In fish farming, temperature needs to be measured in several situations, for instance, the farming water or inside slaughtered fish. One principle utilized to measure temperature is the expansion of certain substances with temperature as in a mercury ther-mometer. The construction of the thermometer is such that even quite a small expansion of the mercury causes a noticeable change in the level in the thin pipe where the reading is taken. Ther-mometer manufactures calibrate the scale according to known values: a bath with freshwater and ice is 0°C at normal atmospheric pressure and is the boiling point of freshwater is 100°C under the same conditions. The Celsius scale divides the interval between 0 and 100 into equal degrees. Other sub-stances besides mercury can be used in thermometers using this principle
That the electrical resistance in materials changes with temperature may also be used to measuring temperature. What actually is measured is the elec-tric current going through a circuit of which the resistance material is an integral part; if such material is exposed to temperature variation its resistance will change together with the electric current passing through the circuit. Platinum is often used in digital thermometers which measure electrical resistance.
The difference in voltage occurring between two different materials may also be used for measuring temperature. If two different metals are soldered together, for instance copper and constantan, a current will flow, the size of which will be proportional to the temperature. If the point where the two materials are soldered together is affected by a temperature gradient, the electrical current will also vary. This equipment is called a thermocouple; it is simple but not as accurate as other devices.
A thermistor is also a temperature dependant resistance, but is more complex and includes semi-conductor technology. Higher accuracy is achieved with this principle than with the other methods described. The device is simple and the price reasonable; thermistors are therefore widely used for temperature measurement