In-house humidity calibration: what are your options?

If you regularly use a large number of humidity probes, relative humidity transmitters or hand-held humidity meters, you will understand the importance of regular calibrations to ensure confidence in your measurements for the purpose of:

• Accreditation — They need to prove the validity of their results to meet the requirements of the regulations they are working to.
• Quality — They need to ensure the accuracy of their readings to ensure the quality of their product or results.
   

The most common method is to send the probes away to either a calibration house or the manufacturer. This, however, presents a problem: how can the necessary measurements be made when the probes are away being calibrated?

The typical answer is to purchase additional probes as spares. However, this results in additional cost — and even more probes to keep track of and recalibrate.

In large pharmaceutical facilities, power generation plants, meteorological offices and standards laboratories, it is often more cost-effective to bring this calibration in-house. This might sound like a daunting concept, but with the right guidance and equipment is not complicated.

The fundamentals of humidity calibration

All humidity calibration systems require three elements:

• A source of clean, dry air from a compressor with a pressure swing dryer
• A humidity or dew-point generator as the source of moisture
• A control system or reference such as a chilled mirror hygrometer
   

You also need to understand the measured parameter of your devices — dew point or relative humidity. These are at different ends of the moisture scale, with dew point covering the very low, trace moisture end, and % rh at the wetter end.

These systems vary in size and complexity, from the high-end systems found in national metrology labs like the Michell DCS-80 to compact ‘all-in-one’ humidity verification systems like the Rotronic HygroGen 2.

Dew point or humidity generation

A dew-point generator contains a saturator, which bubbles a proportion of the dry air through liquid water to ‘saturate’ it with moisture — ie, water vapour pressure = saturation water vapour 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.

Process Sensing Technologies offers a large range of customisable humidity and 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.

Reference instruments

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 or humidity, so in order to make a correct assessment of the performance of a sensor, a reliable reference measurement is required to make a comparison against.

Chilled mirror hygrometers are the gold standard of humidity references. They 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 Michell 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 the standard S8000 is recommended. The Michell S8000 RS has two configurations, for calibration requirements to -80 °Cfp or -90 °Cfp.

We can now introduce the new Michell S8000 -100: a high-precision chilled mirror hygrometer offering the lowest possible measurement range down to -100 °Cfp, without the need for external cooling. A flagship product!

A cost-effective option is the Michell Optidew 401, which is a compact chilled mirror hygrometer that can be used as a reference with the Michell or Rotronic calibrators.

For maximum precision of RH measurements, the Michell S8000 Remote features a chilled mirror sensor which can be directly inserted into either the Michell HygroCal100, S904 or Rotronic HygroGen 2. Its 0.1°C dewpoint and temperature accuracy ensures the best confidence in any RH measurement available.

Reference instruments can be supplied either calibrated against our standards traceable to NPL (National Physical Laboratory) and NIST (National Institute of Standards and Technology), or in our ISO17025 accredited laboratory.

Calibrating or validating relative humidity probes using a humidity generator

Complete humidity calibration systems offer a compact and convenient method to validate a variety of probes, transmitters or even hand-held instruments. Many offer the option to include a traceable reference hygrometer.

The Rotronic HygroGen 2 is a complete humidity and temperature probe calibrator, which allows calibration of multiple probes. The leading instrument in its class, it is favoured by the pharmaceutical industry as well as in ISO 17025 calibration labs around the world. The Michell S8000 Remote can be easily connected as a reference for traceable humidity calibrations.

The Michell HygroCal100 is a compact portable system for automatically validating relative humidity probes. It allows the user to install their test probes into the humidity-controlled chamber, connect them to its electrical inputs and monitor their readings via the intuitive colour touch screen display. The Chamber can generate humidity from 5 to 95% with ±0.5% uniformity. Set points can be designated manually, or complete 10-point calibration routines can be programmed and started in seconds. The calibration can be logged to the internal memory, allowing complete automation of the validation process. Also, the device allows the user to connect their own reference hygrometer to the system, and display its readings on the display, therefore importing their own traceability into the system.

All of this is possible in a battery-powered and completely portable unit weighing just 3.2 kg, allowing the unit to be transported to the location of the sensors requiring test.

The S904 is an RH and Temperature controlled chamber, which allows calibrations across a range of temperatures from 10°C to +50°C with excellent temperature uniformity. It can also be specified with an RS232 interface, which allows remote control of the chamber via the supplied application software, providing facility to program full calibration routines and log the output of the probes under test.

The OptiCal is visually similar to the S904, offering the same functionality but with the addition of a built-in Optidew Chilled Mirror Hygrometer Reference.

The Optidew provides fundamental measurement of dew point and temperature to provide a highly accurate measurement of RH. The reference sensor is installed in the rear of the chamber, meaning that useable space inside is not lost in comparison to the S904. This provides the user the facility to carry out traceable calibrations in a transportable package.

For more information:

Products — AMS Instrumentation & Calibration (ams-ic.com.au)

Humidity and Temperature Calibration — Process Sensing Technologies