Old principles, new opportunities

Citation

Monkman, G. (2001), "Old principles, new opportunities", Sensor Review, Vol. 21 No. 3. https://doi.org/10.1108/sr.2001.08721caa.002

Old principles, new opportunities

Old principles, new opportunities

Gareth Monkman

Keywords: Thermodynamics, Measurement

Those who live at high altitudes know only too well how water boils at a lower temperature than it does at sea level. OK for instant coffee but less inspiring for those who enjoy a good cup of tea. The solution to this problem is, of course, a pressure cooker. It is interesting to note that the inventor of the pressure cooker, Papin (1679), was also the first to observe that a negative pressure can be achieved by reducing the temperature of a steam filled vessel (1690). The force generated being capable of moving a piston – a fact that made James Watt's steam engine possible! Perhaps the most interesting example of the interaction between temperature and pressure is the phenomenon known as the triple point. There exists a small range of temperatures and pressures at which water can exist in its three forms: ice, water and steam – simultaneously!

Renaldini (1694) was the first to suggest the freezing and boiling points of water as standard temperature points. Then Celsius (1742) devised a scale with 1008 as freezing and 08 as boiling – at normal atmospheric pressure!. This was reversed by Strömer (1750) to give us the commonly used "Celsius" scale. Unfortunately, this scale is of limited use at the extremes of temperature – namely absolute zero on the Kelvin scale (–2738C) and the many thousands of degrees needed in metal smelting. Similarly, our earthly ambient air pressure of approximately 1 Bar (or 100,000N/m2) is far removed from a perfect vacuum at one end of the scale and the many thousands of Bar used in industrial water jet cutting at the other. In addition, pressure may also be defined as "gauge" pressure – relative to normal atmospheric pressure. Clearly, extremely low pressure can also mean a very small gauge pressure as may be experienced by a sensitive airflow movement or physical tactile sensor.

Modern industrial applications demand modern measurement techniques – though few of the basic principles used are new! The advent of steam power gave rise to a number of crude membrane pressure gauges, followed half a century later by Bourdon's (1837) famous spiral tube – a method still used for measuring gauge pressure to this day. Seebeck, Peltier and Nernst effects, together with concept of the Carnot thermal cycle, were all developments of the nineteenth century. Pyrometric methods of high temperature measurement are usually considered to be the domain of modern technology. However, the first to make use of pyrometry was Joshua Wedgewood (1782) for checking pottery furnace temperatures.

Many of these old techniques have become neglected, if not somewhat obsolete, over the years. Where modern technology is making an impact is in the use of materials and manufacturing techniques previously unsuitable or unavailable. Owing to the very small scales involved, and the materials which may be used at these sizes, micromachining methods have made the use of mechanisms, membranes, bolometers, etc. once again viable as sensor elements. Sensor and circuit integration coupled with advanced signal conditioning algorithms are also helping to overcome many of the problems associated with measuring techniques hitherto considered to be too unlinear or too noisy to be of real use.

One of the major problems in this respect is the direct integration of high temperature sensors with the electronics. Some headway is presently being made in the use of doped diamond, instead of the traditional silicon, as a semiconductor for both sensors and transistors. Though diamond can withstand temperatures of up to 6,000K, there are few other materials which can, leaving the would-be manufacturer in want of suitable passive components and inter-connectors.