Author Topic: Sling Psychrometer  (Read 480 times)

Offline klugesmith

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Sling Psychrometer
« on: June 10, 2024, 04:43:59 PM »
Since flea market yesterday, my instrument shelf includes a sling psychrometer. The gold standard for relative humidity since 19th century, I'd never owned or used or even seen one.  What mercury-free substitute do meteorologists and humidity-detector calibrators use nowadays?

Picture incidentally shows a digital multiplier unit, confirmed to be perfectly accurate, and an analog Vacuum Fluorescent Display with a 6U5 tube, as yet untested by me.

I gave the psychrometer a good whirl after wetting its wick.  Indicated bulb temperatures were 70 °F and 59.5 °F;   a traditional psychrometric chart says that matches RH of 55 percent. https://www.engineeringtoolbox.com/docs/documents/816/psychrometric_chart_29inHg.pdf


Each glass thermometer tube has a molded hook at the head end, so it would not go flying if an attachment clip were to slip.

Offline Twospoons

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Re: Sling Psychrometer
« Reply #1 on: June 11, 2024, 12:29:31 AM »
The environmental chambers we have at work still use a wet bulb / dry bulb technique to measure rel. humidity, only they use thermocouples instead of mercury.  Its a bit easier for a PID controller to read a thermocouple.

Offline klugesmith

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Re: Sling Psychrometer
« Reply #2 on: July 04, 2024, 12:31:40 AM »
That's good to know, Twospoons.   Is the wet "bulb" cooling wick always wet?   Any special water treatment or maintenance requirement?

There is a heat wave this week in my part of California.
At 3 pm today, dry bulb temperature was 105 °F  (40.6 °C).
Wet bulb temperature was 72 °F  (22.2 °C), implying relative humidity between 18% and 19%.
That's how much one could cool down outdoors by getting all wet.
High temperature with low humidity invites catastrophic wildfires.

For more cooling, I put some denatured alcohol on the psychrometer wick.   
Got the wet bulb down to 62 °F (16.7 °C) without much effort to remove water first.
This might be helpful in emergency hyperthermia situations, if one avoids sources of ignition. :)
« Last Edit: July 04, 2024, 12:34:28 AM by klugesmith »

Offline Twospoons

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Re: Sling Psychrometer
« Reply #3 on: July 04, 2024, 11:02:32 PM »
Yes the wet bulb is kept wet with deionised water. Using distilled or deionised water is essential for two reasons: it prevents a build up of crud on the wick, and its more accurate since any dissolved salts will affect the vapor pressure of the water and hence its temperature.

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Re: Sling Psychrometer
« Reply #3 on: July 04, 2024, 11:02:32 PM »

 


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