The Weather

[Gregor 4,846]

I bought this little electronic gadget to monitor the temp and humidity in the garage for painting. I am trying to calibrate it according ti instructions. It has been raing here for 3 hours. In my mind, when it's raining, humidity is 100%. As per weather.com, the humidity here here is 84%. Once again, my mind is mistaken. I don't get it.

[squonk 41,038]

Are you measuring absolute or Realitive humidity? 

 

Absolute humidity is the measure of water vapor (moisture) in the air, regardless of temperature. It is expressed as grams of moisture per cubic meter of air (g/m3).

The maximum absolute humidity of warm air at 30°C/86°F is approximately 30g of water vapor – 30g/m3. The maximum absolute humidity of cold air at 0°C/32°F is approximately 5g of water vapor – 5g/m3.

Relative humidity also measures water vapor but RELATIVE to the temperature of the air. It is expressed as the amount of water vapor in the air as a percentage of the total amount that could be held at its current temperature.

Warm air can hold far more moisture than cold air meaning that the relative humidity of cold air would be far higher than warm air if their absolute humidity levels were equal.

[Gregor 4,846]

[squonk 41,038]

Your device probably reads RH. If it's say 70 deg and raining your meter might read 65%. Now say it's still raining and the temp drop suddenly to 55. Your RH meter may jump to 85% Warm air can hold more moisture before it starts dropping it on what ever is laying around. Non chrome impact sockets for example will rust a bit in a cool shop on a 40 deg day with wet snow outside. When you see fog in the cool AM the air cant hold any more water and it's starting to condense.

 

My former job I had to control temps and humidity in operating rooms. Strict parameters had to be followed and the governing bodies were always changing it. Last I knew it was a low of 20% and a high of 50%. Too much humidity and the sterile envelope is comprimised. To little and the risk of a spark and fire with oxygen in use. OR docs want the rooms cool for the most part. Like 65 or lower because of the equipment they wear and the pressure they work under. 100% outside air is required. Now a typical summer with it's 90 deg temps. Now add in a mid day gully washer T storm. Now all this moisture is getting sucked in to a cool room. The RH would skyrocket. Now to get the room 65 deg I had to run the chiller balls out to wring as much moisture out the air that I could. So I would  to get a discharge temp of like 45 deg and then reheat the air in the AHU to 55 to get the RH in the parameters I needed.  Then a small reheat coil at the room to get my 65 deg room temp and fine tune the RH.

Edited March 23, 2021 by squonk

[Gregor 4,846]

8 minutes ago, squonk said:

Your device probably reads RH. If it's say 70 deg and raining your meter might read 65%. Now say it's still raining and the temp drop suddenly to 55. Your RH meter may jump to 85% Warm air can hold more moisture before it starts dropping it on what ever is laying around. Non chrome impact sockets for example will rust a bit in a cool shop on a 40 deg day with wet snow outside. When you see fog in the cool AM the air cant hold any more water and it's starting to condense.

 

My former job I had to control temps and humidity in operating rooms. Strict parameters had to be followed and the governing bodies were always changing it. Last I knew it was a low of 20% and a high of 50%. Too much humidity and the sterile envelope is comprimised. To little and the risk of a spark and fire with oxygen in use. OR docs want the rooms cool for the most part. Like 65 or lower because of the equipment they wear and the pressure they work under. 100% outside air is required. Now a typical summer with it's 90 deg temps. Now add in a mid day gully washer T storm. Now all this moisture is getting sucked in to a cool room. The RH would skyrocket. Now to get the room 65 deg I had to run the chiller balls out to wring as much moisture out the air that I could. So I would  to get a discharge temp of like 45 deg and then reheat the air in the AHU to 55 to get the RH in the parameters I needed.  Then a small reheat coil at the room to get my 65 deg room temp and fine tune the RH.

I close the door.