A thermometer comparison

B. Geerts

2/02

 Various types of temperature sensors are listed in Section 3.1. They are:

Various thermometers are used in different situations. For instance, mercury thermometers are the standard equipment at surface weather stations, and bimetallic thermometers are used in radiosondes. Thermocouples are used for in situ observations at locations wired to a computer network. Radiometers are used for remote observations.

The table below compares the various thermometers.

advantages

disadvantages

Mercury thermometer

cheap

display is harder to read

durable

does not work below -39C (Hg freezing pount)

accurate

cannot be used for thermograph

easily calibrated

slow response

 

fragile

 

mercury vapour is poisonous

Alcohol thermometer (compared to a mercury thermometer)

lower freezing point (-114 C)

less durable (alcohol evaporates)

larger coefficient of expansion

alcohol can polymerise

less hazardous

fluid loss by evaporation hard to avoid

 

lower boiling point (60 C)

Bimetallic thermometer *

cheap

requires frequent calibration to maintain accuracy

durable

fairly slow response

can be used for thermograph

 

easily calibrated

 

Electric resistance thermometer

display is easy to read

tends to 'drift' after years of use

rapid response

expensive

accurate over broad temperature range

 

Thermocouple

 

display is easy to read

ancillary equipment is expensive

durable

hard to calibrate

can measure temperature variations over a distance of less than 1 cm

measures only a temperature difference

rapid response

 

Radiometer

 

allows remote measurements

very expensive

 

material of emitting surface needs to be known

 

affected by absorption/emission between object and radiometer

 

* Temperatures can be measured cheaply by means of the bending of a strip of 'bimetal', made by rolling different metals together, choosing metals which have very different degrees of expansion on being heated. The same bending can be seen if a strip of sticky tape is fastened to a strip of aluminium foil. The foil expands when it is heated, but the tape prevents the stretching of that side of the foil, so the 'bimaterial' bends, with the sticky tape on the concave side.