What Equipment Do You Need for Reliable Humidity Calibrations?

If you regularly use a large quantity of humidity probes, relative humidity transmitters or hand-held humidity meters calibrating these in-house may save both time and money.

Making the first step to setting up an in-house calibration facility can be daunting at first, but need not be over complicated, especially with expert advice.

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

Our glossary of calibration terms gives useful definitions and explanations.

What parameters are measured in humidity calibrations?

• Measurement parameters
• Measurement range

What equipment do you need for in-house humidity calibrations?

• Air sources
• Humidity and Dew Point Generators
• Control systems

What parameters are measured in humidity calibrations?

Once you have decided that carrying out moisture calibrations in-house is the most appropriate option for your business, it’s important to ensure you specify the correct system. 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 for customers wishing to set up a moisture calibration system.

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
• How to install your devices into the system These are dealt with in the sections that follow.

Measurement Parameter

The process of deciding which calibration system is best suited to your needs is dependent on the devices to be calibrated and their measurement parameters.

Dew Point

If the devices are measuring dew point, then the calibration manifold is usually situated in an ambient temperature environment. As dew-point calibration systems are frequently intended to produce very low moisture contents, the manifold needs to be of a high integrity design; working with the sealing mechanism of the sensor to ensure that moisture ingress from the ambient environment is prevented. For extremely low dew points (<–80 °C (<–112 °F)) it is sometimes necessary, (depending on the ambient conditions) to enclose the manifold in a chamber which can be purged with dry air, thus limiting the effects of ingress.

Relative Humidity and Temperature

There are two different approaches to calibrating relative humidity sensors. One approach is to place the sensors directly into a calibration 'chamber', which is a temperature and humidity controlled self-contained environment. This functions in a similar way to a climatic chamber, only on a much smaller scale and with much greater uniformity. Calibration chambers without temperature control also exist, meaning that the selected relative humidity will be generated at the prevailing ambient temperature— however, it is important to ensure that when these types of generators are used, they are placed into an environment with a stable temperature.

The other approach is to use an external dew-point generator to pass a flow through a manifold into which the sensors are mounted. The manifold is placed inside a larger, temperature-controlled chamber.

The advantages of this second approach are:

• The volume of the manifold is quite small, and there are few ingress points, so step changes tend to happen more quickly
• Using a volumetric mixing dew-point generator allows much lower humidities to be achieved compared with a calibration chamber

The disadvantages are:

• The components involved are physically much larger
• They can be significantly more expensive than a self-contained chamber

Measurement Range

The next deciding factor is measurement range. The questions to ask here are: What is the complete operating range of your devices? (Also consider 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

The range of an RH calibration system depends on the ability to control two separate parameters: the temperature range of the chamber and the relative humidity range (the lowest RH point being the limiting factor in most cases). All Michell relative humidity chambers (S904, OptiCal) are capable of drying their internal volumes to just 10% RH at any temperature within their operating range. It is challenging to dry a comparatively large volume below this point, as the system is simply extracting the air, drying it with a desiccant cartridge and re-injecting it with a slightly lower moisture content. It is possible to humidify the chamber up to 90% RH which is a sensible limit set for the purpose of preventing condensation. If condensation should occur, it would take a long period of time (and a significant strain on the desiccant cartridge) to dry the chamber out again.

The Michell Instruments' HG10 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. 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 to 95% RH.

Dew Point

Dew-point calibration systems generally produce much lower absolute humidities than RH calibration systems. The generation range of dew-point systems depends on two factors:

The output dew point of the pressure swing dryer used to provide the dry air source to the humidity generator (sometimes referred to as 'full dry').

The resolution of the dew-point generator—which is its ability to mix specific quantities of full dry and saturated air together, in stages, to achieve accurate outputs of very low moisture content. Where volumetric flow mixing generators are concerned; the more stages of mixing, the lower the dew point the generator can control to. For example, a single stage DG3 can only control to a minimum dew point of approximately -40°C (-40°F), no matter how dry the input air is; whereas a two stage DG2 can generate dew points to -75°C (-103°F). Three stages of mixing give the capability to generate dew points to -100°C (-148°F).

What equipment do you need for in-house humidity 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°Fdp) 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 it 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.

Dew-Point and Humidity 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.

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 via 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.

Control

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 so there are no calculated
  - Variables that could shift over time.
  - Are inherently 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 have 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 all 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.

Calibration

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 an annual basis.

Manifold

Standardized or customized designs of manifold are available for Michell Instruments' 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.

Integration

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 wanting 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) that we utilize daily in our own humidity and temperature calibration laboratories worldwide.

View our calibration services pages here.