Dear all,

I have been experimenting using a DT85 Series 2 to measure relative humidity using the Wet/Dry bulb method. I have been taking the measurements using a pair of K type thermocouples - one of which is 'wet.'

I have tried using the routine outline in the FAQ through DeTransfer. The problem is that the calculation in the formula is quite different from that using a psychrometric chart of a phone app (both of which roughly agree to within a few percent).

For example: A measurement this morning produced a Dry bulb reading of 19.4 degrees C and a wet bulb reading of 15.0 degrees C.

Using the routine posted on your FAQ, this comes out at RH of 45.1%

The calculations on my phone app came out at 62.6% and our wall chart at 66% (only 3.4% difference between them) which are significantly different from the DeTransfer program on your web site.

Does anyone have a better formula that calculates RH more accurately?

Regards,
Philip

Hi pwarnes,

I will need to recheck the formula in dataTaker FAQ, it does seem give incorrect result. Since the origin of the formula is no longer available, it will take sometimes to make correction.

Meanwhile you have several choices for RH formula which you can use:

1. The old dataTaker DT5xx manual (http://www.thermofisher.com.au/Uploads/file/Environmental-Industrial/Process-Monitoring-Industrial-Instruments/Data-Acquisition/Data-Loggers/DataTaker/UM-0046-A0-DT500-Concise-Users-Manual.PDF) on page 18 explain about RH calculation utilizing polynomial. The result is around 59.98%

2. Australian Bureau Meteorology method: http://www.bom.gov.au/climate/averages/climatology/relhum/calc-rh.pdf.
   Utilize atmospheric pressure in the formula. The result is around 59.45%

3. Calculator City website: http://www.1728.org/relhum.htm. With the result of 62.4%, there is a RH formula in that link.

Best regards,
Rudy Gunawan