Calibration and Adjustment of Hygrometer

Scientists often use relative humidity to describe the amount of water vapor in the air. Simply put, imagine the air as a towel. If you spill a glass of water, you can use a towel to soak up the water. But a towel can actually absorb more water than a cup. Perhaps he can absorb five to ten glasses of water. The amount of water vapor in the air is only a fraction of what the air can hold, so relative humidity is a percentage. When the relative humidity is one hundred percent, the air is saturated. Like a wet towel, the air can no longer hold moisture. Evaporation and deposition are in equilibrium when the relative humidity is 100 percent and the air is saturated. Reaching equilibrium again accounts for the increased amount of evaporation, as moisture precipitates.

What is Humidity?

Humidity indicates the amount of water vapor in a gas, and there are two ways to express absolute humidity and relative humidity. Absolute humidity refers to the absolute amount of water vapor in a gas, and the most commonly used unit is g/m^3. At a certain temperature and pressure, the water vapor content per unit volume has a certain limit, which is called saturated water vapor content. Relative humidity refers to the ratio of the absolute content of water vapor in a gas to the saturated water vapor content in the same volume of gas at the same temperature and pressure, and the commonly used symbol is %R.H. Hygrometers are used in a wide range of applications, such as in ultra-pure metal smelting, textile processing, production processes such as paper and printing and dyeing, as well as food storage and meteorological measurements, where hygrometers are often required to measure or control the humidity of air or industrial process gases.

Hygrometer Accuracy

Like measuring weight and temperature, the first thing to do when selecting a humidity sensor is to determine the measurement range. Except for meteorological and scientific research departments, those engaged in temperature and humidity measurement and control generally do not need full humidity range (0-100%RH) measurement.Measurement accuracy is the most important indicator of the humidity sensor, and every percentage point increase is a step up, or even a higher level, for the humidity sensor. Because to achieve different precision, the manufacturing cost varies greatly, and the selling price also varies greatly. Therefore, users must tailor their clothes according to their needs, and should not blindly pursue "high, precise, and sharp".

If the humidity sensor is used at different temperatures, its indication should also consider the influence of temperature drift. As we all know, relative humidity is a function of temperature, and temperature seriously affects the relative humidity in a given space. For every 0.1°C change in temperature. A humidity change (error) of 0.5%RH will result. If it is difficult to achieve a constant temperature in the application occasion, it is not appropriate to propose an excessively high humidity measurement accuracy.

In most cases, if there is no precise temperature control means, or the measured space is not sealed, the accuracy of ±5%RH is enough. For local spaces that require precise control of constant temperature and humidity, or where humidity changes need to be tracked and recorded at any time, a humidity sensor with an accuracy of ±3%RH or higher is selected.

The requirement of accuracy higher than ±2%RH may be difficult even to calibrate the standard humidity generator of the sensor, not to mention the sensor itself. For relative humidity measuring instruments, even at 20-25°C, it is still very difficult to achieve an accuracy of 2%RH. Usually the characteristics given in the product information are measured at normal temperature (20°C±10°C) and in clean gas.

In actual use, due to the influence of dust, oil and harmful gases, the electronic humidity sensor will age and the accuracy will decrease after a long time of use. The annual drift of the electronic humidity sensor is generally about ±2%, or even worse. high. Under normal circumstances, the manufacturer will indicate that the effective use time of one calibration is 1 year or 2 years, and it needs to be re-calibrated when it expires.

Requirements for Humidity Performance

1. Indication error

According to the actual situation of the production and use of the hygrometer. Humidity indication error is set as: ±5%RH (40%RH～70%RH, 20°C), ±7%RH (below 40%RH or above 70%RH, 20°C).

This is for the following reasons:

A.The ex-factory indicators of industrial mechanical hygrometers produced by domestic manufacturers are generally: ±5%RH.

B. In industry, the actual ambient humidity range is generally between 40%RH and 70%RH, and the temperature is generally around 18°C to 25°C. This range has higher requirements for temperature and humidity.

C. Mechanical hygrometers such as hair are low-grade instruments, generally not suitable for special occasions such as low humidity, high temperature and high humidity.

D. The parameter of relative humidity is dependent on temperature, so the technical index of relative humidity should be given at a certain temperature.

E. The original regulations did not make any regulations on the error of the wet indication value of the instrument, which brings great inconvenience to the user.

2. Humidity hysteresis error

The hysteresis error is set at: 3%RH.In the original regulations, it is called humidity variation, which is 5%RH. According to the experimental results, the hysteresis error of the mechanical hygrometer is only 1%RH.

3. Repeatability

Set the humidity at 2%RH. Obtained from the experimental results.

Calibration Equipment

It mainly refers to the temperature and humidity test chamber. In order to ensure that the uncertainty of the test results meets the requirements, its main technical indicators are specified in detail. Considering that the relative humidity verification must be carried out at a constant temperature, the chamber must have a temperature adjustment function.

The technical indicators of the standard device and supporting equipment are very critical. It must be set just right, which can not only meet the verification requirements of the mechanical temperature and humidity meter, but also meet the actual national conditions. In the process of formulating the regulations, based on the results of the uncertainty assessment, combined with the actual situation of the equipment that the market can provide, we properly adjust the uncertainty components of the standard and supporting equipment, and then determine the technical indicators based on the component values.

Calibration Procedure

Preparations

The probe of the standard is placed in the central position of the temperature and humidity chamber, and the tested instrument is effectively placed in the chamber.In the space, the way and amount of placement should not affect the air circulation in the chamber. The studio of the chamber shall ensure air tightness and shall not place wet or strong hygroscopic materials.

Methods

Use a standard hygrometer to put the hygrometer into the constant temperature and humidity chamber, set the constant temperature and humidity chamber to a certain humidity, and after the humidity is stable, compare whether the humidity inside the constant temperature and humidity chamber is consistent with the humidity of the standard hygrometer. If there are differences, it can be calibrated by adjusting the humidity controller of the constant temperature and humidity chamber. Use a hygrometer of known accuracy to place a hygrometer inside the constant temperature and humidity chamber and set the humidity to the target humidity. After the humidity is stable, compare the humidity in the constant temperature and humidity chamber with the readings of the hygrometer, and adjust the control parameters of the constant temperature and humidity chamber according to the difference until the readings of the two are consistent.

LIB Constant Temperature and Humidity Test Chamber

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