Principle of Kyushu Shengxin Precision Dew Point Meter

Principle of Kyushu Shengxin Precision Dew Point Meter

The dew point method is an ancient method of measuring humidity. From the classic Regnault dew point meter, it has a history of more than 100 years. With the development of science and technology, dew point technology has been perfected. Modern photoelectric dew point instruments use thermoelectric cooling, and can automatically compensate for the zero point and continuously track the dew point. The dew point meter with microprocessor can also convert the dew point temperature into relative humidity and other measurement units at the same time. The accuracy of the high-precision dew point meter in the general humidity range can reach ± ​​1 ℃ dew point temperature. Humidity is defined as the amount of water vapor in the gas and is an important environmental parameter. The basic quantity for measuring gas humidity is the mixing ratio, that is, the ratio of water vapor mass to dry gas mass, which can be achieved with a gravimetric hygrometer. However, it is difficult to measure the mixing ratio by the weight method, and the operation is cumbersome. The actual measurement of the humidity value mostly uses the dew point. This article mainly discusses the views from the principle of the dew point meter.

principle:

When a certain volume of humid air is uniformly cooled at a constant total pressure until the water vapor in the air reaches a saturated state, this state is called the dew point-during the cooling process, the partial pressure of both gas and water vapor remains unchanged. The sensor consists of two parts: a capacitive polymer film humidity sensor and a resistance humidity sensor. The humidity sensor measures the water molecules in the gas being measured to measure the relative humidity; the temperature sensor measures the surface humidity of the humidity sensor, The microprocessor built into the instrument calculates the dew point from these two parameters. The humidity and temperature sensors are closely back-to-back through the metal film, so that on the one hand, the temperature sensor can accurately measure the temperature of the humidity sensor, on the other hand, through the role of the metal film, the induction capacitance caused by the external electric field is greatly reduced In order to improve the measurement accuracy, under low humidity, the reaction time of DRYCAP is 40-240 seconds, depending on the direction and size of humidity change, the response time is shorter when measuring high humidity. The temperature resistance range of DRYCAP is -40- + 180 degrees, and the pressure range is 0-20bar. Its corrosion resistance is also very outstanding, and it has a good adaptability to alkaline and weak acid gases, usually under low humidity, The minimum dew point that can be measured for a relative film humidity sensor to obtain a dew point accuracy of +2 degrees is -9 degrees. Using Vaisal's DRYCAP sensor and automatic calibration patent technology, it can be measured while ensuring the temperature accuracy of +2 degrees dew point. The lowest dew point is -60 degrees, and the dew point of -80 degrees can be reached in the case of slightly lower accuracy. This is because the self-calibration technology makes accurate relative humidity measurement possible.

During the self-calibration process, the temperature measuring resistor heats the probe to 10 degrees above the ambient temperature and then naturally cools. During the cooling process, the instrument measures the real-time temperature and relative humidity. From the formula RH = RH0 + PW / PWS (where RH is the measured value of the instrument. RH0 is the intercept of the straight line on the Y axis. PW is the partial pressure of the water vapor in the gas to be measured at this moment, assuming a certain value. PWS is the saturated water pressure The value of 1 / PWS is a function of temperature.) It can be seen that from several sets of RH and 1 / PWS values ​​at different temperatures, a fitted straight line can be derived, and the intercept RH on the Y axis of the fitted straight line can be derived. That is, the temperature is infinite When high, the relative temperature measured by the sensor is offset from the zero-open value.

After determining RH0, accurate RH calculation can be performed to accurately calculate the dew point. When the relative humidity is lower than 10%, the system automatically performs the self-calibration function. At this time, the last output parameter is locked and the system can output after calibration Measurements. The self-calibration function can also be activated at time intervals (usually 6 hours). If the temperature or dew point measurement value is unstable during the calibration process, that is, the environmental impact cooling process or the assumption that the PW is a constant condition is not met, the self-calibration function will be executed again after the set time interval, and so on until the temperature The true dew point is output only after the temperature of the dew point is stable. Through excellent hardware design and automatic calibration software, accurate measurement of low humidity dew point can be achieved.

Due to the accumulation of certain chemical substances and gas molecules in the humidity sensor for a long time, the measurement accuracy will be affected. In order to ensure accurate measurement, the gain regression software was developed. Its working process is to perform the gain regression function before the zero point automatic calibration software is executed. The sensor is heated to 160 degrees to evaporate the chemical gas molecules accumulated inside, thereby ensuring accurate measurement. At the same time, this method eliminates the problem of oil pollution accumulation affecting the reaction time.

Moisture-sensitive devices are not afraid of condensed water. Natural air-drying after condensation does not affect normal use, but the instrument needs to be taken out when air-drying, which will affect the normal operation of other work. There is a protection function, that is, when the humidity unexpectedly rises above 80, the temperature sensor immediately heats the humidity sensor to reduce the local relative humidity to avoid the formation of saturated water vapor. By using this customer-friendly function, the downtime rate is greatly reduced, thereby improving production efficiency.

The factory setting of the dew point meter is -80 + 20 degree dew point corresponding to 4 ~ 20MA, but the customer can modify the parameters within any range by software, such as: -60 ~~ + 5 degree dew point or -75 ~~ + 15 degree corresponds to 4 -20MA output, which greatly facilitates the cascade control of the subsequent stage. All dew point meters will drift after being used for a period of time. Most dew point meter customers cannot calibrate themselves. They can only be sent to the manufacturer frequently to spend a certain amount of money and time for calibration. In order to facilitate the use of the customer, the dew point meter can enable the customer When you can find the standard humidity, you can make corrections by using the common software in Macrisoft Windows, which greatly saves the cost and time and guarantees the normal measurement. Through the use of a number of cutting-edge technologies, Vaisala's (Vaisala) dew point meter has been used in various industries for long-term stable and accurate use.

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