Humidity measurement: what you don’t know could cost you
Understanding the inner workings of relative humidity measuring devices and how their product specifications can be misrepresented in product datasheets can mean the difference between purchasing an expensive system with inherent measurement flaws and incremental costs, and getting a system you can simply deploy and depend upon between calibrations. Knowing what to look for in product specifications can also initiate incisive questioning of manufacturers about the effectiveness of their humidity measuring systems. Basic knowledge of how these devices function will prove that often, critical information that is not provided by a manufacturer can be more revealing than what is.
One of the hardest parameters to accurately measure, relative humidity 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’re “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 (Initial vs. One Year Later)
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, the device’s accuracy when first deployed, should factor in all known uncertainties, including:
• Calibration Uncertainty
• Temperature Effect & Mathematical Fit
• 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?
Seek and ye shall find
You may be able to find specs on the accuracy of an RH measuring device after a year of typical use and over a wide temperature range, but first you’ll have to know what to look for and second, look at a lot of product spec sheets. As a last ditch effort, you may have to just ask the sales representative. However, this comes with a proviso; the manufacturer should provide documentation that shows the accuracy values of their devices at the end of the calibration cycle, before re-calibrating.
The question is: why is the inclusion of these values on product specifications so rare within the industry? To answer, it’s vital to understand the three main elements that determine sensor accuracy:
• Sensor characteristics
• Sensor Measurement System (Electronics)
A device may have the best RH sensor available; however, as already stated, all RH sensors drift. To maximize overall accuracy, it is crucial to reduce errors that occur during the calibration process and within the Sensor Measurement System. These elements, well controlled, will create a bit of room for the device to drift.
In other words, to anticipate the drift of a device, you must achieve optimal accuracy in the calibration and the Sensor Measurement System. In effect, you need to reduce or virtually eliminate all other sources of error in the manufacture and maintenance of the device.
Other Sources of Error:
All humidity calibration chambers have an associated uncertainty, a major source of which is temperature non-uniformity, which must be factored into a measuring device’s accuracy specification.
Before humidity calibration, manufacturers of humidity data recorders must perform a high-accuracy temperature calibration. Each recorder’s measured temperature is then able to compensate for chamber non-uniformity during RH calibration — greatly reducing this source of error.
Inside some of the best available data recorders, the temperature sensor is placed right beside the RH sensor. This proximity allows both sensors to read the same environment, eliminating discrepancies between their measurements.
Temperature Effect & Mathematical Fit
Most RH measuring devices are calibrated to measure at one specific temperature (typically 25ºC). But, unless the device will only be used to measure humidity at that temperature, there can be significant temperature-related inaccuracies.
To solve this, a manufacturer could include tables that correlate humidity measurement over a wide range of calibrated temperatures in the memory of the device. Ideally, no two data recorders have the same set of tables because each set is calibrated to the unique components of every recorder.
This creates an “intelligent” device, because the tables contain explicit information on how to measure humidity over a wide temperature range. This is particularly important in the case of ICH (stability) applications.
Hysteresis is the tendency of measuring devices to not return completely to their original state after a change has been measured. It’s also a major source of error. Unfortunately, despite its ubiquity, too few data sheets include hysteresis as a factor in their accuracy values. If it appears at all, it’s often de-emphasized by being placed far apart from the total accuracy specification. Hysteresis unmentioned or disconnected from an accuracy value should be considered product data misrepresentation.
Resolution is simply the smallest measurable increment that the device can detect. A good device will feature a 12-bit high-resolution system that detects changes of as small as 0.05%RH.
A significant element that affects a device’s accuracy is its electronic components. Electronics systems are greatly impacted by temperature, which in turn affects overall accuracy. One challenge that manufacturers face is trying to get the electronic system to remain stable over wide temperature ranges.
For example, Veriteq Instruments found that a synchronous bridge measurement system features low power and superior stability. (1) This unique combination greatly reduced the electronics associated error in humidity monitoring devices.
Product specifications, often one of the key pieces of information decision makers use to select a suitable system, must be explicit, easy-to-understand, and straightforward. All of the known influences and sources of error — calibration uncertainty, temperature effect, measurement resolution, and hysteresis — should be included in the accuracy value stated on any data sheet.
If these values are not mentioned on a product data sheet, the consumer is left to ask: have they been included in that product’s stated accuracy? Until consumers are better informed on other factors that contribute to inaccuracy in humidity measuring devices, manufacturers confronted with their own out-of-spec devices upon re-calibration, can always blame drift.
1 For an in-depth description of this system, see “Methods of Accurately Measuring Capacitive RH Sensors” at:
Author: Kevin Bull, CEO, Veriteq Instruments, Inc. 13775 Commerce Parkway, Richmond, BC, V6V 2V4 1-800-683-8374. For further information, please contact firstname.lastname@example.org.
Veriteq-2000-Series-Datasheet.pdf (157.5 KB)