Electronics > Metrology

Relative humidity measurement. Best practices.

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I have questions - How can I accurately measure RH? Short list of requirements are listed below.
   - accuracy +-2% -> 10years
   - working temperature range - T=5...50oC
   - storage temperature  T=0...40oC
   - normal indoor condition - no harsh environment and exposure to elements excluded in sensor datasheets.
   - re-calibration period. TBD - How often and how?
   - initial calibration - Do I need it and what technique/ e.g wet bulb, NaCl_75% or reference instrument? test condition https://www.astm.org/e0104-20a.html

Why I asking this questions?
 - I'm busy with building of component models. Voltage and resistors references,ADC and AFE boards. Models relay on accurate data.
 - The data is gathered in real time from local sensors and internet.
  - I'm struggling to get consistent RH% data. I have tried many consumer and DIY sensors/BME280/SHT 1x/2x/3x. Just added HDC1080 and considering experimenting with SHT45 EVM
  - the trends data is ok, but absolute value has StdDev= 7..10%

Best regards,

I can't think of the documents at the moment but from what I have read RH is a bit of a vague thing. It is not an exact science.

The best accuracy is likely from a cooled missor type instrument, measuring the dew point and difference to the room temperature.
The absorbing sensors like BME280, SHT..  have the problem that they can also be effected from other gases in the air. This can be temporary, but also causing drift over time - here even relatively low levels could be an issue. These sensors would need a more frequent re-calibration - especially initially to get an idea of the drift and scattering.

For the calibration a defined salt solution (not sure of NaCl is the best choice, one may want more test points at least for the initial calibration) should be good enough. The cooled mirror type may get away without from getting a zero power temperature reading to get the RT to dew point zero.

Accurate RH only makes sense if the temperature is also uniform and the sensor also gets the right temperature. 2% or the RH should correspond to some 0.3 K in the temperature. Central Europe usually has relatively high RH - no need to worry about single digits.
With a person in the room the RH can actually scatter quite a bit and 10% variations may be real. The problem is with the temperature and humidity source, especially when relative dry.

miro123, would you show some data?
What is your humidity standard? Using extreme points of 0% RH and 100 %RH at different temperatures?
Or are you logging various sensors against each other?
I think one meaningful test would be logging humidity and temperature in a hermetically closed box. At intermediate values of RH there should be a pretty obvious correlation. I have been using SHT35 and SHT45 sensors and they measure both temperature and humidity to calculate relative humidity.
I have seen 10 or 12 % deviations from another sensor at the "dry" extreme. While the other sensor would yield about 10 % an SHT35 stayed at exactly zero with fresh desiccant for several weeks, which isn't probably true either.

Regards, Dieter

There are some publications on that topic:

NPL 1996, A Guide to the Measurement of Humidity

If you want to use a wetbulb-instrument as reference:

German NMI guide to calibrate Hygrometers
PTB 2019, Kalibrierung von Hygrometern zur direkten Erfassung der relativen Feuchte

If you really need the 2% accuracy, i really think you need recalibration cycles of 6mo or less if you intend to use regular humidity sensors. (capacitive hygrometers)
If you use an dewpointmirror as reference instrument with these uncertainties: Tdew: 0.15K Tair:0.08K, the resulting RH uncertainty is at 10%RH ~0.15%RH, at 50%RH ~0,55%RH and at 90%RH ~0.95%RH (assuming Tair = 23°C)

The drift of cheap sensors can be >4%RH in a year at low RH, or they just go down to 20%RH and then stop measuring further down (keep indicating 20%RH, or loose the sensitivity)

maybe look for humidity sensors with higher surface area, they are more stable, but generally, all capacitive humidity sensors need initial calibration.


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