Filling Tanks - Humidity vs Temperature

[UaVaj]

I am in sunny southeast florida.

this week's temperature during (1) the day is ~80 and (2) at night is ~60
this week's humidity during (1) the day 60% is and (2) at night is 90%

For longevity of filter. When is the best time to fill tanks?



My guess - would be during the day (late late afternoon) when humidity is at it's lowest. Can someone (1) confirm this or (2) correct this.

 

[swamp diver]

I think it would be better to run the compressor when the temperature is lowest as the amount of moisture (mixing ratio) the air can carry vs temperature is an exponential one.

The humidity may be 90 percent in the night but that is 90 percent of a significantly lower mass of water. Looking at this graph and using the temps and RH provided it would appear that there would be 10 grams of water per kg of air at 16 C and 90 % RH and about 15 grams of water per kg of air at 27 C at 60% RH.

Hope that helps.

 

[Doc Harry]

If your house is air conditioned, and if your compressor intake is inside of your house, then you can fill your tanks anytime you want. Just takes a saw and a few minutes of your time to make a hole in the wall...

 

[dannobee]

Lower temperature beats lower humidity.

Cold air and low humidity wins, hands down. Air conditioning cools and dries the air.

As air cools, humidity goes up until the moisture comes out of suspension (reaches the dew point).

 

[bleeb]

dannobee touched on the key issue. As far as moisture content goes, temperature and relative humidity are only indirect parameters. The real value of interest is dew point, which is (more or less) the absolute amount of water the air is holding, since that's what your system has to cope with. If the night time relative humidity isn't too high, the dew point can actually remain roughly constant throughout the day, with rh falling in lock step with the rise in temperature and vice versa. However, high humidity at night tends to drive water out of the air in the early hours of the morning, lowering the absolute water content until daytime heating starts to warm up things at ground level. So as swamp diver numbers implied, if there's significant temperature swing, night is often better than day, and morning is generally better evening since it gives time for the effects of the temperature change to take effect. Of course, if the forecast says it's going to rain in the morning, you might want to reconsider, since you're now dealing with air that's no longer 'local'.

On top of this, as several people have commented, air conditioners help even more, assuming the air conditioner is set for a nice frigid temperature, and therefore, low absolute humidity.

 

[h90]

I think the most important point is the temperature of the condensate separator.
Anyway there will be far more than 100 % and it will separate to 100 % at that temperature.
so between 50 and 100 % moisture you will only have more water in the separator but not in the filter.
so go with the lower temperature.

 

[UaVaj]

I think it would be better to run the compressor when the temperature is lowest as the amount of moisture (mixing ratio) the air can carry vs temperature is an exponential one.

The humidity may be 90 percent in the night but that is 90 percent of a significantly lower mass of water. Looking at this graph and using the temps and RH provided it would appear that there would be 10 grams of water per kg of air at 16 C and 90 % RH and about 15 grams of water per kg of air at 27 C at 60% RH.

Hope that helps.

Let me try to understand what you are saying.

Base on your chart.
60 degree (16 C) = 12g / kg
80 degree (27 C) = 25g / kg

90% humidity at 60 degree (night condition) = 11g * 90% = 9.9g / kg
60% humidity at 80 degree (day condition) = 25g * 60% = 15g / kg

The difference is 5 grams of water per kg of air.

 

[Indian Valley Scuba]

That's exactly why we installed both our compressors inside the air conditioned portion of our dive center, to take advantage of the conditioned air no matter when we fill the banks

 

[UaVaj]

What is the formula for figuring out amount of water vapor in air?
I can do the math.

 

[bleeb]

What is the formula for figuring out amount of water vapor in air?
I can do the math.

Most people find it more convenient to read it off a psychrometric chart.
There are online calculators out there. (Apparently the math can be a bit messy.)