Expert: James Yee - 2/1/2008

Question
Mr. Yee:

Recently, I purchased a Csonka digital hygrometer.  The company claims that the hygrometer is accurate to +1/-1% relative humidity.  Are you familiar with these hygrometers?  

I performed the salt test because I wanted to ensure accuracy.  I mixed a tablespoon of salt with only enough water to make a paste.  Does the amount of salt in relation to the amount of water have any real effect on the test?  

I placed the salt mixture along with the hygrometer in a glass mason jar and left it for over 24 hours.  The temperature was around 70 degrees for most of the test.  The highest humidity reading was 70%.  From what I have read about the salt test, the test is suppose to provide an accurate 75% reading.  

Am I doing something wrong or do I have an innacurate digital hygrometer?  Do you have any other suggestions?  The reason I bought this hygrometer was for the accuracy that the company claimed and now I am doubting that claim.  

Thanks in advance,
Jerry

Answer
Hi Jerry,

To answer your questions, I know of Csonka hygrometers and it's a reputable brand but I've never used them before.  First, check any documentation that came with your hygrometer for calibration and accuracy issues.  Many digital hygrometers are factory programmed and therefore do not need calibration but there are some digital hygrometers that instruct the user to calibrate it themselves to ensure true accuracy.  If your Csonka hygrometer requires you to calibrate it yourself, simply adjust the humidity reading to 75% after the salt test.  If it's displaying 70% after the test, then it's 5% out of sync so adjust it to 75%.

The salt test procedure you did sounds correct but instead of a mason jar, try the salt test again but use a Ziploc bag instead of a mason jar.  Sometimes the salt test can be inaccurate when the hygrometer is spaced too close to the salt paste and placed in a narrow but vertically tall container such as a jar or tupperware container.  Digital hygrometers have super sensitive components so be sure to space the salt paste and hygrometer at least 3-4 inches apart while in the bag and do not let any of the salt touch the hygrometer at all.  

Some digital hygrometer manufacturers even advise that digital hygrometers should not be subjected to a salt test because of the sensitive components so as an alternative, if you have an analog hygrometer, calibrate that one with the salt test and then leave it in a fully-charged humidor with the digital hygrometer for 24 hours.  Check on the hygrometers after 24 hours and if the digital hygrometer's reading matches the analog hygrometer, then it doesn't need to be calibrated.  If the digital hygrometer reading doesn't match the analog reading, calibrate accordingly.

If all else fails, contact the manufacturer and they might be able to provide more help.  If the hygrometer is still inaccurate at this point, then it may most likely be defective but it's always good to contact the manufacturer first.

Hope this answers your question.

Regards,
James