The 2364-SPM03A uses the dew point of -20°C as a boundary to classify the measured gases into high and low humidity gases. Dew point temperature refers to the temperature at which air cools to saturation under the condition that neither the water vapour content nor the air pressure changes. Figuratively speaking, the temperature at which the water vapour in the air becomes dew is called the dew point temperature. Dew point temperature is a temperature value, but why use it to indicate humidity? This is because when the air is saturated with water vapour, the temperature is the same as the dew point temperature. When the air is not saturated with water vapour, the air temperature must be higher than the dew point temperature. So the difference between the dew point temperature and the air temperature can indicate the degree of water vapour saturation in the air. The discrete statistical relationship between PPM and dew point temperature is shown in Table 1.

The value of 2364-SPM03A is sent from the special transmitter to the LM3310B high-precision input module of the LM series PLC of HELITHI. According to the correspondence in Table 1, PLC calculates the dew point temperature value and displays it on the text display, and at the same time, the average value of multiple dew point temperature values is used as the key parameter to participate in the discrete control and process control of the large system.

The relationship between the PPM and the dew point temperature value in the 2364-SPM03A is 55 discrete points. If the PLC characteristic curve instruction (CHARCURVE) is used, the number of coordinate points cannot exceed 11. So the method of curve fitting discrete points is used to draw the discrete point graph and fit the curve equation. Practice shows that one curve is not enough to meet the accuracy due to the nonlinear characteristics of the curve, so it is divided into 5 intervals and 5 formulas are used to express it. These 5 formulas are input into PLC, which calculates the dew point temperature value according to the formulas.