We have just added an addition to our range.  This is a kit for calibrating humidity sensors and hygrometers.  For further technical details please  click below:


You may or may not be aware that humidity is a real problem when it comes to calibration; temperature/pressure/speed/viscosity/colour all can be simulated and measure relatively cheaply.  Humidity requires a delicate test set costing $10k along with a trained technician.  Our testing kit allows the job to be done much quicker and cheaper.


Humidity sensors require frequent calibration and continual monitoring, they are, along with pH sensors, the most prone to error sensors in general facility use. Where as most sensors measure to within (plus or minus) 1% and have extremely low drift rates.  Most competitively priced humidity sensors measure to within (plus or minus) 3% at mid range and 5-7% at the top and bottom of their ranges, and have a significant drift rate.

Where the humidity is controlled to protect a regulated product it is essential that your humidity sensor is not allowed to drift out of calibration.  Calibration periodicity should therefore start at 4/6 months and be extended as you’re calibration history (trend analysis) establishes a justification for extending the period between calibrations.

Where the humidity is controlled for creature and equipment comfort and protection, it is false economy to not calibrate frequently.  These sensors do drift, and if in the AHU (air handling unit) this occurs then the air is over chilled for moisture removal.  This over-chilled air has then to be reheated for room use.  This can be extremely severe and add $xxK to the annual energy costs of each AHU.


Below are two real live incidents that we have had with faulty humidity sensors. An annual maintenance and calibration service has been set up to support these Humidity Calibration Kits.

To Purchase please click here:- Humidity Calibration Kit.



No humidity instrument stays accurate on its own. From time to time, the performance of these instruments must be checked against a reference.  When calibration is done in-house, the use of salt solutions as a reference is a proven and affordable method.

National Physical Laboratory (NPL)

Saturated (or unsaturated) salt solutions, and certain other chemicals, can be used to generate an environment of a particular relative humidity in an enclosure.  salt solutions, with solid salt present, have the special property that a stable concentration is maintained - and hence a constant relative humidity - even if water migrates to or from the solution.

OMEGA Process and Measurement Control.

Salt Solutions: A very convenient method to calibrate humidity sensors is the use of salt solutions. At any temperature, the concentration of a salt solution is fixed and does not have to be determined.


All relative humidity (RH) measurement points were created with a salt solution system. The measurements were done with the same salt system each time. The salt systems are susceptible to temperature differences within the system. In addition there are other known sources of uncertainty. [2] In the described experiments the major source of error is believed to be the temperature differences measured between the salt column and the adjacent air. The variations in the laboratory temperature were believed to be the second most significant source of error. Taking into account these two sources of error it has been estimated that the measurements uncertainty is ±0.3RH% at 75%RH (salt solution) and ±0.4 RH% at 97%RH (salt solution).

Past Cases.

ProblemSuffolk Power Station.

During the construction of Sizewell B nuclear power station, the quality assurance system specified that all control system instruments arriving on site, must be re-calibrated on site prior to installation.  This involve many thousands of instrument and sensors.

The hygrometer transmitters arrived (160 of them) and during calibration large errors were found (35%).  Since they arrived already calibrated and with NIST traceable certification (which had incurred a substantial premium charge), questions were asked, and a full investigation was undertaken.  Our on-site calibration standards and methods were subjected to a full verification at a UKAS approved calibration centre.  They were found to be accurate, and no reason or cause was ever found for these huge errors.  Had they not been subjected to this on site calibration the ensuing problems would have been horrendous.


Problem South Wales New Facility.

When starting the validation of a Building Management System, for a new facility consisting of eight process rooms, in South Wales, the system was found not to control room conditions smoothly.  It was over compensating every time a correction or change was made.  On closer investigation the main air ducting from the AHU’s (air handling units) was found to be covered in ice.  When the temperature of this air in this duct was measured it was found to be -22 centigrade.  It was also found that the steam injection (for humidity control) was injecting almost continually and that the room entry heat exchangers steam-valves were fully open.  

This was very disturbing as the main HVAC contractor had signed off the system and left the site for good.  We eventually discovered that due to a faulty hygrometer sensor. The system was sensing that the in-coming air was saturated.  This powered up the chilling coils in the AHU (in fact it set the cooling refrigerant valve to 100% open).  The duct air leaving the AHU was now at 10% RH and minus 22 degrees centigrade.  Steam injection was then being over supplied and the room entry heaters were working at close to 100% to as they attempted to maintain the rooms at 20 degrees centigrade and 40% RH.  The HVAC contractors had played around with the loop offset values until they achieve a form of balance.  As stated before this was highly unstable and resulted in a massive increase in energy requirements.  We re-calibrated all the sensors (changing the faulty hygrometer sensor) and rebalanced the system.  The company’s estates manager report showed that the system was now using 85% less energy, and went on to state, that had the system been allowed to go into production in the state the contracts signed it off in, there would have been an additional annual energy cost of £68k.   


The SOP for Equipment Validation continues to be an extremely popular document. This document leads you through the validation process, from the URS to the final P2Q.
Purchase your copy now at Special Price of $16.00.

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