Supco Humidity Sensor Calibration

You don't place the sensor in the salt solution, you put the probe in the salt solution. Just rinse it when you are done.

From one of their manuals:

11Plug the sensor probe into 33% salt bottle. Hold down and under normal mode to enter 33% calibration(Fig.17). "CAL" and calibrating value (32.7% if at are blinking on the LCD with current temperature at the left.Meter is now calibrating, and will finish in about 60 minutes when "CAL" and humidity stop blinking. (Fig.18) o25 C) The meter defaults to be calibrated the humidity with 33% and 75% salt solution. The ambient condition is recommended oto be at 25 C and stable humidity(better to be close to the calibrating value). To abort calibration, hold down for more than 1second at any time. RH CALIBRATION %%OOFig. 17%%OOFig. 18After 33% calibration, plug the sensor probe into 75% salt bottle, then press to enter 75% calibration (Fig.19)

>Can I prepare myself the solution with purified NaCl and distilled water?

If you can figure out how to make a saturated solution at a specific % RH, you can, but you won't have a valid calibration certificate certifying the solution.

GA. I was confused by my own answer, when I pasted the info from the manual.

This explains the term, "Plug the probe into the bottle".

I'm a little surprised that the company doesn't include the test bottle with the sensor, along with adequate instructions for calibration.

You might find that 'salt' in this context is a general word. Unless it clearly says so, it probably isn't NaCl.

Horace40,

I didn't want to pay for access to the academic journal article that discussed the various salt solutions, but the abstract that I viewed on-line did mention NaCl as one of the solutions that was used for certain %RH measurements. It also said that the article looked at both saturated solutions as well as non-saturated ones. You are correct that many other salts besides NaCl are used for achieving the various %RH values.

--John M.

Damn time out.

Horace40 Post should end '....for 33%'

Tail end of sentence got clipped and lost in the copy and paste routine.

A known amount of 'salt is dissolved in a known amount of water in an enclosed container with air space.

The probe is exposed to the air in the space above the solution.

As you have been advised, it is only the gas above the saturated solution that is used for humidity measurements. Also, the 75% solution is for NaCl only. To prepare a 33% solution, you will be advised to use MgCl.

Thank you !

Evidently, it turned out my doubt was not without reason !

The use of saturated salts to calibrate humidity sensors is very easy and straight forward. The problem is that we have a lot of good folks, each with good intentions, that are confusing you. Get a pile of sodium chloride and mix it with water to make a slurry. There must be enough sodium chloride to saturate the solution (more than will dissolve in the water). Put this mixture in a bowl. Place your sensor in the space between the solution and a sealed lid. Use a tight fitting plastic lid or saran wrap with an elastic band to create a seal. The air between the cover and the salt slurry, the RH will reach equilibrium(75%RH) in about an hour. No psychrometer is needed because moisture calibration using saturated salt is more accurate than a psychrometer. You could calibrate with a chilled mirror sensor, but it's not necessary. Repeat the same process with magnesium chloride for 33% RH. The air inside your chamber will contain the required water vapor. Just don't get any of the liquid salt solution in contact with your sensor.

Thank you very much ! Have not encountered the subject of RH sensors calibrating till now.