Automated Environmental Control Systems In Composting And Mushroom Growing

Temperature Measurement



The main characteristic of measuring instruments is an error, i.e. a measurement result deviation from a true value.

The maximum measuring error of glass thermometers corresponds to the value of scale division. Thermistors error depends on the type of thermistor. Semiconductor sensors error is 0.5 Celsius degrees, as a rule. RTDs error is determined by its accuracy class.

When measuring temperature by means of RTDs, thermistors or semiconductor temperature sensors, the measuring error consists of sensor error and device error, i.e. the error converting the output signal into temperature. Accuracy of measuring instruments is specified in the device certificate.

Typically, the certificate contains either a relative error or an accuracy class. The relative error is the ratio of the maximum permissible error of conversion to the measuring range. Accuracy class corresponds to a relative error in percentage.

For example:

  • the relative error of 0.01 at an operating range of 50 to +200°C corresponds to an absolute error of 2.5°C;
  • the relative error of 0.5% at an operating range of 0 to +100°C corresponds to an absolute error of 0.5°C;
  • the accuracy class of 0.2 corresponds to a relative error of 0.2% and an operating range of 0 to +100°C corresponds to an absolute error of 0.2°C.

Mushroom growers should use the devices with a narrow working range and a high accuracy. For clarity, let us consider a few examples.

The 1-st device has accuracy class 1.0 with conversion range from 45 to +195°C (for copper RTDs). Consequently, the absolute error of conversion will be 2.5°C. Adding the temperature sensor error, we get the full temperature accuracy 2.53 ° C. Such a device is clearly not suitable for mushroom growing.

The 2-nd device with the copper RTDs has a working range of 50 to +200°C, accuracy class 0.5. Thus, this device brings an additional conversion error 1.25°C. The total error of temperature measurement will be 1.3°C which is also unacceptable.

It is particularly should be noted that in the measuring instruments the digital resolution does not correspond to the absolute measuring accuracy.

For example, when measuring the temperature with a semiconductor sensor with PC data transfer function the temperature measurement can be displayed to the nearest tenth, hundredths and even thousandths of a degree but the measurement error may be 0.5 degrees or more.

The device # 1 shows the temperature to within 1°C, with accuracy 2.53°C. Device # 2 shows the temperature to within 0.1°C, with accuracy 1.3°C.

Our enterprise manufactures measuring instruments specifically for the mushroom growing field, with an operating range 0 … +100°C and an accuracy class of 0.1. The absolute conversion accuracy of such devices is not more than 0.1°C.

Comparative Table of Device’s Accuracy

Device Accuracy Class Measuring Temperature Range Absolute Measurement Error Absolute Measurement Error with RTD Class B Resolution
#1 1.0 -50 to 200 °С 250*1.0/100=2.5°С 2.53 °С 1.0 °С
#2 0.5 -50 to 200 °С 250*0.5/100=2.5°С 1.3 °С 0.1 °С
IТ-8 0.1 0 to 100 °С 250*0.1/100=2.5°С 0.4 °С 0.1 °С