Knowing what to look for in product specifications can initiate incisive questioning of manufacturers about the effectiveness of their humidity measuring systems.

One of the hardest parameters to accurately measure, relative humidity (RH) is a pivotal factor across a broad spectrum of industries and often entails the potential to impact critical applications and public safety. In calibration, stability testing, or quality assurance processes, the intrinsic uncertainty of humidity measurement can be a major source of unnecessary cost, skewed data,and lost revenues.

FACT: ALL HUMIDITY SENSORS DRIFT
It’s an immutable law of RH measurement; relative humidity sensors drift. They do so for the simple reason that they are “air breathers.” Unlike temperature sensors, the internal structure of the humidity sensor must be in direct contact with the environment, which is constantly changing temperature and contains countless airborne contaminants. Both fluctuating temperatures and contaminants significantly affect the accuracy of any RH sensor, more so over time. This is why, even if the calibration process were perfect (it isn’t), once exposed to the real world, the measurement accuracy inevitably degrades.

A TALE OF TWO CALIBRATIONS
There are two key accuracy values that must be considered when looking at any RH measuring device’s product specifications. The first is “initial accuracy”; the other is one year accuracy. Initial accuracy should factor in all known uncertainties, including:

• calibration uncertainty
• temperature effect and mathematical fit
• hysteresis
• measurement resolution

If you don’t see these variables on a product’s specifications, they may or may not have been included in calculating that device’s accuracy.

ONE YEAR LATER: HOW LONG HAVE YOU BEEN OUT OF SPEC?
One year accuracy is the accuracy of the device after a year of normal use, one year being the typical interval between calibrations. Although a critical value, a device’s projected accuracy value after exposure to the environment is rarely included on product specifications for humidity measuring instruments. However, this percentage is actually more important than initial accuracy because all data gathered since the last calibration is based solely on its accuracy upon re-calibration.

For example, if your RH measurement device is found out-of-spec when you go to re-calibrate, you will be faced with some hard questions. What products or tests were affected and to what extent?