Chilled Mirror Instruments

Precision Humidity Solutions

Accuracy you can rely on. Water vapor is a critical factor in many processes. Michell Instruments’ range of precision hygrometers is designed to measure and control moisture levels in diverse applications - from trace moisture in laboratory settings to hot, saturated gases in heavy industrial processes.

Our precision hygrometers are based on the fundamental chilled-mirror principle to give you accurate and repeatable measurements of moisture vapour you can trust - whatever your needs. The S4000 and S8000 ranges are suitable for use as national standard reference instruments, providing a firm foundation for your moisture control procedures, while the Optidew range is designed to withstand demanding industrial settings with no loss of accuracy.


  • Fundamental measurement technique
  • Excellent measurement stability
  • Wide measurement range –100 to +130°C dew point (-148 to +266°F)
  • Dynamic contamination correction automatically balances optics to ensure consistent accuracy
  • Comprehensive range for process and calibration applications

Why use Chilled Mirror Sensing Technology?

Sensitivity - Michell’s chilled mirror products detect minute changes in moisture down to the ppb range.
Accuracy - Chilled mirror technology is accurate to 0.1°C (0.18°F) (S8000 and S4000 model).
Drift free - A chilled mirror sensor will measure consistently every time.
Calibration - The S8000, S4000 and Optidew can be used to calibrate relative humidity and dew point instruments. All Michell’s products offer traceability to the major national and international standards laboratories.
Rugged - Michell’s chilled mirror products are designed to withstand industrial use.
Fundamental - The optical condensation principle of dew point measurement has been established for centuries as the most fundamental method of determining the moisture content of a gas.

Contamination Correction

All Michell’s chilled mirror instruments incorporate an automatic compensation system that periodically balances the optics to compensate for any loss of light intensity from contamination.

Background to Michell Instruments

Michell Instruments is the international leader in the field of moisture and humidity measurement solutions. With over 30 years experience, Michell designs and manufactures a wide range of transmitters, instruments and system solutions capable of measuring trace moisture, humidity, dew point and oxygen in a vast range of applications and industries including compressed air, power generation, process, oil and gas, pharmaceutical and many more.

With a fast growing international subsidiary and distribution network, the Michell Group provides solutions in moisture and humidity for the most demanding applications worldwide. Michell uses four key dew point measurement technologies in its products:

  • Impedance method using a ceramic tile which delivers unrivalled speed of response and robustness.
  • Chilled Mirror method which has been incorporated into advanced, precision instruments for industrial and laboratory use.
  • Dark spot technology, which was developed in partnership with Shell, to offer a world-class solution for the measurement of hydrocarbon dew point.
  • Quartz crystal technology for a high precision, fast responding moisture measurement with self-calibration.

The Technology

The chilled mirror principle works by passing the sample gas over a polished mirror surface - the temperature of which is controlled by a peltier heat pump.

The mirror is cooled until moisture condenses on its surface. An optical system is used to detect the point at which this occurs, and this information is used to control the mirror temperature and maintain it at the dew point.

The system operates by illuminating the mirror with an LED. The light reflected back is measured by a photodetector and this amount of light is recorded as a datum point. As moisture builds up on the mirror the level of light reflected will decrease. By comparing this signal with the datum point at any time, the system will be able to control the peltier drive circuit to either heat or cool the mirror in order to fi nd and maintain the dew point.

The core of the mirror contains a PT100 platinum resistance thermometer which accurately measures the mirror surface temperature - this temperature is equal to the dew point.