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Thread: Wet Bulb Temperature - important knowledge

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    Default Wet Bulb Temperature - important knowledge

    We have threads on the record high temperatures that have gripped much of western North America. And they have included discussion of relative humidity in how uncomfortable a person feels at a given temperature. But I don't think we have touched on the measurement of human survivability.

    To get wet bulb temperature you have to throw ambient temperature, relative humidity, and atmospheric pressure into a complex formula that fortunately has been reduced to plug in calculators available online.

    https://www.easycalculation.com/weat...calculator.php

    What it amounts to is that above a certain wet bulb temperature the human body cannot sweat enough to cool and with continued exposure will succumb to organ failure. Turns out that wet bulb temp is around 95 degrees Fahrenheit (35 Celsius).

    Here is a good article in the Washington Post on the subject. There may be a paywall but the info is widely available on "the google."

    https://www.washingtonpost.com/weath...2fda945a15e117

    It may be more relevant now because the heat dome is predicted to extend eastward to much of the rest of the country this week including to areas with typically high humidity rates.
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    Wet bulb temp. It's relevant in HVAC service as well.

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    Want to know why is called "wett bulb"?
    Knowing why and how will help in understanding.

    I have seen the instrumented. It has two thermometers, you know, the glass tube with the fluid inside (was probably mercury, I saw these in mid 1970s) with the bulb on end with the resivor of the fluid. One thermometer has gauze fabric around the bulb, the other does not. THe gauze fabric is wetted with distilled water. Both thermometers are mounted to a narrow frame and hangs on a short chain and handle. CAn hold by the handle and twirl the entire thing around, thereby creating some air flow past the bulbs (more like the termometers traveling thru the air).
    If the air is dry, there will be more evaporative cooling and will be a large difference between the temp of the "wet bulb" and the "dry bulb". THe wett bulb will be lower temperature. If the air is more humid, there will be less evaportive cooling and will be less of a differnce, the "wet bulb" temp will be closer to the dry bulb temperature. If humdity is close to 100% the wet bulb temp will be about he same or equal to the drybulb temp. Then can look up the dry temp and the wet temp and from that can determine the relative humidity and/or dew point.

    THe thought process in understanding "wet bulb" temp, is about the same idea as understanding "dew point".

    I find it interesting that weather reports tend to report in terms of relative percent when humidity is low (SanDiego was often single digits and people complain if was 12 or 15 instead of 4%), and report in terms of dew point when humidity is high.
    Last edited by Redman; 07-25-2021 at 06:36 PM.

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    Quote Originally Posted by Redman View Post
    I have seen the instrumented.
    I have used the instrument. And your explanation is very good, Dave.

    When we first introduced automated surface mount assembly into our circuit board manufacturing, humidity was an issue.
    During early manufacturing trials, I used a wet bulb thermometer to chart the humidity in our assembly area.

    This would be somewhere in the 80s or 90s....
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    Quote Originally Posted by Redman View Post

    THe thought process in understanding "wet bulb" temp, is about the same idea as understanding "dew point".
    I thought dew point was when the wife got annoyed with my procrastination. heh
    I went to the grocery store and there was a sign at the door. It said no food or drinks inside.
    ........So I went home.

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    Quote Originally Posted by earlfor View Post
    I thought dew point was when the wife got annoyed with my procrastination. heh
    No....That is the "honey do point"...LOL...
    Bob T.

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    What do you do when the wick on the wet bulb freezes and the temperature starts going up on it? It usually happens at a higher temperature than the dry bulb reads.
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    Good stuff, the wet bulb temp reminds me of the wind chill research although very different. It's all about how humidity, temperature and air movement factor together. If humidity is low and your sweat can evaporate freely, you can take a lot more heat. When it is humid and hot you can get in trouble a lot faster. The heat and humidity combined can shut down your built in evaporative cooling system. Yeah you really have one of those. Also think or your dog they cant sweat, they have to pant to get some evaporation. Watch your dogs/pets in the heat, it can easily kill them too.
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    https://advances.sciencemag.org/content/6/19/eaaw1838


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