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December 21 - December 24, 2020

What are the reasons for different dew points from a sling psychrometer and a digital dew point meter at the same site, and what should be done?

Selected Answers

From adrian marius neagu of DAMEN on January 10, 2021:
We always use the Sling Psychrometer, Bacharach 0012-7012 Red Spirit Filled, instead of Digital Dew Point Meters, when temperatures are above 0 degrees Celsius. However, two Bacharach show us different values! Bachelor 1: Ta = 10, Tw = 7, Ts = 11, RH = 65.6%, Dp = 3.8, T? = 7.2 Bachelor 2: Ta = 9, Tw = 6, Ts = 11, RH = 64.3%, Dp = 2.6, T? = 8.4 In this case we call a third Bacharach ... lol!

From Rob Francis of R A Francis Consulting on December 23, 2020:
There is quite a significant uncertainty in relative humidity readings. Even weather bureau RH readings are +/-3% or so, so expect +/-5% or so in readings in the field from the electronic or whirly gauges. This error carries on to dew point which is calculated from RH.

From Tom Swan of M-TEST on December 23, 2020:
I actually presented a paper on this at an SSPC conference about 20 years ago and I would like to thank the 8 people that attended. (First talk in the morning) Back in the old days of Mercury slings there was much greater agreement between the two. Assuming the thermometers are accurate, up to about 85F there is good general agreement between slings and electronic meters. As the temperature increases over 85F slings become very inaccurate and at about 90 to 95 degrees just throw a dart at a dart board with a sling and you will get just as accurate a number. To prove this in hot weather do a sling in a normal manner then repeat the reading holding the sling in front of a fan. You will get two completely numbers. The one in front of the fan is the correct number. You might be able to find the original paper from the Dallas conference in the SSPC archives or you can find it on our website under technical information. It gives all the data. Bottom line - if you have a red spirit thermometer in a sling, usually the electronic meter will be the most accurate. Remember when using either, you must acclimate the meter to the climate. This includes allowing the internal electronics to reach ambient temperature before using. This can take 20 to 30 minutes to get accurate readings.

From Michael Halliwell of Thurber Engineering Ltd. on December 22, 2020:
Both are supposed to do the same thing....a sling psychrometer and hygrometer are after the same info, just one does it with liquid thermometers and one does it with electronics. The electronic version are generally faster and more convenient, but may be less accurate if they aren't stabilized to conditions before starting and the sensor needs to be maintained (can dry out) as well....but of course the psychrometer needs to be properly set up and wet (so they don't work in sub-freezing temps) to get that higher accuracy. If you're not getting the same readings (within a small degree of variation) and it is above freezing, I'd check a few things. Are the liquid thermometers reading the same before being wet? For the digital version...give it a good amount of time to stabilize at the site before making a reading and has the sensor been kept moist per manufacturer's recommendations? Not everything, but good places to start.

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Tagged categories: Asia Pacific; Coating Application; Dew-point; EMEA (Europe, Middle East and Africa); Latin America; North America

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