Site Navigation Links : Please click on SITE MAP:
The measurement of temperature has a long and distinguished history. Some of the techniques we use today have been available for more than a century and those sensors fabricated from semiconducting materials date back for more than four or five decades.
If one cares to glance back into a Practical Physics book, say, that written by Worsnop and Flint (Methuen 1942, seventh edition) one can see that the Platinum Resistance thermometer held pride of place ( full description on page 511). Since the resistance of any metal increases with temperature, a resistance measurement will also be a temperature measurement and the metal, platinum ( though currently a very expensive material), gives a very stable and reproduceable sensor for precision work. There is almost a linear relationship between resistance and temperature so data handling is simple. Unfortunately, added to the expense, the sensor is relatively bulky, typically several centimetres long and perhaps one or two centimetres in diameter and so thermocouples, which have very small sensing volumes, typically millimetre dimensions, were used for the majority temperature measurements (page 545 in same text).
An informative discussion on the topic of temperature measurement has been given in Wikipedia
http://en.wikipedia.org/wiki/Temperature_measurement and on this page only a few selective sensors will be introduced.
Platinum Resistance Thermometers - Althought the author has no practical experience with this type of sensor it is widely used for accurate measurements. In a recent Newsletter from Pico Technology Ltd ( www.picotech.com ) this thermometer is on their product list:
Thermocouples - As mentioned before, thermocouples have always been popular temperature sensors although the thermoelectric voltages are small and they vary in a nonlinear fashion with respect to temperature. The arrival of the Analog Devices chip AD594 made thermocouple use much simpler than that described by Worsnop and Flint. This IC is a complete instrumentation amplifier with cold junction compensation and gives a linear output of 10 mV per degree C. The data sheet lists a temperature range of -200 to + 1250 degree C when the AD594 is attached to a Type K thermocouple and this is likely to satisfy more measurement situations. (Note - the original Patent was taken out in 1977 so this IC has been available for many years).
Thermistors - These are sensors which have a temperature dependent resistance but the variation is more rapid than in the case of metallic resistors and hence there is an enhanced sensitivity. They often have a negative temperature coefficient (NTC) so that the resistance reduces as the temperature increases. In functional form we have :
R = R25 exp(B(1/T - 1/298)
R25 is the resistance at 25 degree C , B is a constant and T is in degree Kelvin
ATC Semitec Ltd have manufactured a series of clip-on sensors for water pipes www.atcsemitec.co.uk and these have proved useful in monitoring temperatures of inlet and exit pipes in solar water heating installations. (The temperature - resistance graph for these sensors is given below).
As one can see the resistance changes with temperature in a very non-linear manner so data manipulation is necessary to display temperature. A complete system is shown in the illustration given below (Device from Rapid Electronics Ltd, part number 26-7472).
The instrument had facilities for both conductivity and temperature measurement but only the temperature sensor was connected. (With the conductivity sensor being left as an open circuit a reading of zero conductance was given.)
Infra red (IR) sensors - Any hot body gives off radiation and thermometers can be fabricated based on IR sensors. The Fluke 61 is a compact IR thermometer which has a laser "pointer" to accurately specify the site under investigation.
At a somewhat higher price tag a vast selection of thermal imaging cameras are available and these will map the temperatures of any surface.
With temperature being colour coded it is easy to see that the roof is at a low temperature, the windows are hot and the wooden cladding is at an intermediate temperature.
Miscellaneous - The above gives only the briefest of glimses into temperature measurement. Semiconductor sensors like the LM 35 are very convenient for laboratory use and the more exhotic optical fibre sensors are useful for probing microwave ovens where any form of conductor is not allowed.
Temperature control is a very important topic in all aspects of engineering. Clearly, measuring temperature is the first step in attempting this control. In one's home detecting temperature is very often carried out by a bi-metallic sensors which are essentially a sensor and switch combined ( see Pressing Problems : this shows that an electric iron controls temperatures for synthetic or cotton fabrics by switching on and off the heating element https://sth-se.diino.com/f.thompson/migrated_data/EandH )
Safety is an issue in all homes and many gas appliances have a temperature sensing unit which switches off the gas supply if a pilot light is extinguished.
Summay Temperature measurement is a rapidly evolving area as it is a topic at the heart of energy efficiency analysis.
| Index Page | Top of Page | Next Page |