What are the types of moisture calibration?

What is meant by moisture calibration?

Moisture, dew point and relative humidity calibration is the process of comparing a measuring instrument against an authoritative reference to identify any bias or systematic error in the readings. Reference instruments should have a significantly lower uncertainty of measurement. Traditional accuracy multipliers used in electrical, or pressure calibrations can be misleading with moisture calibration due to the number of other contributors to the calibration (including stability, temperature and thermal gradients). Generally, a moisture calibration will be performed by repeating the process of comparison at a representative selection of points across the measurement range.

Calibration is often interpreted as ‘adjustment of an instrument to read correctly’ but this is a misconception. Calibration is technically only the comparison of an instrument to a reference (it defines the instrument errors). Adjustment is an additional step to correct the instrument and remove the error. If adjustment is performed a repeat calibration is required to confirm the new (potentially lack of) errors. Here the terms 'as found calibration' and 'as left calibration' come into play.

Why is Humidity Calibration Important?

Reputable manufacturers will provide calibration certificates with instruments or sensors at the time of purchase. However, this is not a guarantee of the measurement performance throughout the entire lifetime of the equipment. Over time, the ageing of mechanical and electronic components can cause changes in the characteristics of sensor or instruments. More critically, once the device has been used in the field, and possibly introduced to contaminants which can have an impact on accuracy, it is difficult to say with any degree of certainty whether or not the original calibration is still valid.

In order to minimize uncertainty and have confidence in measurements made with the instrument, it is important to have the quality assurance of a regular, accredited calibration procedure.

How is the calibration data used?

When the readings provided by the sensor or instrument are critical to the process or testing procedure, it is vital to know that the device is still performing within its original specifications, or if the correction factors have changed. When an instrument is recalibrated, a report is provided showing the calibration corrections before, and after, any adjustment was performed. For many hygrometer users, these ‘as found’ are crucial, as they provide a figure for measurement drift between calibrations and give insights to the readings prior to calibration. For example if the efficiency of a drier has reduced this could be due to sensor drift, calibration identifies this as one of the reasons for this drop in performance.

Calibration standards and traceability

The best way to ensure accurate measurements is through traceability to attested standards.

This leads to consistent measurements across different instrumentation, locations and users. ‘Traceability’ is an unbroken chain of calibrations which relate a working hygrometer back to a national standard.

Most countries hold their own authoritative standard for dew point and humidity which forms the primary measurement standard. Alternatively, reference standards are shared across a region. The reference instruments of calibration facilities in the country or region are calibrated against this primary standard and are then used as references or ‘transfer standards’ to calibrate other instruments.

This approach is represented by the pyramid graphic below:

How is a humidity calibration carried out?

Humidity calibrations involve generating a stable level of humidity in a sample gas and measuring this with both a suitable fundamental reference instrument and the instrument under calibration, and then making a comparison of the readings.

A humidity calibration systems comprises:

• A dry air source
• A humidity generator to provide the desired amount of moisture content
• A reference instrument which is more accurate than the instrument under test – to provide an authoritative measurement
• A manifold to supply calibration gas to the sensors/instruments under test

If the calibration is being performed in terms of relative humidity and temperature, then the manifold must be temperature controlled or placed in a temperature-controlled environment in order to determine the relative humidity by associating the generated dew-point value with the temperature.

We are the only supplier to offer complete dew-point calibration systems and individual calibration components, all of which are designed and built in-house.

How to read a moisture calibration certificate

A humidity calibration certificate will contain information about the instrument under test (IUT) and reference used, along with calibration points and information about the measurement uncertainty.

In the example certificate below, information is displayed in a table with the columns laid out as follows:

Column 1: The measured value indicated by the reference instrument.
Column 2: Shows the measured value indicated by the instrument under calibration.
Column 3: The correction that should be applied to any measured values obtained from the instrument under calibration.
Column 4: The measurement uncertainty for each set point.

Generally, if the instrument under calibration has multiple ways to report the measured value (i.e. built-in display, digital output, analogue voltage or current outputs) then the values taken from these will also be shown on the certificate at each calibration point.

Other information that should be included on the calibration certificate:

• Customer details, sometimes including the customer’s address, customer number or order number
• Any serial numbers that identify the instrument, and associated measuring components
• The date of the calibration, and sometimes the date when the subsequent calibration is due
• A unique reference number identifying the certificate or calibration run
• A list of calibrated equipment used during the calibration or a list of unique identifiers corresponding to that equipment
• An estimation of uncertainty over the range of the calibration

What are the advantages of in-house humidity calibration vs using external calibration laboratories?

In simple terms, it’s a question of volume: the more humidity equipment you use, the more cost-effective and convenient an in-house laboratory will become.

Because of the volume of instruments being calibrated, the cost of setting up the laboratory is soon offset against the savings of sending instruments out to a commercial laboratory and the downtime waiting for them to be returned. Typical companies that opt for setting up their own in-house facilities include:

• Meteorological organizations
• Standards laboratories
• Pharmaceutical manufacturing plants

However, purchasing and operating a dedicated humidity calibration system requires a significant commitment in terms of initial cost, personnel training, record keeping and, often, physical space. For companies that use a small number of humidity instruments, sending these to a commercial calibration laboratory for regular calibration is the most cost-effective solution.

What next?

If in-house calibration seems like a good option for you,discover our ranges of moisture calibration equipment from cost-effective portable humidity calibrators to fully equipped dew-point calibration systems.

Contact us to find out more about our accredited humidity calibration services.