Humidity is a tough measurement. It’s very hard to get a repeatable measurement with low uncertainty. This post discusses seven best practices that will help you make a more accurate, repeatable, and reliable humidity measurement. The recommendations are primarily based on the NPL document, “A Guide to the Measurement of Humidity.”
1. Choose the correct instrument. It may surprise you to know that many of the problems we help customers solve are caused by them using the wrong instrument in their particular application. For example, it’s impossible to get an accurate measurement of dry air (–40 dew point temperature) with a resistive humidity sensor. Choosing the correct instrument for measuring humidity is not as easy as it seems. Temperature range of the application is one of the more important things to consider when choosing an instrument. Other factors that significantly affect the choice include expected range of relative humidity, temperature difference between the application and the ambient environment, pressure of the system, presence of chemicals in the air, composition, how will the output be used, and the availability of power. Check out this Vaisala technical note for more information about choosing the correct humidity instrument.
2. Follow the manufacturer’s instructions. We highly recommend reading the manual before trying to use or install the instrument. Manufacturers are the best source of information about their respective products. Typically, most of us read product manuals as a last resort; however, I highly recommend reading the manual at the earliest possible point in your product relationship.
3. Ensure proper calibration. The first question that this statement begs is the definition of a proper calibration. As a general rule, a proper calibration is performed in accordance with the manufacturer’s recommendations, uses a standard that is traceable to international standards, specifies the uncertainty, and is performed within the recommended time interval. Ideally, your organization has written, and follows, an instrument maintenance standard operating procedure that delineates the process as defined by your organization. ISO 17025 is an excellent guide for an instrument calibration standard operating procedure. For more information about humidity, temperature, and pressure calibration, download the Vaisala Calibration Handbook.
4. Keep records. Records document the life of your hygrometer. Calibration records are the obvious place to start; however, you should also maintain records about repairs, significant events that occur around the instrument, sensor replacement, power surges, and similar incidents. Records like this will help identify the causes of measurement problems.
5. Spot check performance. Suppose your calibration interval is 12 months. What happens between the calibration event? As the saying goes, “stuff happens” that could cause an instrument to suddenly go out of tolerance. Power surges, condensation events, contamination of the gas, large pressure variations, or an errant forklift driver, among other myriad “stuff” happens from time to time and could cause various problems with your hygrometer. To mitigate the problems, put into place a spot check program as part of your standard operating procedures. Depending on your level of acceptable risk, you may spot check monthly, quarterly or semi-annually. A spot check is a quick comparison to a trusted standard vs. a full and proper calibration.
6. Be knowledgeable about the parameter. It’s important to know what parameter your humidity sensor is measuring. For example, your instrument may report dew point temperature, but it does not directly measure dew point. Instead, the instrument measures relative humidity and temperature, then calculates dew point. The considerations for a proper measurement are quite different for a true dew point sensor vs. a relative humidity sensor. It’s just good practice to know how your instrument comes up with the reported parameter. Know the conversion formulas your instrument manufacturer uses. Download the list of conversion formulas used in Vaisala instruments.
7. Be aware of potential external factors. Depending on the sensor type, certain external factors will affect the measurement. For example, if the sensor is measuring RH, temperature has a direct effect on the measurement. Sunlight shining directly on an instrument with a RH sensor will significantly distort the reading because of the heat generated. Pressure differences significantly affect a true dew point sensor. A person who takes a reading standing too close to a RH sensor will distort the measurement. Temperature leakage caused by temperature differences between the measured environment and the external environment are quite typical in causing measurement problems.
If you follow these seven best practices, you will make a better humidity measurement. For more in-depth information, check out the Vaisala on-demand webinar, Best Practices in Humidity Measurement, or sign up for a Vaisala Humidity Seminar in a city near you.