The number of controlled variables in food processing is quite large and often a considerable number of different sensors are available for measuring each parameter. Some of the most common types of measuring instruments will be described in this section. The output signal must be of a kind that can be transmitted to and read by the controller. Very often, a ‘converter ’has to be used to transform the measurement signal to the desired type without distortion. In modern control systems, the desired format is a digital signal. In transmission, the signal must be protected against electrical and electromagnetic disturbances.
The range of measurement and the sensitivity must meet the process requirements. The measurement has significance only if the measured parameter falls within the range of the sensor. Thus, if the measurement range of a thermometer is 20 to 100°C, a temperature of 120°C will be read as 100°C and a temperature of 10°C as 20°C. Sensors can be on-line, at-line or off-line. On-line measurements, whenever feasible, are preferable. At-line measurement refers to rapid tests that can be performed on samples, near the production line.
Sensors can be contacting or non-contacting. Remote sensing, whenever possible, is the generally preferred type. On-line sensors are physically inserted in the process line and often come in contact with the food. In this case, the sensor and its position in the line must comply with the strict rules of food safety. Remote sensors eliminate this problem, but the number of commercially available remote sensing elements is still limited.
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