HUMIDITY CALIBRATION.

Introduction to Relative Humidity.

Relative humidity is a measure of the amount water vapour in the air compared to the maximum amount of water vapour air is capable of containing at any specified or specific temperature, and is expressed as a percentage.  

The amount of water vapour that air can contain varies with the temperature of the air, as warm air can hold more moisture than cold air.

A relative humidity of 100 percent indicates that the air is holding all the water it can at the current temperature.   A relative humidity of 50 percent means the air is holding half the amount of moisture that it is capable of holding.  

If the temperature decreases, while the amount of moisture in the air remains constant, then the relative humidity increases (since the maximum amount of moisture that the cooler air can hold is reduced).

Significance of Humidity in Industry.

The manufacturing process for many regulated pharmaceutical products specifies specific atmospheric conditions that are mandatory at all times.  This usually applies when a product that is sensitive to room air moisture content level has to be exposed to it.  Then it is essential that the humidity stays within acceptable levels.  This mandates the validation and humidity calibration of all the associated RH control and indication systems.

However humidity is often controlled just for creature comfort and the well being of equipment in general.  Our experience in a new facility inSouthWales highlights some of the problems that arise as hygrometer sensors drift out of humidity calibration.  Of all HVAC sensors they are the most inclined to go out of calibration.

Hygrometer and hygrometer sensors must be routinely calibrated and their calibration history must be subjected to a predictive analysis; for product safety it is mandatory and for creature comfort it is absolutely essential for energy conservation.

Calibration of Humidity & Hygrometer Sensors.

Humidity sensors require frequent calibration and continual monitoring, they are, along with pH sensors, the most prone to error sensors in general facility use. They should be calibrated against a range of Chemical mixtures that establish a specified humidity calibration level in small atmospheric containers.  These chemical mixtures are issued with UKAS traceable humidity calibration certification.  They are supplied in a kit that includes three certified standards covering the low mid and high sectors of the relative humidity calibration percentage range.

The kit includes three ready to use enclosures (with certification of their values), a range of adaptors to suit your sensors, a sample FDA compliant calibration certificate and a complete detailed method statement on exactly what to do.  All these items are contained in a robust neat and easy to handle carrying case.

Recommended Calibration Methods.

The normal cost of test equipment for the humidity calibration of sensors is around $10K.  This precludes most companies from setting up in-house calibration.  Subsequently this calibration is usually performed by an external calibration company.  However there are alternative calibration methods.

We have used certified saturated salts solutions for nearly twenty years and they are very reliable and, just like the column of water or mercury, they are basic standards.  The only variable is temperature.  So with your saturated solution and a temperature gauge you can calibrate any humidity sensor or hygrometer.

VAISALA news

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 saturated 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.  Saturated 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.

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

Vaisala

All relative humidity calibration points were created with a saturated 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 (saturated salt solution) and ±0.4 RH% at 97%RH (saturated salt solution).

Vapour Pressure, Dew Point and Humidity Calculator.

Negligent Humidity Calibration Can Cost A Fortune.

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 humidity 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 the 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 departed 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.   

HUMIDITY TESTING AND CALIBRATION