How does raising temperature reduce humidity?

It is a  scientific fact that warmer air is able to hold more moisture than cooler air, so warming air increases it’s ability to hold moisture. The table below shows how much water air can hold at a variety of temperatures. This amount of water indicates 100% RH at those temperatures.

Temperature °FMax Water Content g/m³

Look at the massive difference between the top and bottom of the chart. At -13°F the air can’t even hold a single gram of water and yet at 104°F it can hold more than 50 grams. If you look at the difference in capacity between the 68° and 86° points, you can start to see how increasing the temperature will produce the desired results.

Most people  depend on  the principle of rising temperature lowering humidity almost everyday of their lives without even realizing it. Both a clothes dryer and hair dryer work on this principle.

Where does the water go?

If your clothes go into a dryer wet and come out dry, where does the water go? There’s no drain on a dryer. Moisture will always move from an area of high concentration to an area of low concentration until the levels even out. In a dryer, the wet clothes are placed in an environment of warm/hot dry air. The increase in temperature gets the water molecules moving and they rapidly move from the clothes to the air and out the dryer vent. Go outside on a cold day and notice the water vapor that exits the vent. That is where the water is going. Hot, very low humidity air keeps coming into the dryer and drawing the moisture out of the clothers until they are completley dry. The Guardian Rod does exactly the same thing with the cash in an ATM, just in a much more gentle fashion. Warm, low humidity air passes the money and draws the moisture out of it. It rises and exits out the openings in the upper parts of the machine.

Sometimes looking at things from a different perspective helps with understanding. Remember the term is relative humidity, not absolute humidity. 50% RH means the air is holding half the water vapor that it is capable of at a specific temperature.

Imagine we have a 5 gallon bucket. If we put 2.5 gallons of water into it, it would be 50% full. If we removed 2 gallons of the water from the bucket, leaving only 0.5 gallons, the bucket would be 10% full. Removing water from the air is exactly like this and that is how a traditional room dehumidifier works.

Let’s go back to having  all 2.5 gallons of water, but let’s replaced the bucket with a 10 gallon bucket. The new, larger bucket would be 25% full. No water removed, the bucket just got bigger.

That is the principle that makes the Guardian Rod work. By increasing the air’s capicity to hold water we are preventing excess moisture from being absorbed by the cash or contributing to accelerated wear and tear on belts, rollers, and other internal components.


The question that pops up now is:

“This sounds so simple. Does it really work?”

The answer is yes! We used two of our most troublesome ATM’s as guinea pigs to see what would happen. These are machines that routinely have trouble with currency jamming. We have spent untold hundreds of dollars on service calls and parts over the years to keep them up and running. As the days became weeks and the weeks became months and we still had no problems, we started to get excited!

We wanted to get some data on how humidity affects currency, so we subjected a  stack of 1,000 one dollar bills to some extreme humidity levels. We weighed them at regular intervals during the testing and discovered that the cash will absorb about 10% of it’s weight in water. The cash felt significantly different to the touch as well. It was not only sticky, but it swelled up noticably too. In a precision machine like an ATM, it was easy to see how problems could arise.

We have found that raising the temperature inside an ATM by 10 degrees will lower the RH by about 20 percentage points. In most cases this will be the difference between frequent jamming to uninterrupted service.

These photos show the weight of a stack of 1,000 one dollar bills before and after they spent a few days inside a roughly 80% humidity enclosure.

Translate »