How do I use the RHCalc online calculator?

  Added by: ranger2000  Last edited by: ranger2000  Viewed: 567 times  Rated by 11 users: 10.00/10
RHcalc - A Tool for Helping with Humidity Issues

Contributed by: Ranger2000

What is it?
The Relative Humidity Calculator (RHCalc) is a web page the uses javascript to let you make relative humidity (RH) calculations. It can be used to diagnose several humidity related problems. You can open up a copy of RHCalc by like clicking here.

What is Relative Humidity?
Relative Humidity is a term that tells you how much water is in the air. As air temperature increases, the amount of water it can hold will go up. The hard part about humidity is that an increase in air temperature does not lead to a linear increase in the amount of water it can hold.
Figuring out how much water air can hold at a certain temperature is difficult, which is why we need a calculator. The word 'relative' means "in relation to something else." In this case, 'relative' means "in relation to the most water air at this temperature could hold if totally saturated." To be absolutely accurate, we also need to know the air pressure when determining the amount of water air can hold. It turns out that for air temperature and pressure for which plants and animals are comfortable, the air pressure matters very little. To make life easy, RHCalc assumes 29.921 inches of mercury, the standard air pressure at sea level.

How do I use the RHCalc?
The page is broken up into three columns. The two columns on the left are where you enter your data. The final column is where you find your answers. You can press the 'Calculate' button at any time to find the answers. The simplest way to use RHCalc is to find your initial conditions and enter these in the left column. Next, you can enter a different temperature in the middle column. This is necessary if you are heating (or cooling) the air and want to find the new RH. Otherwise, don't worry about the middle column. The following items describe the various things you can do with RHCalc.

What are the limits on RHCalc?
The javascript code has been set to check values you enter. You are not allowed to enter temperatures below 33 degrees or above 130 degrees Fahrenheit. Centigrade limits are 1 and 54 degrees. You can't enter a humidity value above 100% or below 0%. These values simplify calculations and should allow you have no problems with using RHCalc as it was intended.

How do I use the "Poor Man's psychrometer?"
A psychrometer is a device that can be used to measure RH in the air. Many
people find these difficult to locate in stores or don't have enough money to get one. RHCalc will let you do find some of the answers without investing in the equipment.
Assume you are drawing air from the outside. It is 57 degrees Fahrenheit and has a RH of 60%. You can find these values from any weather report or web page, so the information is free ( I like Now, let's assume your growing area is around 80 degrees. What happens to humidity if you pull the air in and raise the temperature? Using RHCalc, you enter 57 degrees and 60 percent in the first column, 80 degrees in the second column, and press 'Calculate'. The answer is RH in your growing area drops to 27 percent. This is probably a bit low for most horticulturists and you would probably want to raise the RH by adding moisture to the air in your growing space.

How do I convert Fahrenheit to Centigrade and vice versa?
One of the neat tricks you can do with RHCalc is use it to convert from metric to english units. Just enter the desired temperature in one of the temp boxes and click the new units value. All displayed temperatures will now be in the new units, rounded to the nearest integer. Going back and forth several times, you may notice you don't get the same answer you started with. That is because RHCalc always rounds to the closest whole degree on any conversion, so you may gain or loose a small amount in the process.

My exhaust air is going to the cold outside. How can I tell if condensation will form?
Condensation forms when moist air is cooled below the point where the air can still maintain the amount of moisture in gas form. The point at which humidity hits 100% is known as the "Dew point". Any excess moisture will revert back to liquid (or solid) form and you will see condensation or fog.
Again, RHCalc makes finding this point easy. Assume you have exhaust air that is at 85F and with an RH of 60%. Enter these two values in the first column. Since you don't care about changing temperatures, just ignore the second column and click 'Calculate'. You will find that the dew point is 70 degrees. If you are expelling air into an area where the temperature is at or below this value, you will see fog. If you have a wall surface that is at or below this temperature, water droplets will condense on it and you will get runoff. Keeping your exhaust above the dew point keeps it in vapor form and invisible to normal sight.

I have too much humidity, and I don't know where it's coming from. How can I find the source?
This is one of the more advanced things you can do with RHCalc. Assume you have an outside temperature of 62F and 84%. The inside of your house is at 75F and 76%. Your growing area is at 82F and 60%. You want to lower the RH in your growing room, but you don't know how.
The easiest thing to do is determine where it is coming from and control at the source. Using RHCalc, you can see the outside air has 11.78 grams per cubic meter, inside the house you have 16.25 grams per cubic meter, and the growing area has 15.94 grams per cubic meter. Obviously, you got a big humidity boost when the air came in to the house. Do you have an unvented gas stove? Do you have any open water in the living area that can be covered? Find the source of water in the living area and your growroom humidity will come down. Another big hint would have been the condensation you are probably getting around your windows or window frames. RHCalc shows the dew point for air in the living area to be higher than the air outside. As we talked about previously, this is the condition that causes condensation.

How much water should I expect to use if I'm humidifying?
This is another advanced thing that RHCalc can help you figure out. For this, you need to know all the temp/RH values and you need to know the rate at which you exchange air. Let's assume your air source is 73F and 22% RH. Your growing area is 83F and 50% RH. Finally, you are exchanging air at 30 cubic feet per minute. RHCalc tells you that the air source has 4.41 grams of water per cubic meter. Your growing box has 13.28 grams of water per cubic meter. There are about 35 cubic feet per cubic meter, so your fan is exhausting about .86 cubic meters per minute. Water vapor is coming into the area at 3.8 grams per minute (.86 x 4.41 = 3.8) and leaving at 11.4 grams per minute (.86 x 13.28 = 11.4). If you subtract incoming from outgoing (11.4 - 3.8) you find you are adding 7.6 grams per minute. A gram of water is exactly 1 milliliter. A bit more math (7.6 g/minute x 60 minutes/hour) shows you have to add 456 milliliters per hour (about 1/2 liter per hour) of water to your growing area to maintain the desired humidity.

I have a sealed room/house, it is summer, and I have to use AC to cool it. How do I know if I have an air leak in the house and how big it is?
Good question!! This one can save you lots of money on your cooling bill. I'm trying it now, but don't know the answer. This answer is rough and doesn't account for water sources (like open toilets) in the house. If you can, keep toilet lids shut and empty any standing water during the test. Here's what I think you have to do:
  • Figure out an estimate for the amount of cubic meters in the house. We are only talking about the cooled area here. It should be within 10% or better. Use Length x width x height of each room if you have to. Make sure you convert to cubic meters, as that is what we will be using later on.

  • Get a bucket and capture all water coming from the AC outflow. This is condensation and is coming out of the house. If things weren't condensing, there would be no water in the bucket, would there? If you weren't getting new air into the house, the old air would dry to a point it won't condense at. Thus, condensate means you have an air leak or evaporation within the house.

  • Measure how much is coming out in gallons per day, or liters per hour, or whatever. Make a chart and record the amount of water that came out every 4 hours or so. More often is more accurate, but more of a pain. You must also record the Temp/RH pair inside and outside at the time you make the water measurement. You should record the time accurate to the nearest minute.

  • Convert to liters, then milliliters, then grams.

  • For every time period, figure out how much water vapor entered the house during the period. Use the following procedure, and then add the number up to find cubic meters of air per day:

    • Figure out grams/meter^3 (absolute humidity) for inside and outside. Subtract outside minus inside. Assuming you are cooling, if outside is always smaller than inside during the entire test, then you have an evaporating water source in the house/room and it is contaminating the measurement.

    • Figure out cubic meters of air the water difference gives. This is the leakage during your unit of time.

    • Divide cubic meters of air by size of house (in cubic meters). This tells you how many air changes per day you are getting.

These are just some of the things I was able to come up with for using RHCalc. I really did use RHCalc to help someone find the source of mysterious humidity. We were able to determine it came from an unvented propane heater rather than the plant water as he originally thought. I have not actually tested the "how much water to humidify?" application, but I will probably get a chance this winter. I'm sure there are other ways to use RHCalc. If you can think of a question that involves humidity, RHCalc may be able to help you solve it. If you think of other uses or questions, please feel free to contact me (ranger2000) and we can discuss it. If we come up with another use, I'll add your idea and credit it to you.
  Last modified: 15:52 - Aug 21, 2002 

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