Chilled Mirror Hygrometers for Calibration Laboratories

Chilled Mirror Hygrometers for Calibration Laboratories

Why calibrate dew point or humidity?

Maintaining the accuracy of moisture measurement instruments through regular calibration is paramount. Whether conducted on site or by an independent calibration laboratory, ensuring the precision of field devices used for measuring trace moisture or dew point and relative humidity is critical. 

Failure to routinely calibrate may not manifest immediately during normal operations, but can lead to changes in process conditions, affecting product quality, efficiency, safety, and compliance over time. In critical scenarios, the financial and reputational repercussions could be severe.

Learn more about calibration for moisture measurement.

The role of dedicated calibration laboratories

Calibration laboratories play a crucial role in maintaining measurement accuracy, reliability and quality in many industries. These specialized facilities are equipped to perform calibration on a wide range of dew-point, temperature and relative humidity instruments, ensuring they meet, or exceed, the standards required for their specific applications. The calibration process involves comparing the outputs of the device under test against a known reference, which has been calibrated and is traceable to national or international standards.

The calibration process, particularly for precise and sensitive measurements like trace moisture or dew point, represents the critical need for accuracy and traceability in scientific and industrial instrumentation. Calibration against highly accurate reference instruments, like a chilled mirror, is a cornerstone of this process, ensuring that field-based devices produce valid, reliable data. 

The use of chilled mirrors in calibration

A typical dew-point or humidity laboratory calibration set-up requires a source of controlled moisture and accurate reference instrumentation. The set-up itself will depend on the levels of moisture being generated; in other words, the range within which the instruments are being calibrated.  

Chilled mirror hygrometers are the instruments for choice for setting reference standards in dew-point and humidity calibration.

Humidity calibration 

Calibration chambers, like Rotronic’s HygroGen2, are the most common solution when calibrating relative humidity instruments. These are usually temperature-controlled and generate a stable environment at a range of moisture levels and temperatures.  

HygroGen2 - Hygrometer for Calibration Laboratories


Moisture levels generated in humidity calibration chambers are usually much higher than the levels needed to calibrate dew-point sensors and rarely go below -40 oCdp.  

The chilled mirror hygrometer – for example, Michell’s Optidew or S8000 Remote – is inserted into the calibration chamber alongside the instruments under test, providing a high-accuracy reference to calibrate the instruments against.  

Dew-point calibration 

Calibration of dew-point sensors, especially at low dew-point levels, requires the use of more sophisticated dew-point generators and dryers. 

Clean compressed air is passed through a dryer to ensure a steady supply of dry air; it is then funnelled through a dew-point generator such as the Michell DG2 or ADG400, to create a gas stream with known dew point. The air is then directed into a chamber or manifold and supplied to both the device under test and a chilled mirror reference. This process enables a comparison of readings and, if needed, an adjustment is carried out to bring the device being tested into specification.  

In both cases, the accuracy and consistency of the reference instrument is crucial.  This is where our chilled mirror hygrometers come into play.   

A chilled mirror hygrometer uses a temperature-controlled mirror, which is cooled until the moisture present in the air sample creates a condensate layer on its surface. A highly sensitive optical sensor observes the presence of condensation by detecting the change in reflectivity of the mirror. This moment, when condensation forms, corresponds to the dew-point temperature, providing a precise measurement of the air's moisture content. This method is known for its accuracy and reliability in measuring humidity and dew point, especially in applications requiring high precision.  

S8000 RS chilled mirror technology offers greater accuracy and flexibility

Our S8000 RS chilled mirror hygrometer represents a significant advancement in dew-point measurement technology and is designed specifically to meet the high demands of calibration laboratories for speed, precision, and flexibility. 

S8000 RS - Chilled Mirror Hygrometers for Calibration Laboratories

S8000 RS

The S8000 RS boasts remarkable specifications, achieving an accuracy of ±0.1oC at temperatures as low as -90oC frost point, which is equivalent to 95.4 parts per billion by volume (ppbV). Its stability and reproducibility are equally exceptional. At a -90oC frost point, the device ensures a reproducibility of ±0.05oC dew point, which improves to ±0.025oC dew point at -80oC frost point, alongside a stability of ±0.05oC at -90oC frost point. Such high-level performance is crucial for calibration laboratories that depend on consistent and repeatable measurements.

The use of a gold-plated copper mirror and the Dynamic Contamination Correction (DCC) further enhances the accuracy of measurements by ensuring a uniform condensate layer.  

The new S8000 RS is available with a choice of communication protocols, including analog mA and digital Modbus outputs, to enable communication with a wide range of existing systems, and internal data logging.

Learn more about the S8000 RS

Michell Instruments’ Calibration Laboratories 

Our UKAS-accredited (ISO 17025) laboratories offer dew-point, relative humidity, and temperature calibration services. With our chilled mirror references being calibrated at The National Physical Laboratory (NPL), we ensure that the calibrations are traceable to the national standards and, thus, compliance for our customers.  

Learn more here. 

With half a century of pioneering work and innovation, we have established ourselves as the leaders in the field of trace moisture, humidity, and dew-point technology. 

If you would like to discuss your requirements, please contact our team today.

Transfer standards are widely referenced by calibration laboratories.  But, what exactly do we mean by a ‘transfer standard’?

In simple terms, a transfer standard for moisture measurement relates to a reference instrument used to calibrate or verify the accuracy of other moisture-measuring devices, especially those used daily in industrial, laboratory and process applications. 

A transfer standard uses a reference instrument that is calibrated to national or international measurement standards like ISO 17025. The most used reference instruments are highly accurate chilled mirror hygrometers such as those in our S8000 range.

The reference instrument is typically more accurate than the devices being calibrated and effectively serves as the intermediary or ‘transfer’ point in the calibration process. 

Learn more about calibration for moisture measurement.

PST offers comprehensive solutions for dew-point and relative humidity calibration. With our deep-rooted expertise and a legacy spanning 50 years in moisture, humidity, and dew-point technology, we are uniquely positioned to meet the most demanding calibration requirements across various industries. 

Our range of calibration equipment includes Rotronic HygroGen2 calibration chambers, Michell chilled mirrors, pressure swing dryers, generators, and entire calibration systems.  

Learn more about PST’s calibration equipment here.

Related Information

PST Dew Point Calibration Services

PST Relative Humidity Calibration Services

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What are the Types of Moisture Calibration?

Use of Chilled Mirror Hygrometers as Reference Instruments in Calibration Laboratories

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