Humidity Calibration | Calibration of RH Controller

Calibration of relative humidity hygrometers is generally performed in a humidity chamber.

The most used methods to generate humidity are the two pressure method, the split flow method and salt and water solutions. The latter is one of the least expensive methods to perform a

calibration, this method generally covering a wide range of relative humidity. According to, this method we can obtain similar results to those obtained with a humidity chamber. Some disadvantages with these methods are the possible contamination of the sensor, the long times of

stabilization and the limitation of only being able to calibrate the instrument to defined values.

This work describes the calibration process of relative humidity hygrometers using as

moisture generation system, salt-water solutions and validate the results with a system

of generation of humidity of two pressures..

INTRODUCTION

The three most used methods to generate humidity in the air are: the two pressures method

, the split flow method and salt-water solutions. The mixture of salt and water allows to generate humidity relative from about 3% to 98%. This wide range of humidity is useful for calibrate hygrometers as well as for others Applications. The main advantage of this method is the reduction in cost. The humidity value generated depends on the type of Salt, table 1 shows the most used Salt solutions in the calibration.

Salt-water solutions and their values Relative humidity reported in ASTM

Type of salt	HR(%)	t(ºC)
LiCI	11,3	25
MgCl2	32,8	25
NaCl	75,3	25
BaCl2	90	25

Due to the low cost of salt-water solutions to generate moisture and based on the values to be considered as fixed reference points, these are widely used by manufacturers, calibration laboratories, and other users. However, recently, differences have been found regarding the values reported by Greenspan , OIML  and ASTM, so it is convenient to characterize these solutions with a suitable technique or instrument (direct measurement of vapour pressure, meter dew point temperature).

A disadvantage is that there is still no reliable and repeatable calibration procedure based on salt solutions. Among the most important aspects to consider in the preparation of salt solutions are the following:

1) Material of the container containing the aqueous solution,

2) The purity of the components of the solution,

3) The preparation technique,

4) The volume and design of the calibration chamber,

5) the measurement and control of temperature inside the chamber 6) Characterization

The container should be made of a non-hygroscopic material, resistant to corrosion. Teflon, stainless steel and glass are commonly used.

The salt used must be analytical reagent grade. The water used in the solution must be distilled or de-ionized.

A portion of salt must be completely dissolved in water and maintain an undissolved amount (excess salt) which can be 30% to 90% compared to the weighed sample.

Another important factor to consider is the volume and design of the test chamber. The stabilization times, the volume of water-salt to be used and the temperature and humidity gradients depend on this.

The temperature and the variation in the solubility, constitutes the main factor of influence in the solutions. Temperature instability at ± 0.1 ºC, can cause relative humidity instabilities of ± 0.5% RH of the generated value.

As already mentioned, depending on the design and volume of the test chamber, it is possible that relative humidity and temperature gradients are generated, therefore it is necessary to carry out a characterization to determine these gradients and their effect on uncertainty. The amounts of water and salt required were calculated from the solubility of each of the salts at a temperature of 23 ºC.

Calibration:

Measuring

In this method, the measurand is the resulting correction value of the instrument under calibration (IUC)

CR = [(HRp+CR´ )- (HR) IUC] + [δ (HR)t + δ (HR)hist + δ (HR)stb ]

Where: CR is the correction value resulting from the IUC reading (in% relative humidity); (HRp + CR´) is the corrected reading of the pattern; CR 'is the correction value of the standard reading; (HR) IUC is the instrument reading under calibration; (HR) t is the temperature effect correction value; (HR) camera is the stability correction value in the camera; (HR) hysteresis is the correction value for hysteresis effect

Process

The calibration procedure is based on the “Technical Guide on traceability and measurement uncertainty in the calibration of relative humidity hygrometers” and is summarized in the following diagram. The selected calibration points were described in table 

Figure  shows a diagram summarizing the calibration procedure.

The salts were characterized before starting the calibration process. The volume of the container was considered to be small, so the effect of gradients is neglected and only stability in relative humidity is considered.

UNCERTAINTY CALCULATION

The sources of uncertainty considered in this calibration system are

Pattern uncertainty Up

Camera uncertainty U _camera

Uncertainty of the readings U _reading

Temperature Uncertainty U _Temperature

Hysteresis uncertainty U _Hysteresis

The standard uncertainty is that reported on your calibration certificate. Chamber uncertainty includes stability and gradients in temperature and humidity.

The uncertainty of the readings is given by:

Where HR1 is the value of relative humidity measured upwards; HR2 is the value of relative humidity measured downwards.

This method is adequate if the necessary care is taken into account such as:

The purity of the materials.

The preparation of the solutions.

Characterization of the calibration zone.

Temperature control.

Humidity measurement with calibrated instrument.