A thermometer comparison
|
B. Geerts |
2/’02 |
Various types of temperature sensors
are listed in Section 3.1. They are:
- Mercury thermometer
- Alcohol thermometer
- Bimetallic thermometer *
- Resistance thermometer
- Thermocouple
- Radiometer
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 -39ºC (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 ºC) |
less durable (alcohol evaporates) |
|
|
larger coefficient of expansion |
alcohol can polymerise |
|
|
less hazardous |
fluid loss by evaporation hard to avoid |
|
|
|
lower boiling point (60 ºC) |
|
|
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.