Overview of Temperature Measurement

1
Overview of Temperature Measurement

• Figures are from www.omega.com Practical
  Guidelines for Temperature Measurement unless
  otherwise noted

2
Outline

• Thermocouples
• overview, reference junction, proper connections,
  types, special limits 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

3
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 larger DTs, non-linearities may occur.

4
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 voltage will be proportional to the
  difference between J1 and J2 (and hence T1 and
  T2). Note that this is Type T thermocouple.

5
External Reference Junction

• A solution is to put J2 in an ice-bath then you
  know T2, and your output voltage will be
  proportional to T1-T2.

6
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.

7
Isothermal Block

• The block is an electrical insulator but good
  heat conductor. This way the voltages for J3 and
  J4 cancel out. Thermocouple data acquisition
  set-ups include these isothermal blocks.
• If we eliminate the ice-bath, then the isothermal
  block temperature is our reference temperature

8
Software Compensation

• How can we find the temperature of the block? Use
  a thermister or RTD.
• Once the temperature is known, the voltage
  associated with that temperature can be
  subtracted off.
• 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 software
  compensation built in. To use Labview,youll need
  to know if you have a thermister or RTD.

9
Hardware Compensation

• With hardware compensation, the temperature of
  the isothermal block again is measured, and then
  a battery 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 voltmeter instead of a thermocouple
  reader. You need a separate ice-point reference
  for every type of thermocouple.

10
Making Thermocouple Beads

• Soldering, silver-soldering, butt or spot or
  beaded gas welding, crimping, and twisting are
  all OK.
• The third metal introduced doesnt effect results
  as long as the temperature everywhere in the bead
  is the same.
• Welding should be done carefully so as to not
  degrade the metals.
• If you consistently will need a lot of
  thermocouples, you can buy a thermocouple welder
  you stick the two ends into a hole, hit a button,
  and the welding is done.

11
Time Constant vs. Wire Diameter
12
Time Constant vs. Wire Diameter, cont.
13
Thermocouple Types
If you do your own calibration, you can usually
improve on the listed uncertainties.
14
Thermocouple Types, cont.

• Type B very poor below 50ºC reference junction
  temperature not important since voltage output is
  about the same from 0 to 42 ºC
• Type E good for low temperatures since dV/dT
  (a) is high for low temperatures
• Type J cheap because one wire is iron high
  sensitivity but also high uncertainty (iron
  impurities cause inaccuracy)
• Type T good accuracy but low max temperature
  (400 ºC) one lead 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
  elevated temperatures for long periods

15
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 of interest. Outside the medium of
  interest it can be reduced.
• Sometimes the bead is exposed and only the wire
  is covered by the sheath. In harsher
  environments, the bead is also covered. This will
  increase the time constant.
• Platinum wires should be sheathed in non-metallic
  sheaths since they have a problem with metallic
  vapor diffusion at high temperatures.

16
Sheathing, cont.

• From J. Nicholas D. White, 2001, Traceable
  Temperatures An Introduction to Temperature
  Measurement and Calibration, 2nd ed. John Wiley
  Sons.

17
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 cold
  temperatures, wire properties can change due to
  diffusion of insulation or atmosphere particles
  into the wire, cold-working, or annealing.
• Inhomogeneities in the wire these are especially
  bad in areas with large temperature gradients
  esp. common in iron. Metallic sleeving can help
  reduce their effect on the final temperature
  reading.

18
Potential Problems, cont.

• Shunt impedence
• As temperature goes up, the resistance of many
  insulation types goes down. At high enough
  temperatures, this creates a virtual junction.
  This is especially problematic for small diameter
  wires.
• Galvanic Action
• The dyes in some insulations form an electrolyte
  in the water. This creates a galvanic action with
  a resulting emf potentially many times that of
  the thermocouple. Use an appropriate shield for a
  wet environment. T Type thermocouples have less
  of a problem with this.

19
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, this may be a
  problem.
• Use small wire (with a small thermal mass) to
  help alleviate this problem. Small-diameter wire
  is more susceptible to decalibration and shunt
  impedence problems. Extension wire helps
  alleviate this problem. Have short leads on the
  thermocouple, and connect them to the same type
  of extension wire which is larger. Extension wire
  has 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.

20
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 than most other types since one of the leads
  is made of copper which has a high thermal
  conductivity.
• Inaccurate ice-point

21
Data Acquisition Systems for Thermocouples

• Agilent, HP, and National Instruments are
  probably the most popular DAQ systems
• Example National Instruments DAQ setup for
  thermocouples and costs

22
Things to Note During System Assembly

• Make sure materials are clean, esp. for high
  temperatures.
• Check the temperature range of materials.
  Materials may degrade significantly 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 only change 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).

23
RTDs (Resistance Temperature Detectors)

• Resistivity of metals is a function of
  temperature.
• Platinum often used since it can be used for a
  wide temperature range and has excellent
  stability. Nickel or nickel alloys are used as
  well, but they arent as accurate.
• In several common configurations, the platinum
  wire is exposed directly to air (called a
  bird-cage element), wound around a bobbin and
  then sealed in molten glass, or threaded through
  a ceramic cylinder.
• Metal film RTDs are new. To make these, a
  platinum or metal-glass slurry film is deposited
  onto a ceramic substrate. The substrate is then
  etched with a laser. These RTDs are very small
  but arent as stable (and hence accurate).
• RTDs are more accurate but also larger and more
  expensive than thermocouples.

24
RTD geometry

• From Nicholas White, Traceable Temperatures.
• Sheathing stainless steel or iconel, glass,
  alumina, quartz
• Metal sheath can cause contamination at high
  temperatures and are best below 250ºC.
• At very high temperatures, quartz and high-purity
  alumina are best to prevent contamination.

25
Resistance Measurement

• Several different bridge circuits are used to
  determine the resistance. Bridge circuits help
  improve the accuracy of the measurements
  significantly. Bridge output voltage is a
  function of the RTD resistance.

26
Resistance/Temperature Conversion

• Published equations relating bridge voltage to
  temperature 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 yourself is to use a constant temperature
  bath and NIST-traceable thermometers. You then
  can make your own calibration curve correlating
  temperature and voltage.

27
Accuracy and Response Time

• Response time is longer than thermocouples for a
  ¼ sheath, response time can easily be 10 s.

28
Potential Problems

• RTDs are more fragile than thermocouples.
• An external current must be supplied to the RTD.
  This current can heat the RTD, altering the
  results. For situations with high heat transfer
  coefficients, this error is small since the heat
  is dissipated to air. For small diameter
  thermocouples and still air this error is the
  largest. Use the largest RTD possible and
  smallest 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 be
  subtracted off.

29
Thermistors

• Thermistors also measure the change in resistance
  with temperature.
• Thermistors are very sensitive (up to 100 times
  more than RTDs and 1000 times more than
  thermocouples) and can detect very small changes
  in temperature. They are also very fast.
• Due to their speed, they are used for precision
  temperature control and any time very small
  temperature differences must be detected.
• They are made of ceramic semiconductor material
  (metal oxides).
• The change in thermistor resistance with
  temperature is very non-linear.

30
Thermistor Non-Linearity
31
Resistance/Temperature Conversion

• Standard thermistors curves are not provided as
  much as with thermocouples or RTDs. You often
  need 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 fit
  easily.
• Thermistors do not do well at high temperatures
  and show instability with time (but for the best
  ones, this instability is only a few millikelvin
  per year)

32
Infrared Thermometry

• Infrared thermometers measure the amount of
  radiation emitted by an object.
• Peak magnitude is often in the infrared region.
• Surface emissivity must be known. This can add a
  lot of error.
• Reflection from other objects can introduce error
  as well.
• Surface whose temp youre measuring must fill the
  field of view of your camera.

33
Benefits of Infrared Thermometry

• Can be used for
• Moving objects
• Non-contact applications where sensors would
  affect results or be difficult to insert or
  conditions are hazardous
• Large distances
• Very high temperatures

34
Field of View

• On some infrared thermometers, FOV is adjustable.

35
Emissivity

• To back out temperature, surface emissivity must
  be known.
• 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 more
  accurate emissivity value.

36
Ways to Determine Emissivity

• Measure the temperature with a thermocouple and
  an infrared thermometer. Back out the emissivity.
  This method works well if emissivity doesnt
  change much with temperature or youre not
  dealing with a large temperature range.
• For temperatures below 500F, place an object
  covered with masking tape (which has e0.95) in
  the same atmosphere. Both objects will be at the
  same temperature. Back out the unknown emissivity
  of the surface.
• Drill a long hole in the object. The hole acts
  like a blackbody with e1.0. Measure the
  temperature of the hole, and find the surface
  emissivity that gives the same temperature.
• Coat all or part of the surface with dull black
  paint which has e1.0.
• For a standard material with known surface
  condition, look up e.

37
Spectral Effects

• Use a filter to eliminate longer-wavelength
  atmospheric radiation (since your surface will
  often have a much higher temperature than 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 flames or
  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 of
  your desired surface and glass temperatures.

38
Price and Accuracy

• Prices range from 500 (for a cheap handheld) to
  6000 (for a highly accurate computer-controlled
  model).
• Accuracy is often in the 0.5-1 of full range.
  Uncertainties of 10F are common, but at
  temperatures of several hundred degrees, this is
  small.

39
Non-Electronic Temperature Gages

• Crayons You can buy crayons with specified
  melting temperatures. Mark the surface, and when
  the mark melts, you know the temperature at that
  time.
• Lacquers Special lacquers are available that
  change from dull to glossy and transparent at a
  specified temperature. This is a type of phase
  change.
• Pellets These change phase like crayons and
  lacquers but are larger. If the heating time is
  long, oxidation may obscure crayon marks. Pellets
  are also used as thermal fuses they can be
  placed so that when they melt, they release a
  circuit breaker.
• Temperature sensitive labels These are nice
  because you can peel them off when finished and
  place them in a log book.

40
Non-Electronic Temperature Gages, cont.

• Liquid crystals They change color with
  temperature. If the calibration is know, color
  can be determined very accurately using a digital
  camera and appropriate image analysis software.
  This is used a fair amount for research.
• Naphthalene sublimation (to find h, not T) Make
  samples out of naphthalene and measure their mass
  change over a specified time period. Use the heat
  and mass transfer analogy to back out h.

41
Thin-Film Heat Flux Gauge

• Temperature difference across a narrow gap of
  known material is measured using a thermopile.
• A thermopile is a group of thermocouples combined
  in series to reduce uncertainty and measure a
  temperature difference.

From Nicholas White, Traceable Temperatures.
42
Thin-Film Heat Flux Gauge, cont.

• Fig pg a-26

43
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 the temperature difference
  measurement can be large.
• Watch where you place them. If the effective
  conductivity of the gauges is different than the
  conductivity of the material surrounding it, it
  will be either easier or harder for heat to pass
  through it. Heat will take the path of least
  resistance, so if you dont position the gauge
  carefully, you may not be measuring the actual
  heat flux.

44
Temperature Controllers

• Consider the following when choosing a controller
• Type of temperature sensor (thermocouples and
  RTDs are common)
• Number and type of outputs required (for example,
  turn on a heater, 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
  (where temperatures take a long time to change)
  and for alarms.

45
Proportional controllers

• Proportional controllers
• Power can be varied. For example, in a heating
  unit the average power supplied will decrease the
  closer one gets to the set point.
• Power is often varied by turning the controller
  on and off very quickly rather than using a VFD
• Some proportional controllers use proportional
  analog outputs where the output level is varied
  rather than turning the controller on and off.

46
PID

• Combines proportional with integral and
  derivative control.
• With proportional control, the temperature
  usually stabilizes a certain amount above or
  below the setpoint. This difference is called
  offset.
• With integral and derivative control, this offset
  is compensated for so that you end up at the
  setpoint. This provides very accurate temperature
  control, even for systems where the temp. is
  changing rapidly.

47
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

48
International Standards

• North America
• NEMA (National Electrical Manufacturers
  Association), UL (Underwriters Laboratories), CSA
  (Canadian Standards Association

49
Enclosure Ratings

• Type 1 general purpose indoor enclosure to
  prevent accidental contact
• Type 2 indoor use, provides limited protection
  from dirt and dripping 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 ice formation
• Type 4 add splashing or hose-directed water to
  3
• Type 4x add corrosion
• Type 6 add occasional submersion to 4x
• etc.

50
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 be taking a lot of abuse

51
Simplified Uncertainty Analysis for Lab 1

• Random (precision) error
• For temperature measurements, this typically
  includes fluctuations in the electronics of the
  data acquisition units as well as fluctuations in
  the quantities measured
• Bias (fixed) error
• For temperature measurements, this typically
  includes the finite resolution of the A/D card
  (if one is used), the use of a curve fit for the
  thermocouples, reading of calibration
  thermometers, and conduction and radiation
  errors.
• Total uncertainty is found using the root mean
  square of these two errors

52
Random Error

• 95 confidence interval 95 of temperature
  readings will fall in this range
• /- 2 standard deviations
• For your lab, during calibration, take at least
  35 data points (N35) at one temperature. Then
  calculate the average and standard deviation
  using the equations below.
• Excel can also be used.

53
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 your bias error.
• In ME 120 youll learn a lot more about
  calculating uncertainties!