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Dew Sensors & Meassurements

Equipment Dew Heater Dark Site
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Jerry Gerber avatar
Jerry Gerber avatar
I have a 130mm refractor at a dark sky site in the New Mexico desert, about 4200 feet in elevation.   There are indicators for dew point and humidity in several places–in NINA, with the Pegasus Powerbox Advanced 2, on the MGP box and at the dark sky site itself.  None of them concur.  The Pegasus Powerbox shows the humidity as being far higher than NINA, the MGP Box or the site's weather info gives yet even these do not agree, often differing by 10 degrees. 

Which one should I trust?

Thanks,
Jerry
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David Moore avatar
David Moore avatar
Relative humidity and dewpoint are difficult measurements to measure accurately, especially near saturation. The one we used at work in meteorology cost over £1000. They also do not like being kept at saturation. Shading of the instrument to provide the true air temp is also important to give an accurate reading in the daytime but also at night to a lesser degree. You could expect daytime dry air RH and dewpoint reading to differ a lot due to poor shading.  The dark sky site readings ought to be better I would have thought depending on the instrument used and their housings. Presumably you are only interested in nighttime readings when the RH is high?  Do you get fog? If so watch the various RH reading climb as fog forms and see which holds near 100% the best.
There are instruments that measure dew point directly but they cost a lot but they are the best and give low degrees of uncertainty. I am assuming they use electronic sensors. Forgive me if you do not get fog. We do not have deserts here.
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Jerry Gerber avatar
Jerry Gerber avatar
David Moore:
Relative humidity and dewpoint are difficult measurements to measure accurately, especially near saturation. The one we used at work in meteorology cost over £1000. They also do not like being kept at saturation. Shading of the instrument to provide the true air temp is also important to give an accurate reading in the daytime but also at night to a lesser degree. You could expect daytime dry air RH and dewpoint reading to differ a lot due to poor shading.  The dark sky site readings ought to be better I would have thought depending on the instrument used and their housings. Presumably you are only interested in nighttime readings when the RH is high?  Do you get fog? If so watch the various RH reading climb as fog forms and see which holds near 100% the best.
There are instruments that measure dew point directly but they cost a lot but they are the best and give low degrees of uncertainty. I am assuming they use electronic sensors. Forgive me if you do not get fog. We do not have deserts here.

Thanks David for explaining this to me.  I'd better lower my expectations in regard to accuracy in measuring dew levels, as every device/sensor seems to give a different measurement. 

Best,
Jerry
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David Moore avatar
David Moore avatar
There is one instrument that is used as a ref instrument for dew point and that is a mirror dew point  hygrometer that uses a peltier cooled chilled mirror and a beam of light bounced off, it to measure the dew point directly. It stands on a pole and costs about £5000 from memory. It has low uncertainties and is reliable. Not much help though just of interest!
Tony Gondola avatar
Tony Gondola avatar
I lived in southern and northern New Mexico for may years and I can tell you that I never had a issue with dew formation on a refractor with a proper shield. I won't say it can never happen, I've just not experienced it in that environment.
Jerry Gerber avatar
Jerry Gerber avatar
Tony Gondola:
I lived in southern and northern New Mexico for may years and I can tell you that I never had a issue with dew formation on a refractor with a proper shield. I won't say it can never happen, I've just not experienced it in that environment.

That's good to know as my refractor is near Cotton City NM.  I'm probably overly concerned about dew  as this is my first fall/winter at the site and don't know what to expect yet.
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Tony Gondola avatar
Tony Gondola avatar
It looks like your scope is sited in a very dry and hot part of the state, great site, good luck with it, you'll enjoy it!