistors

Thermistors also measure the change in resistance withtemperature.

Thermistors are very sensitive (up to 100 times more than RTDsand 1000 times more than thermocouples) and can detect verysmall changes in temperature. They are also very fast.

Due to their speed, they are used for precision temperaturecontrol and any time very small temperature differences must bedetected.

They are made of ceramic semiconductor material (metal

oxides). The change in thermistor resistance with temperature is very

non-linear.


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Thermistor Non-Linearity


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Resistance/Temperature Conversion

Standard thermistors curves are not provided asmuch as with thermocouples or RTDs. You oftenneed a curve for a specific batch of thermistors.

No 4-wire bridge is required as with an RTD.

DAQ systems can handle the non-linear curve fiteasily.

Thermistors do not do well at high temperatures andshow instability with time (but for the best ones, thisinstability is only a few millikelvin per year)


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Infrared Thermometry

Infrared thermometers measure the amount ofradiation emitted by an object.

Peak magnitude is often in the infrared region.

Surface emissivity must be known. This can add a lotof error.

Reflection from other objects can introduce error aswell.

Surface whose temp youre measuring must fill the

field of view of your camera.


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Benefits of Infrared Thermometry

Can be used for Moving objects

Non-contact applications

where sensors wouldaffect results or bedifficult to insert orconditions are hazardous

Large distances

Very high temperatures


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Field of View

On some infrared thermometers, FOV is adjustable.


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Emissivity

To back out temperature, surface emissivity must beknown.

You can look up emissivities, but its not easy to get

an accurate number, esp. if surface condition is

uncertain (for example, degree of oxidation). Highly reflective surfaces introduce a lot of error.

Narrow-band spectral filtering results in a moreaccurate emissivity value.


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Ways to Determine Emissivity

1. Measure the temperature with a thermocouple and an infraredthermometer. Back out the emissivity. This method works wellif emissivity doesnt change much with temperature or yourenot dealing with a large temperature range.

2. For temperatures below 500F, place an object covered withmasking tape (which has e=0.95) in the same atmosphere.Both objects will be at the same temperature. Back out theunknown emissivity of the surface.

3. Drill a long hole in the object. The hole acts like a blackbody

with e=1.0. Measure the temperature of the hole, and find thesurface emissivity that gives the same temperature.

4. Coat all or part of the surface with dull black paint which hase=1.0.

5. For a standard material with known surface condition, look upe.


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Spectral Effects

Use a filter to eliminate longer-wavelength atmospheric radiation(since your surface will often have a much higher temperaturethan the atmosphere).

If you know the range of temperatures that youll be measuring,you can filter out both smaller and larger wavelength radiation.Filtering out small wavelengths eliminates the effects of flamesor other hot spots.

If youre measuring through glass-type surfaces, make sure that

the glass is transparent for the wavelengths you care about.Otherwise the temperature you read will be a sort of average ofyour desired surface and glass temperatures.


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Price and Accuracy

Prices range from $500 (for a cheap handheld) to$6000 (for a highly accurate computer-controlledmodel).

Accuracy is often in the 0.5-1% of full range.Uncertainties of 10F are common, but attemperatures of several hundred degrees, this issmall.


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Non-Electronic Temperature Gages

Crayons You can buy crayons with specified meltingtemperatures. Mark the surface, and when the mark melts, youknow the temperature at that time.

Lacquers Special lacquers are available that change from dullto glossy and transparent at a specified temperature. This is atype of phase change.

Pellets These change phase like crayons and lacquers but arelarger. If the heating time is long, oxidation may obscure crayonmarks. Pellets are also used as thermal fuses; they can beplaced so that when they melt, they release a circuit breaker.

Temperature sensitive labels These are nice because you canpeel them off when finished and place them in a log book.


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Non-Electronic Temperature Gages,

cont.

Liquid crystals They change color with temperature.If the calibration is know, color can be determinedvery accurately using a digital camera and

appropriate image analysis software. This is used afair amount for research.

Naphthalene sublimation (to find h, not T) Makesamples out of naphthalene and measure their mass

change over a specified time period. Use the heatand mass transfer analogy to back out h.


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Thin-Film Heat Flux Gauge

Temperature difference across a narrow gap ofknown material is measured using a thermopile.

A thermopile is a group of thermocouples combined

in series to reduce uncertainty and measure atemperature difference.

From Nicholas & White, Traceable Temperatures.


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Thin-Film Heat Flux Gauge, cont.

Fig pg a-26


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Thin-Film Heat Flux Gauge, cont.

Difficulties with these gauges The distance between the two sides is very small, so the

temperature difference is small. The uncertainty in thetemperature difference measurement can be large.

Watch where you place them. If the effective conductivity ofthe gauges is different than the conductivity of the materialsurrounding it, it will be either easier or harder for heat topass through it. Heat will take the path of least resistance, soif you dont position the gauge carefully, you may not be

measuring the actual heat flux.


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Temperature Controllers

Consider the following when choosing a controller

Type of temperature sensor (thermocouples and RTDs arecommon)

Number and type of outputs required (for example, turn on aheater, turn off a cooling system, sound an alarm)

Type of control algarithm (on/off, proportional, PID)

On/off controllers

These are the simplest controllers.

On above a certain setpoint, and off below a certain setpoint

On/off differential used to prevent continuous cycling on and off. This type of controller cant be used for precise temperature control.

Often used for systems with a large thermal mass (wheretemperatures take a long time to change) and for alarms.


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Proportional controllers

Proportional controllers Power can be varied. For example, in a heating unit the

average power supplied will decrease the closer one gets tothe set point.

Power is often varied by turning the controller on and off veryquickly rather than using a VFD

Some proportional controllers use proportional analogoutputs where the output level is varied rather than turningthe controller on and off.


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PID Combines proportional with integral and derivative control.

With proportional control, the temperature usually stabilizes acertain amount above or below the setpoint. This difference iscalled offset.

With integral and derivative control, this offset is compensatedfor so that you end up at the setpoint. This provides veryaccurate temperature control, even for systems where the temp.

is changing rapidly.


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How to Choose a Temperature Control

Device or System

Things to take into account Standards

Cost

Accuracy Stability over time (esp. for high temperatures)

Sensitivity

Size

Contact/non-contact Temperature range

Fluid


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International Standards

North America NEMA (National Electrical Manufacturers Association), UL

(Underwriters Laboratories), CSA (Canadian StandardsAssociation


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Enclosure Ratings

Type 1 general purpose indoor enclosure to preventaccidental contact

Type 2 indoor use, provides limited protection from dirt anddripping water

Type 3 outdoor use to protect against wind-blown dust, sleet,rain, but no ice formation

Type 3R outdoor use to protect against falling rain but no iceformation

Type 4 add splashing or hose-directed water to 3

Type 4x add corrosion

Type 6 add occasional submersion to 4x

etc.


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Choice Between RTDs, Thermocouples,

Thermisters

Cost thermocouples are cheapest by far, followed by RTDs

Accuracy RTDs or thermisters

Sensitivity thermisters

Speed - thermisters Stability at high temperatures not thermisters

Size thermocouples and thermisters can be made quite small

Temperature range thermocouples have the highest range,followed by RTDs

Ruggedness thermocouples are best if your system will betaking a lot of abuse


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Simplified Uncertainty Analysis for Lab 1

Random (precision) error

For temperature measurements, this typically includesfluctuations in the electronics of the data acquisition unitsas well as fluctuations in the quantities measured

Bias (fixed) error

For temperature measurements, this typically includes thefinite resolution of the A/D card (if one is used), the use ofa curve fit for the thermocouples, reading of calibrationthermometers, and conduction and radiation errors.

Total uncertainty is found using the root mean square ofthese two errors

22errorbiaserrorrandomU


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Random Error

95% confidence interval 95% of temperature readingswill fall in this range

=+/- 2 standard deviations

For your lab, during calibration, take at least 35 data points(N=35) at one temperature. Then calculate the average andstandard deviation using the equations below.

Excel can also be used.

2

1

N

1i

2

iT

N

1i

i

TT1N

1S

T

N

1T


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Bias Error

Conduction and radiation errors should be negligible.

For our lab, we will do a simplified analysis.

Once you have a calibration curve fit, find the

deviation between the curve fit and each data point.Use the magnitude of the maximum deviation as yourbias error.

In ME 120 youll learn a lot more about calculating

uncertainties!


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Thank you

Documents

Overview of Temperature Measurement