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Emerald Group Publishing Limited
Copyright © 2005, Emerald Group Publishing Limited
Our themes for this issue are temperature and flow measurement. Both are easy to measure approximately, but both can also be very difficult to measure with any great precision.
One difficulty lies in obtaining a precise measurement without influencing what you are measuring, and another problem is that both can be highly localised. For example the flow of liquid in a tube will vary across the diameter of the tube due to surface effects and the physical form of upstream and downstream pipes. Also the temperature of an object will vary across its surface and within its volume, again caused by a mixture of internal and external influences.
In this issue we have contributions that show how the very best and most reliable measurements can be obtained. However it is clear from these, and also from our Viewpoint by Robert Bogue, that we still have a long way to go.
We can measure time and distance and mass to ridiculous levels of accuracy, but flow rates are doing well to be able to claim 1 per cent accuracy. We already have a bewildering array of measurement techniques at our disposal; each with its own strengths and weaknesses. Perhaps there is still scope for a totally new – more unifying – approach?
It is always worth considering how the very act of taking a measurement influences the measurement itself. For example some flow meters work by heating the gas or liquid with a certain amount of energy and then measuring the temperature rise a bit further downstream. A paddle wheel or rotating vane will offer some resistance to the flow as will sensors based on the Venturi principle.
What about Doppler shift systems that measure flow velocity? Even these will cause small temperature and/or pressure changes which distort the readings, in addition to any physical disturbances that the sensor's structure might create.
Optical scattering from lasers looks useful as our article “New optical sensor poised to revolutionise airspeed measurements” clearly shows.
I consider that semiconductor technology will take an ever increasing role in flow and temperature measurement if for no other reason than the devices can be made very small.
If it is inevitable that your sensing mechanism will affect your readings then the least you can do is minimise the impact by using micro and nano technology sized sensors. Far better to measure flow using a 1nW heater than a 1W heater. Also, being small you can fit more of them into a given space, and so take multiple readings that enable a meaningful average to be obtained.