What Equipment Do You Need for Reliable Humidity Calibrations?

Reliable Humidity Calibrations Illustration

Taking the first step to setting up a calibration facility can be daunting, whether it’s an accredited relative-humidity/dew-point calibration laboratory or in-house calibration department, but it doesn’t need to be overly complicated, especially with expert advice.

This detailed article covers everything you need to consider when designing or choosing a humidity calibration system and gives examples of the equipment and components required.

Our glossary of calibration terms of calibration terms gives useful definitions and explanations.

What parameters are measured in relative-humidity and dew-point calibrations?

  • Relative humidity
  • Temperature
  • Dew point

  • What equipment do you need for in-house relative humidity and dew-point calibrations?

  • Air sources
  • Humidity / dew-point generators
  • Calibration chamber
  • Reference instrument

  • Once you have decided that carrying out calibrations in house is the most appropriate option for your business, it’s important to ensure you choose equipment that suits your needs.

    This guide is designed to help you understand more about the options available; however, guidance from an expert in the field is also strongly recommended. We offer a full support service to ensure the set-up works for you.

    The key elements for you to consider before choosing a system are:

  • The measured parameter of your devices
  • The measurement range of your devices
  • How much automation is required

  • Measurement parameter

    It is important to understand your calibration needs.

    Relative humidity, temperature, and dew point are usually the parameters you will focus on when calibrating relative-humidity or dew-point instruments.

    These parameters are critical in various industries, including pharmaceutical, manufacturing, and environmental monitoring, where precise control and measurement can significantly impact product quality, safety, and compliance with regulations.

    Dew point

    Setting up a dew-point calibration rig requires specialized equipment and careful planning to ensure accurate calibration of instruments. This is especially important for extremely low dew point (-100 oCdp).

    The equipment required depends, of course, on the level of dew point at which you need to calibrate the instrument and whether the calibration needs to be done at ambient or specified temperature.

    The calibration is usually achieved by mounting the sensors into a manifold (sometimes placed inside a temperature-controlled chamber) connected to a dew-point generator; sometimes this would include a dryer.

    For extremely low dew point, a more sophisticated dew-point generator, manifold, and chilled mirror reference will be needed.

    Relative humidity and temperature

    There are a few different approaches to calibrating relative-humidity sensors. In each case, it is important that the equipment is used in an environment with a stable temperature; ideally, a temperature-controlled room.

    The most common approach is calibrating both the relative-humidity and temperature circuits of the device. This is done by placing the sensors directly into a calibration chamber, which generates required humidity and temperature conditions. The chamber provides stable conditions, which leads to a more dependable calibration and lower uncertainties.

    Relative-humidity sensors can also be calibrated at ambient temperature; in this case, only the humidity circuit of the sensor is being calibrated. This can be achieved by using either saturated or non-saturated salt solutions or humidity validators that utilize saturation and desiccant reservoirs.

    Measurement range

    The next deciding factor is the measurement range. The questions to ask here are:

  • What is the complete operating range of devices being calibrated? (Also consider the temperature range if the probes in question are measuring relative humidity.)
  • Do you need to calibrate across the complete range, or do you have a specific area or areas of interest?

  • Relative-humidity calibration in a temperature-controlled chamber

    Temperature-controlled chambers are the most common solution when calibrating relative-humidity instruments. These generate a stable environment at a range of humidity levels and temperatures and, when paired with a chilled mirror hygrometer as a reference instrument, contribute to increased confidence in the measurement.

    At PST, we offer a diverse selection of calibration chambers, including Rotronic’s HygroGen2 with extended ranges of-5…60 °C and 2…99 %rh or Michell's OptiCal with built-in chilled mirror reference.

    Another option from PST’s range is the Michell Instruments HG10. This complete calibration system uses an external humidity generator to feed a stream of air, with a precisely controlled humidity, directly into a manifold within a temperature-controlled chamber and chilled mirror reference. The humidity generator is fed with fresh, dry air from a pressure swing dryer and can achieve very low humidities at a specific temperature, with a total range of 1…95 %rh.

    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.

    A clean compressed air is passed through a dryer to provide a constantly regulated source of dry air, then the dry air is passed through a dew-point generator to produce a moisture-laden gas flow, which is then mixed – in varying proportions, in multiple stages – to create air samples with different concentrations of moisture. This is then fed into a manifold or chamber and then to both the device under test and the chilled mirror reference.

    We offer a wide selection of pressure swing dryers, generators and chilled mirror hygrometers; the options are described below.

    What equipment do you need for dew-point calibrations?

    Air sources

    Dew-point calibration systems require clean and dry compressed air to operate correctly. The required specification of this air varies depending on the model of pressure swing dryer selected, but typically it should be at a pressure of approximately 7 barg (100 psig). For calibration systems designed to generate <-80 °C (<-112 °F) dew point (generally those utilizing the PSD4), the supply air will need to be pre-dried to <-40 °C (<-40 °F) dew point in order to maximize the effectiveness of the pressure swing dryer.

    For users who do not have a supply of compressed, or instrument, air readily available on site, Michell Instruments can provide compressors suited to each type of system, and a pre-dryer for use with systems designed for <-80 °Cdp (-112 oFdp) capability.

    Pressure swing dryers

    Our air dryers operate on the 'pressure swing' principle. Two desiccant columns are connected to each other in parallel. Compressed air from the dryer inlet is passed through the first desiccant column to remove virtually all the moisture present. Most of the dry air from this column is partially expanded, to further reduce the dew point, and then directed to the dryer outlet. The remaining dry air is used to purge the second, off-line, desiccant column to sweep away the moisture collected during its on-line cycle to the atmosphere.

    After a pre-determined period, the function of the two columns is switched – the first column is re-generated while the second column is on-line, producing a flow of dry air. As part of the changeover, the off-line column is rapidly de-pressurized, which causes the moisture adsorbed by the desiccant to be released and purged away. One cycle of this operation is represented diagrammatically in Figure 1.

    The dryers require minimal maintenance and, under normal operating conditions, only require a desiccant change approximately once every 5 years. The highly efficient purge/regenerate system enables the dryer to operate at the same high-performance levels throughout the lifetime of the desiccant.

    We offer two models of dryer:

  • The Michell PSD-2 gives an output of -80 °C (-112 °F) dew-point air or better
  • The Michell PSD-4 gives an output of -100 °Cdp (-148 °Fdp) or better
  • Reliable Humidity Calibrations Illustration

    Dew-point generators

    A dry gas source is fed to the generator from a pressure swing dryer and split into two streams. One stream is bubbled through liquid water via a sintered glass nozzle, ensuring it is completely saturated with water vapor, while the other stream remains dry. The two gas streams are then mixed at atmospheric pressure, in a single or multi-stage process, to generate the target humidity level. The entire enclosure is insulated and temperature controlled, ensuring the saturation, and therefore the output, is always consistent.

    Reliable Humidity Calibrations Illustration

    A single stage of mixing provides a coarse adjustment, limited to around -40 °Cdp (-40 °Fdp). To generate drier dew points, the output of this first stage is mixed with the dry gas source a second time, providing finer adjustments for low moisture concentrations down to -75 °Cdp (-103 °Fdp). For trace moisture levels, a third stage can be added, where the output of the second stage is again mixed with the dry gas source, giving the possibility to generate dew points as low as -90 °Cdp (-130 °Fdp). The -100 °C (-148 °F) dew point is taken directly from the output of the dryer.

    The Michell DG Series dew-point generators are based on the volumetric mixing of dry and wet gases. This gives the fastest response when changing between set points, when compared to other dew-point generation technologies (such as two-temperature, two-pressure, or the combination of both). The mixing is either controlled by flow-metering valves, for a manual control of the target dew point, or automated using a bank of pre-set metering valves, selected by actuating combinations of solenoids to switch between the different wet–dry mixing ratios.

    Manual mixing – DG2 & DG3

    The DG3, with manual single-stage mixing, generates dew points ranging from -40 to +20 °Cdp (-40 to +68 °Fdp). Drier dew points, down to -75 °Cdp (-103 °Fdp), can be reached by the DG2 which has a second stage of gas-flow mixing. The great strengths of the DG2 and DG3 are their ease of use and flexibility in manually generating an accurate target dew point by fine tuning the gas mix via the flow-metering valves. A table of nominal flows is supplied with the generator to guide the user in setting the metering valves appropriately for each desired set point.

    Automatic mixing – ADG400

    The ADG400 uses a two-stage flow-mixing system with mass flow controllers to mix dry air and saturated air in precisely pre-metered proportions. This allows the operator to generate a range of dew-point levels from -80 to +20 °C (-112 to +68 °F). The generator comes factory programmed with 11 calibration set points at 10 °Cdp intervals across its range, with the facility to store 13 additional user set points. The generator can be driven by the HMI with manual control or timed calibration profiles, PC software or via serial commands using the USB interface.

    Automatic mixing – VDS

    To generate dew points down to -100 °C (-148 °F), a more sophisticated system is required. The software-controlled Vapor Delivery System (VDS) generator gives precise, repeatable, and flexible control of the generated dew point. Individual, three-stage, mass flow controllers select precise proportions of wet and premixed air.

    Humidity injection is achieved by a liquid mass flow controller and controlled evaporation system. The entire system is controlled by dedicated PC software, allowing automatic calibration programs to be created, or set points to be triggered manually.


    There are four options of set-point control, which vary between models of generator. This is an important factor to consider, as some systems may have a greater requirement for automation. This is especially applicable if the system is being designed to calibrate many sensors:

  • Manual flow mixing (Michell’s DG3 and DG2) – The wet and dry flows are manually altered by metering valves on the front panel of the generator. On the DG2, these are monitored by means of a flow meter for each valve. A table of nominal flow values for each set point is provided, and full analog adjustability is possible across the complete range of the device.
  • Locally controlled automatic flow mixing (Michell’s ADG400) – Humidity set points can be selected from the factory or user calibration tables, using the HMI on the front panel.
  • Timed calibration profile mode (Michell's ADG400 and VDS3) – The user can define a sequence of set points that are activated on a timed basis for an unattended calibration.
  • Remote-controlled automatic flow mixing (Michell’s ADG400 and VDS3) – Control is implemented by sending serial commands to the generator via RS232, or USB, either through dedicated control software (which can run user-defined sequences) or via the customer's own system.
  • Reference instruments

    Chilled mirror hygrometers are precision instruments for critical measurement and control applications. Chilled mirror sensors measure a primary characteristic of moisture – the temperature at which condensation forms on a surface. This means that chilled mirror hygrometers:

  • Have very low drift: the temperature at which condensation forms is measured directly.
  • There are no calculated variables that could shift over time.
  • Are inherent repeatable, giving reliable results every time.
  • Chilled mirror operation

    The chilled mirror sensor consists of a temperature-controlled mirror and an advanced optical detection system. A beam of light from an LED is focused on the mirror surface with a fixed intensity. As the mirror is cooled, less light is reflected due to the scattering effect of the condensate formed on the mirror surface. The levels of reflected and scattered light are measured by two photo detectors and compared against a third reference detector measuring the intensity of light from the LED.

    The signals from this optical system are used to precisely control the drive to a solid-state thermoelectric cooler (TEC), which heats or cools the mirror surface. The mirror surface is then controlled in an equilibrium state, whereby evaporation and condensation are occurring at the same rate. In this condition, the temperature of the mirror, measured by a platinum resistance thermometer, is equal to the dew-point temperature of the gas.

    Choosing a reference instrument

    The Michell Instruments' range of chilled mirror reference hygrometers has measurement capabilities matched to the performance of each of the different rh and dew-point generator options. For reasons explained earlier in this guide, a high accuracy reference is a necessity for performing traceable, credible calibrations.

    Ambient temperature measurement accuracy of most Michell Instruments' chilled mirror products is ±0.1 °C (±0.18 °F). When the calibration parameter is relative humidity, then a measurement of ambient temperature is also necessary as the other input to the equation which determines this from dew point: Vapor pressure (e) is determined by solving the Sonntag (1990) formula for the current dew-point temperature. Saturation vapor pressure (es) is found by repeating the process for the ambient temperature. This provides the relative humidity in %rh.

    This calculation is recognized and published in the National Physical Laboratory's 1996 publication, 'A Guide to the Measurement of Humidity' . Its use will, in most cases, still yield lower uncertainties of measurement than can be achieved with hygrometers which directly measure relative humidity.


    Although Michell Instruments' chilled mirror hygrometers are fundamental and have very low drift, in order to maintain the traceability of your reference we suggest you return it to us to be calibrated against one of our transfer standards on a regular basis.


    Standardized or customized designs of manifold are available for Michell Instruments' dew-point sensors depending on how many sensors are intended to be calibrated on the system at any one time. We can also design custom manifolds to accept non-Michell sensors or instruments; the optimal configuration is designed from the dimensions of the device and its mounting arrangement.


    We can integrate various system components and functions, such as logging sensors under test, the reference instrument, and other enhancements. Please contact us for further details.

    We are uniquely qualified to advise our customers who wish to carry out their own humidity calibrations. We manufacture a wide range of humidity calibration equipment (from fully integrated bench-top calibrators to the separate components needed for a customized system) which we utilize daily in our own humidity and temperature calibration laboratories worldwide.

    Calibration System Selection Guide

    Calibration System Selection Guide

    View our calibration services pages here.

    Related Categories

    Humidity and Temperature Calibration

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    PST Dew Point Calibration Services

    PST Relative Humidity Calibration Services

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    Use of Chilled Mirror Hygrometers as Reference Instruments in Calibration Laboratories

    Meeting the Challenge of Measuring Very Low Dew Points with a Chilled Mirror Hygrometer

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