Relative humidity measurement. Best practices.

[dietert1]

Today i supplemented my ambient monitoring kit with a TI HDC1080 sensor. When it arrived its results were about 8 % high in comparison to the SHT35, e.g. 81 %RH instead of 73 %RH. Datasheet specs are +/- 2 % typical for the SHT35 and +/- 3 % typical for the HDC1080 in this humidity range. In the HDC1080 manual they write that a systematic shift is expected after an extended period of high humidity (above 70 %RH).
Right now i am using the built-in heater with about 4.5 °C temperature increase. As a result the HDC1080 reads less than 70 %RH and is slowly recovering. The first 2 % of the initial 8 % shift is already gone after about one hour.
Appears like in a high humidity ambient the heater should be normally on and turned off only for taking a valid measurement. For the time being this would be my conclusion.

Regards, Dieter

[Anders Petersson]

Some relevant Sensirion documents.

Recommended RH testing conditions (stringent!):
https://sensirion.com/media/documents/17D5076D/6350F402/Sensirion_Humidity_Sensors_Testing_at_Ambient_Conditions.pdf

Using the built-in heater of SHT4x for decontamination:
https://sensirion.com/media/documents/FEE9F039/62459F54/Application_Note_Heater_Decontamination_SHT4xX.pdf

[dietert1]

Thanks. As they propose about 1 hour at 110 °C as temperature for reconditioning, i will try with an external heater for the HDC1080 to accelerate the process a bit, using soldering iron. The risc should be small, as the sensor is monitoring its temperature. Currently it's running at about 70 °C, measuring 20 %RH.

Regards, Dieter

[dietert1]

I think i want to put a stronger heater onto the HDC1080 and operate it at a higher temperature adjusted such that it outputs a relative humidity of 50 %RH. The 50 %RH temperature will be somewhat similar to the dew point except not at 100 %RH. Then a little calculation should give relative humidity at ambient temperature. Maybe that's a good method to use those sensors, at least in a high humidity environment like here.

Regards, Dieter

[CorporateReference]

Hello, this may help with your quest. We calibrate around 60 Fluke 2626 sensors a year (Used with the 1620A).  The 2626-S and -H sensor are exactly the same, they just use tighter calibration points. They both use the Honeywell HIH-4000 Sensor. Fluke does not pre-age these, however we have found after a year they are more stable (The Fluke manual references the initial calibration should be 2x in the first year to support this).  When our contract started, we had some sensors that had not been calibrated for 10 years and they were only out of spec by <5%.  From our experience after soldering, the Sensirion SHT45 and SHT85 require a bit more processing for stability (drying out and re-humidifying) vs the HIH-4000.

For calibration in field, we use a check/reference probe (HC2A-SH calibrated w/ Thunder Scientific PP Chamber) together with saturated salt solutions.  We use Lithium Chloride, Potassium Acetate, Potassium Carbonate, and Sodium Chloride - All high purity ACS/Reagent grade mixed with DI water. Make sure to have more solute in the solution to maintain the equilibrium in case of excess moisture.  As long as you are able to keep them at a fixed temp (We keep and use these in a temperature chamber or dry well calibrator) and allow 30min-1hr time to settle after inserting the probe this method has proven very stable and repeatable within <0.5%RH.

Refer to the NIST article https://nvlpubs.nist.gov/nistpubs/jres/81A/jresv81An1p89_A1b.pdf for your solution of choice.

We use Pyrex 1399 Bottles to keep the salt solutions. During calibration, we use the 195-45HTSC caps with a silicone septa that has had a hole punched through to seal the sensor. For odd sensors, we also have this silicone seal (DWK Kimble Silicone Lid) that can be put over - see photo.

[miro123]

I would like to everyone for valuable feedback.
I already start improving my system. Results are promising. Lessons learned.
 - temperature and humidity probing are the main source of errors - Applying knowledge from my project "sub mK TC oven" was helpful.
 - consumer grade IC are not robust enough e.g humidity creeping and many other side effects
https://sensirion.com/media/documents/A88858C9/629626D4/Application_Note_Creep_Mitigation_SHT4x.pdf
 - trick learned from local meteo administrator - use forced air to measure Temperature and humidity.  It achieve at least two goals 1.dry the humidity sensor polymer. 2. Equalize the ambient air.
 - industrial grade capacite sensors are step up from consumer garde BMExxx/SHTxx, HTH seems to be somewhere in the middle.
 - Ulitimate performes is aschived with dew point/chilled mirror/ hygrometers - proposed from Conrad Hoffman. COTS Michell S4000
 - the heater on consumer sensors solves one problem but introduce another - small heater works only in thermal insulated configuration. Result is wrong temperature measurements and we are back to initial problem again
  - last but not least - frequent calibration and calibration history is a must.
-------------
Some hyperlinks
https://sensirion.com/media/documents/A88858C9/629626D4/Application_Note_Creep_Mitigation_SHT4x.pdf
https://www.knmi.nl/kennis-en-datacentrum/publicatie/knmi-humidity-sensor-test
https://cdn.knmi.nl/system/data_center_publications/files/000/068/576/original/p325_bloeminkms_netherlands.pdf?1495621276
https://www.vaisala.com/en/search?k=&content_group=products%20and%20services&product_type%5B0%5D=2171&measurement%5B0%5D=776
https://www.rotronic.com/en/humidity-measurement-feuchtemessung-temperaturmessungs/humidity-measurement-feuchte-messung/probes-filters
http://www.michell.com/us/products/s4000_rs_and_trs.htm

Best Regards,
Miro

[dietert1]

While waiting for the real thing, i made two test bottles with desiccant bags. In one bottle the desiccant generates about 20 %RH (after drying bags in oven), in the other one there is about 55 %RH. These bottles maintain sensor measurements constant within +/- 0.5 % over several days.
Today ambient has 46 %RH, everything at temperatures of 19°C. The diagram shows how fast the sensors react after going from one bottle to the other and how slow nominal humidity in the second bottle recovers after mixing with ambient air.
Hope it will be faster with liquid chemistry.

Regards, Dieter

[Anders Petersson]

I guess the last seven data points are the steady-state after a long delay.

The sensor response time and the chamber RH recovery time are somewhat sped up by adding a fan. See the fan in my earlier post https://www.eevblog.com/forum/metrology/relative-humidity-measurement-best-practices/msg4670443/#msg4670443
But the recovery time will still take minutes or hours also with chemicals. I don't bother waiting until perfect equilibrium occurs.

I don't think the sensor response time is a property that most people care about. I do however... attachment shows my module with a fast-response RH sensor. The large sensor area gives a 0.3 second response time. By the way, PTB has built a test rig for RH response time.

[dietert1]

I guess the last seven data points are the steady-state after a long delay.
..

Yes, after about 2 hours.
I posted the diagram to show how the steady state of the SHT35 agrees to the 1 minute response of the HDC1080 (samples 32..34). The observed differences may result from the HDC1080 calibration procedure.

Regards, Dieter

[dietert1]

Last week i made a small setup with some I2C sensors currently not in use. Also i got five new SHT45 sensors (the ones i mentioned above). They agree very well to each other. The SHT35 (about 2 years old ) is about 5 % off and matches the HDC1080 results better.
The table is from 6 hours of data logging with the setup inside a box. 1 sample per second, no apparent self heating.
Of course this can't replace a calibration, but it's a preparation. Chemistry will arrive soon.

Regards, Dieter

[CorporateReference]

Airflow/small movements can affect most of these sensors in the fractional percent range, as well as sources of bright light. For more stable readings you can try placing a small amount of foam over the sensor (as Fluke does) or some type of plastic shield/tubing around the sensor such as in Ander's photo. If it will be in a sealed/protected area then it doesn't matter.