Selecting the Right Humidity Testers for Clean Rooms and Calibrating Them Correctly

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4. Resistance-type hygrometer

The resistance-type hygrometer utilizes the principle that electrical resistance varies in a material that absorbs moisture. Special resistive sensors measure the electrical resistance between the electrodes. They are suitable for measurements in mass production and are used primarily in home appliances and consumer products. However, the readings taken by these devices are not particularly accurate in environments subject to very low or very high humidity.

5. Capacitance-type hygrometer

A capacitance-type hygrometer measures humidity by detecting the change in capacitance of a thin polymer film. This type of sensor is sufficiently accurate in most cases and is used frequently in industry.

Additional Information

From experience

Selecting the calibration service provider

Before selecting a calibration device, the user should check that it is fit for purpose. Every provider of calibration services should be able to provide evidence of an effective traceability chain. At least one working reference must have been calibrated in an external laboratory and then used for internal calibration. Some commercial service providers will only show uncertainty estimations in their calibration certificates if they are specifically ordered, while others are not even able to calculate uncertainties of measurement.

National laboratories maintain traceability at national calibrated measurement standards and develop this further. They are also responsible for providing the greatest possible calibration accuracy. In certain circumstances these laboratories are limited to the calibration of ultraprecise primary standards.

Commercial service providers offer calibration services for lower level standards and measurement equipment, e.g. measurement equipment manufacturers or laboratories which provide general calibration services for any measurement equipment. Non-accredited establishments account for the majority of services, but their competence is not confirmed. An in-house calibration department can be practical, e.g. if it is difficult to move the calibration equipment. If introducing an in-house service, it is advisable to set up a corresponding institution.

6. Mirror dewpoint hygrometer

A mirror dewpoint hygrometer utilizes the occurrence of condensate at dewpoint temperature when air containing water vapor is cooled. A mirror is cooled until the dewpoint of the gas in question is reached. As condensation forms, the light reflected from the mirror changes. When the mirror surfaces reaches an equilibrium state whereby evaporation and condensation are occurring at the same rate, the temperature of the mirror is equal to the dewpoint temperature of the surrounding gas. This type of sensor is often used in research institutes. The sensors most frequently used in clean rooms are the resistance-type hygrometer, the capacitance-type hygrometer (dew indicator) and the mirror dewpoint hygrometer.

Conclusion: All the sensors listed are able to measure normal ambient humidity. Sensors 5 and 6 are also used to measure low dewpoint temperatures. When selecting a suitable instrument, price and product specifications are important criteria but so are the measurement accuracy, the manufacturer’s knowledge of fields of application, and the services provided.

It is these factors in particular that will help to determine whether the equipment will be suitable. Regular and traceable calibration is important because the levels indicated by the instruments should be reliable and accurate — in the short as well as in the long term.

(ID:20987540)

Humidity in Check

Selecting the Right Humidity Testers for Clean Rooms and Calibrating Them Correctly

4. Resistance-type hygrometer

5. Capacitance-type hygrometer

6. Mirror dewpoint hygrometer