Use of chilled mirror hygrometers as reference instruments in calibration laboratories

For large pharmaceutical manufacturing plants, power plants and other industrial/process plants which operate large quantities of dew point sensors, it is usually more cost-effective to calibrate these in-house rather than sending them away to a calibration lab. This also gives the user the option to perform error checks on probes at any time.

Meteorological offices & Standards Laboratories will operate several generation systems which are used to calibrate ‘transfer’ standards (either within the lab or the hygrometers of other institutions) against the primary standard of the laboratory. Working instruments can then be calibrated against the transfer standard; this provides a chain of traceability from the instrument in the field to the primary standard.

Dew point generation

Michell Instruments offers a large range of customizable dew point calibration equipment, which caters for a variety of different calibration ranges. These start from manually operated volumetric flow mixing generators, up to fully automatic mass flow-controlled systems.

all the necessary components for humidity calibration including generators, dryers and reference instruments
Typical components of a humidity calibration system

A dew-point calibration system consists of the following components, which can appear in a few different packages; a clean compressed air supply is required to feed a pressure swing dryer, this dryer provides a constant source of dry air, which forms the lowest calibration point, and the base from which all other points are generated.

A dew point generator contains a saturator, which bubbles a proportion of the dry air through liquid water to ‘saturate’ it with moisture – i.e., water vapor pressure = saturation water vapor pressure at the temperature of the saturator. This saturated air is then blended in different proportions (and often in multiple stages to increase resolution, depending on the device) to create air with different moisture contents. The entire generator assembly is usually temperature controlled to ensure stability of the device and a known saturation quantity.

The air which is output from the generator is then fed into a manifold or chamber which houses the devices under test.

Fundamental reference hygrometers

When calibrating sensors, very little real information can be gained by simply monitoring the readings of the sensors under test against the set point on the generator. As the generators only use mixing techniques, they cannot produce a definite dew point, so to make a correct assessment of the performance of a sensor, a reliable, fundamental reference measurement is required to make a comparison against.

Michell Instruments’ range of high-precision chilled mirror hygrometers measure a primary characteristic of moisture – the temperature at which condensation forms on a surface. This means that chilled mirror instruments are inherently repeatable and so are integral for laboratories looking to achieve low measurement uncertainties.

The S8000 range provides a number of options for dependable reference measurements all of them specified at ±0.1 °C frost point(fp)/dew point (dp). For calibration requirements down to -60 °Cfp we have the standard Michell S8000 (otherwise known as the Integrale). The Michell S8000 RS is also available in two different configurations, for calibration requirements to -80 °Cfp or -90 °Cfp.

metallic bench top chilled mirror hygrometer
Michell S8000 RS High Precision Chilled Mirror Hygrometer

And Introducing the S8000-100 a new high precision chilled mirror hygrometer offering the lowest possible measurement range down to -100 °Cfp.

 high-precision chilled-mirror hygrometer
Michell S8000 -100 Low Dew-Point Hygrometer

The S8000 -100 is the result of 40 years’ experience of developing chilled mirror technology. As one of the world’s largest producers of high-quality dew-point sensors, we use the S8000-100, along with other instruments in the Michell chilled mirror range, as the workhorses of our manufacturing and calibration operations.

Need help with in-house humidity calibrations? Contact Us for advice.

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Related Products

High Precision Chilled Mirror Hygrometer - Michell S8000 RS
High Precision Chilled Mirror Hygrometer – Michell S8000 -100
Precision Chilled Mirror Hygrometer - Michell S8000

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