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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
Mechanical	Bimetallic strip -70 to 500	- -	- medium - good	low
Thermoelectric (medium temperature devices)	Thermocouple type T -200 to 400	- 51 µV/°C - 0.5 to 1.4	- very fast - low	low
	Thermocouple type J -180 to 750	- 56 µV/°C - 1.5 to 3	- very fast - low	low
	Thermocouple type E -270 to 870	- 80 µV/°C - 1.5 to 3.2	- very fast - low	low
	Thermocouple type K -270 to 1270	- 41 µV/°C - 1.5 to 4	- very fast - low	low
	Thermocouple type N -270 to 1300	- 38 µV/°C - 1.5 to 4	- very fast - high	medium
Thermoelectric (high temperature devices)	Thermocouple type S -50 to 1600	- 11.4 µV/°C - 1.5 to 2.5	- very fast - good	high
	Thermocouple type R -50 to 1600	- 12.9 µV/°C - 1.5 to 2.5	- very fast - good	high
	Thermocouple type B 0 to 1750	- 10.6 µV/°C - 1.5 to 4.25	- very fast - good	high
	Thermocouple type W5Re/W26Re 0 to 2310	- 15 µV/°C - 4.4 to 23	- very fast - drift at high temperature	high
	Thermocouple type W3Re/W25Re 0 to 2310	- 17 µV/°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:

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