Construction of measuring instruments
The construction of measuring instruments depends on the measuring principles used and the signal transfer. One classification is mechanical, hydraulic, pneumatic, electrical and electronic, where the last is being increasingly used.
A measuring instrument often comprises three major parts (Fig. 19.1):
A sensor or probe
A transmitter to transfer the signal
A display or another type of indicator (connected to the transmitter).
In some instruments the three major parts are connected within the same unit, while in other instruments the parts are separate and connected via cables for electric signals or another principle for transfer of the measured values. Measuring equipment can either give continuous signals (analogues), or on/off signals (digital). An example of the first case is an oxygen meter that shows the concentration of oxygen. Flow indicators that register if there is water flow or not (on/off) are an example of the latter case.
The sensor in the unit is used to record the phys-ical conditions in the medium, such as the probe in an oxygen meter. The transmitter can either be electrical or mechanical and translates the signal coming from the sensor to a scaled signal that as is further transported to the display or indicator where the results are shown in an understandable way. In the display the physical conditions of the medium are shown. Signals may also go directly to a recording unit such as a computer for storing the results, or can be used to control a regulator.
A short description of the measuring instruments most used in aquaculture facilities is given below. Equipment is separated into that used for measuring water quality and that used to measure physical conditions.