Animal fat thickness
One of the more specialized uses of sonar distance meters has been in measuring the thickness of the layer of fat under the skin of animals, pigs in particular. The speed of sound in a fatty layer is different from the speed in the more dense meat, so that an ultrasonic beam will be reflected from the place where fat ends and meat starts. By making the usual measurement of time elapsed between sending the ultrasonic pulse and receiving the reflec- tion, the thickness of the fat layer can be measured painlessly.
Water purity
There is no single instrument that can assess all aspects of water purity, because this would require the assessment of both ionizing and non-ionizing contaminants, bacterial and viral presence, dissolved gases, colour and taste; hardlya task for which any one instrument is suited. One reading that can be a very useful guide, however, is electrical conductivity. This does not need to
be carried out on an absolute basis, measuring the conductivity in units of Siemens per metre, but can be comparative, measuring the current flow in an electrometer cell at a fixed voltage, as compared to a sample of deminera- lized or distilled water under the same conditions. So many common impurities such as nitrates, nitrites and other salts will increase the conductivity of water, that this figure can be used as one which will indicate when a more complete analysis should be undertaken.
Readings of pH are also useful as they can help to establish what type of contaminants are likely to be present, but since perfectly non-conducting water must also be perfectly neutral, the pH reading is of rather less importance compared to conductivity. A useful secondary test is clarity, using an optical gauge to assess the transparency of a long column of water. Many contaminants that do not affect the conductivity of water have a noticeable effect on its transparency.
The most difficult contamination problems, from the point of view of continuous monitoring, are the biological and non-ionizing types. Although there is a possibility of manufacturing biological transducers which will provide an electromotive force in the presence of specific bacteria, we are a very long way from any device that will provide an electrical readout for the whole range of bacteria and virus contaminations that can be found in water. Solvents present another type of problem, because being electrically non-conducting they do not alter the conductivity of water and are very difficult to detect by other than chemical methods.