Measurement of Temperature Thermocouples Resistance (RTD) Thermistors Pyrometers

Thermocouples A thermoelectric temperature sensor consists of two

dissimilar metallic wires, e.g., one chromel and one constantan, coupled at the probe tip (measurement junction) and extended to the reference (known temperature) junction.

The temperature difference between the probe tip and the reference junction is detected by measuring the change in voltage (electromotive force, EMF) at the reference junction.

The absolute temperature reading can then be obtained by combining the information of the known reference temperature and the difference of temperature between probe tip and the reference.

Three Wire Layouts of Typical Thermocouples

Common Specifications Common commercially available thermocouples

are specified by ISA (Instrument Society of America) types.

Type E, J, K, and T are base-metal thermocouples and can be used up to about 1000C (1832F).

Type S, R, and B are noble-metal thermocouples and can be used up to about 2000C (3632F).

ISA Material(+ & -)

Temperature Range

C (F)

Sensitivity@ 25C (77F)

V/C(V/F)

Error* App.

EChromel & Constantan(Ni-Cr & Cu-Ni)

-270~1000(-450~1800)

60.9(38.3)

LT:1.67C(3F)HT:0.5% I,O

JIron & Constantan(Fe & Cu-Ni)

-210~1200(-350~2200)

51.7(28.7)

LT:2.2~1.1C(4~2F)HT:0.375~0.75%

I,O,R,V

KChromel & Alumel(Ni-Cr & Ni-Al)

-270~1350(-450~2500)

40.6(22.6)

LT:2.2~1.1C(4~2F)HT:0.375~0.75%

I,O

TCopper & Constantan(Cu & Cu-Ni)

-270~400(-450~750)

40.6(22.6)

LT:1~2%HT:1.5% or 0.42C(0.75F)

I,O,R,V

R

Platinum & 87% Platinum/ 13% Rhodium(Pt & Pt-Rh)

-50~1750(-60~3200)

6(3.3)

LT:2.8C(5F)HT:0.5% I,O

S

Platinum & 90% Platinum/ 10% Rhodium(Pt & Pt-Rh)

-50~1750(-60~3200)

6(3.3)

LT:2.8C(5F)HT:0.5% I,O

B

70% Platinum/ 30% Rhodium & 94% Platinum/ 6% Rhodium(Pt-Rh & Pt-Rh)

-50~1750(-60~3200)

6(3.3)

LT:2.8C(5F)HT:0.5% I,O

LT = Low temperature range, HT = High temperature range

I = Inert media, O = Oxidizing media, R = Reducing media, V = Vacuum

Constantan, Alumel, and Chromel are trade names of their respective owners.

Pros:- Low cost. - No moving parts, less likely to be broken. - Wide temperature range. - Reasonably short response time. - Reasonable repeatability and accuracy.

Cons:- Sensitivity is low, usually 50 V/C (28 V/F) or less. Its low voltage output may be masked by noise. This problem can be improved, but not eliminated, by better signal filtering, shielding, and analog-to-digital (A/D) conversion. - Accuracy, usually no better than 0.5 C (0.9F), may not be high enough for some applications. -Requires a known temperature reference, usually 0C (32F) ice water. Modern thermocouples, on the other hand, rely on an electrically generated reference. - Nonlinearity could be bothersome. Fortunately, detail calibration curves for each wire material can usually be obtained from vendors.

Resistance Temperature Detector (RTD)

uses the fact that the resistance of metals increases with temperature

The resistance of commercially available RTDs ranges from 10 to 25,000 . More common ones are 100, 200, and 1000 strain-free platinum (>99.999%) probes and 10 copper probes.

Generally, the higher the resistance, the less affected the RTD will be due to small resistance/voltage fluctuations in the lead wires and circuit.

Common metals used in RTDs include platinum, copper, nickel, BalcoTM (70% Ni-30% Fe), and tungsten.

Material TemperatureRange Note

Platinum (Pt) -260~1000 C(-440~1800 F)< 550 C (1022 F) in most applications

Copper (Cu) -200~260 C(-330~500 F)

Nickel (Ni) -200~430 C(-330~800 F) Linearity is not good

Balco(70% Ni-30% Fe)

-100~230 C(-150~450 F)

Linearity is not good; cheap to fabricate; high resistance

Tungsten (W) -100~1200 C(-150~2200 F)

Pros:Stable and accurate. Linearity is better than thermocouples. Higher signal-to-noise ratio.

Cons:More expensive. Self heating. Requires a current source. Response time may not be fast enough for some applications.

Thermistor (Bulk Semiconductor Sensor)

Similar to Resistance Temperature Detectors(RTD)

Uses resistance to detect temperature. However, unlike an RTD's metal probe where the resistance increases with temperature, the thermistor uses ceramic semiconductingmaterials which respond inversely with temperature.

Pros: High accuracy, ~0.02 C (0.36F), better than RTDs, much better than thermocouples. High sensitivity, ~10 times better than RTDs, much better

than thermocouples. As a result, lead wire and self-heating errors are negligible. Small in size compared to thermocouples. Response time

shorter than RTDs, about the same as thermocouples. Reasonable long term stability and repeatability.

Cons: Limited temperature range, typically -100 ~ 150 C (-148 ~

302 F). Nonlinear resistance-temperature relationship, unlike

RTDs which have a very linear relationship.

Pyrometer Also called radiation thermometer non-contact instrument that detects an object's

surface temperature by measuring the temperature of the electromagnetic radiation (infrared or visible) emitted from the object.

The wavelength of thermal radiation ranges from 0.1 to 100 m (4 ~ 4,000 in), i.e., from the deep ultraviolet (UV) across the visible spectrum to the middle of the infrared region (IR).

Pyrometers are essentially photodetectors which are capable of absorbing energy, or measuring the EM wave intensity, at a particular wavelength or within a certain range of wavelengths.

Optical Pyrometer (Brightness Pyrometer or Disappearing Filament Pyrometer) Designed for thermal radiation in the visible

spectrum. Utilizes a visual comparison between a calibrated

light source and the targeted surface. When the filament and the target have the same

temperature, their thermal radiation intensity will match causing the filament to disappear as it blends into the targeted surface in the background.

When the filament disappears, the current passing through the filament can be converted into a temperature reading.

Infrared Pyrometer Designed for thermal radiation in the infrared

region (0.75 ~ 1000 m; 30 in ~ 0.04 in) usually 2 ~ 14 m (80 ~ 550 in)

Constructed from pyroelectric materials, e.g., triglisine sulfate (TGS), lithium tantalate(LiTaO3), or polyvinylidene fluoride (PVDF).

Pros:-Non-contact measurement -Fast response time -Good stability

Cons:-Expensive -Accuracy maybe affected by suspended dust, smoke, and thermal background radiation

Measurement of TemperatureThermocouplesCommon SpecificationsResistance Temperature Detector (RTD)Thermistor (Bulk Semiconductor Sensor)Pyrometer