Measurement methods

Back

There are two main types of contact thermometer, either mechanical or electrical:

-   mechanical thermometers are based on the expansion of a fluid (liquid or gas) or solid (usually metals) with temperature

-   electrical thermometers use the change of the sensing element electrical resistance with temperature, or the electromotive force (emf) generated at the junction between two conductors when heated (as in the thermocouple)

The most popular contact temperature thermometers are:

-   the liquid-in-glass thermometer; comprising a reservoir and capillary tube in which the working liquid expands on heating

-   the resistance thermometer; the resistivity of the sensing element changes with temperature. The sensing element can be a conductor (RTD) or a semiconductor (thermistor)

-   the thermocouple; comprises two dissimilar conductor wires. An emf is generated when heat is applied to the junction between the two conductors (the hot junction), while the other ends of the wires (the cold junction) is kept at a constant temperature, usually 0 C

To choose the most suitable thermometer for an application consider the following criteria:

-   accuracy (is a few degrees uncertainty acceptable or is it critically important to be right?)

-   sensitivity (how small a temperature change do you need to measure?)

-   temperature range (too high a temperature can damage the thermometer)

-   environment (chemical, physical, electrical; the wrong environment, e.g. acidic, could damage the thermometer)

-   response time (is a thermometer with a quick response needed, e.g. in a quality or safety-critical situation)

-   convenience (will a portable, hand-held device do or do you need something more sophisticated and flexible that may be more bulky and require operator expertise?)

-   cost (being clear about your essential requirements will help you choose the right device for the application)


Comparative table of the different contact thermometers

Device type Contact thermometer & useable range C Sensitivity, accuracy C Response time, stability Cost
Thermodynamic Gas thermometer nitrogen, hydrogen, CO2
-260 to 700
-
- Standard
- slow
- high
Very high
(none commercially available
Mechanical Liquid in glass thermometer, metallic or organic liquid
-40 to 600
-
- 0.02 to 10
- slow
- good
low
Bimetallic strip
-70 to 500
-
-
- medium
- good
low
Thermoelectric (medium temperature devices) Thermocouple type T
-200 to 400
- 51 V/C
- 0.5 to 1.4
- very fast
- low
low
Thermocouple type J
-180 to 750
- 56 V/C
- 1.5 to 3
- very fast
- low
low
Thermocouple type E
-270 to 870
- 80 V/C
- 1.5 to 3.2
- very fast
- low
low
Thermocouple type K
-270 to 1270
- 41 V/C
- 1.5 to 4
- very fast
- low
low
Thermocouple type N
-270 to 1300
- 38 V/C
- 1.5 to 4
- very fast
- high
medium
Thermoelectric (high temperature devices) Thermocouple type S
-50 to 1600
- 11.4 V/C
- 1.5 to 2.5
- very fast
- good
high
Thermocouple type R
-50 to 1600
- 12.9 V/C
- 1.5 to 2.5
- very fast
- good
high
Thermocouple type B
0 to 1750
- 10.6 V/C
- 1.5 to 4.25
- very fast
- good
high
Thermocouple type W5Re/W26Re
0 to 2310
- 15 V/C
- 4.4 to 23
- very fast
- drift at high temperature
high
Thermocouple type W3Re/W25Re
0 to 2310
- 17 V/C
- 4.4 to 23
- very fast
- drift at high temperature
high
Electrical resistance Platinum Pt100
-253 to 1000
- 0.4 Ω /C
- 0.03 to 1
- fast
-
medium
Nickel RTD
-150 to 300
- 0.4 Ω /C
- 0.15 to 1.45
- fast
-
low
Thermistor NTC
-80 to 250
- 10mV/K
- 0.01 to 0.05
- fast
-
high

Below are descriptions of the operating principle and characteristics of the most commonly used thermometers:



    Back