Can RH Be >100% In The Return Air Duct??

The spec for a typical RH sensor is only +/- 2% of reading, please note that this is for a limited range say 10% to 90%

So anything outside of that RH say above 90% can not be relied upon. Also many RH sensors can not operate satisfactorily if they are wetted, so your sensor may need drying out and recalibrating... In fact some of the adsorbing types can not tolerate some chemicals.

Either way check the calibration of your instrument at its end points near 90% etc...

John.

Show me a Psycrometric chart that shows you can get more then 100% RH.

More then 100% RH and you get condensation

If you looked at the basics probably you may not have asked this question.

Relative humidity in simple terms is defined as amount of moisture in a given air volume of air divided by amount of moisture in same amount of air volume when air is saturated with moisture.

So by definition RH cannot go above 100%. If RH is 100% and if you coll air further excess moisture will condensate.

Remember lower the temperature you need less moisture to saturate air.

Obviously something is wrong with your meters or readings are out side the limits.

Hello waterlucky,

100% RH is fully saturated, and the air cannot hold any more unless warmed, which then alters the RH, of course....

If it was really 102%, it would be pouring rain inside that return duct, and time to look around for a pump and a boat.

Kind Regards.....

I would concur that your RH unit is out of calibration or out of the range of specified accuracy. Greater than 100% RH is not possible, rather you should see signs of condensation on the unit.

Friends,

I have to disagree with the logical answer given. Supersaturation is a fairly common event. There are two conditions which drive this effect--the lack of a nucleus on which to condense and the speed of temperature change. A supersaturated gaseous solution is unstable and therefore short-lived, so it would be difficult to observe or measure. Therefore an unstable condition of RH being above 100% is quite reasonable.

Beyond the above phenomenon, the basic question in the post was well answered by noting the limits of calibration and accuracy of the sensor involved.

Regards--JMM

Hello JMM,

I do agree with your explanation of Supersaturation, and this is often seen in New Zealand's Southern Alps, where Supersaturated air runs into a snowcapped mountain peak, chills instantly, and the cascade effect means extremely heavy rain, for a short time.

Mount Egmont, all alone away from other hills or mountains, 8000+ foot volcanic peak, in NZ's Taranaki region of the North Island, has also the same effect.

Many climbers at or near the peak of Mt. Egmont, have been caught out, with a clear sky and within a few seconds heavy rain or snow falling - a few have lost their lives, because of this effect - The weather front is not actually seen, until after it has arrived.

I have seen a 45 degree sloped mountainside, in the Southern Alps, running 2+ inches deep with water, and the sheer volume of water falling, made me thank God, that He designed noses the way they are, similar to an umbrella.

At that time, 12 miles further down the valley, the hotel rain gauge overfilled, and the rainfall there was over 12 inches in under 9 minutes.

I have yet to see a return air duct big enough to have clouds forming inside, although there are hangars and NASA assembly buildings in the US, where rain has fallen inside, due to the lowering of the Saturation point....

"I have yet to see a return air duct big enough to have clouds forming inside, although there are hangars and NASA assembly buildings in the US, where rain has fallen inside, due to the lowering of the Saturation point....

"

They have a big enviro chamber hangar in florida. The USAF wanted to do cold weather start tests on an SR-71 there. The Lockheed guys asked if the air conditioning in the building could handle the SR's engines and were assured it could. The idea was that the exhaust would be ducted/sucked out to keep the hangar cold.

So, they did the test... and collapsed the ducting in the hangar. The SR-71's engines blew air out faster than the building ducts could replace it and they got sucked down flat - like when the milk shake gets frozen solid and you cant suck it up. The Egland AFB guys were pretty embarrassed, but they had tested big cargo planes and didn't figure a little measly 150,000GTOW plane could hurt them - it was apparently pretty funny

Actually, I think that you are mistaken, even though "super saturation" events as you describe could possibly exist. RH is abreviation for relative humidity with no definitive answer for actual grains of moisture and is described as the present amount of moisture in the air compared to the MOST it can hold at that temperature ( dewpoint.). 100% is "tops". Any attempt to add more moisture results in the moisture "raining out". Same holds true for lowering air temp which decreases ability to hold water, but the result is the same....100% RH.

An interesting observation is to place a hygrometer / thermometer in a supply air duct during AC operation. Typically you would see an 18° to 22° F drop in discharge air temp but the RH may RISE by 20% to 30% ( in the duct only ). But when it mixes with the room air, the resultant will be lower temp and lower RH after some period of time.

I like to limit duct RH to 90% in hope of never condensing moisture and growing mold.

Sniccus,

I would tend to agree with both yourself and Agua-Doc, that RH by definition ranges between 0 and 100%. Certainly in the design and installation of HVAC ductwork and systems you want the 100% point with its condensation to occur only at the evaporator coils where a drain is readily available. You are correct to note that the %RH is strictly a function of the air temperature so long as the absolute humidity does not change during that time.

However, at the microscopic level two molecules of water do not magically find each other and immediately form a drop when that magic dew point is reached, which we call 100% RH. There is almost always some very small particle of dust or other material in the air which becomes the nucleus around which the water can condense back to a liquid. In highly purified air, where these particles are considerably less common, you can cool the air noticeably below the saturation temperature before even micro-drops (fog) form. And, as I said in my post, if you cool typical outdoor or indoor air quickly there will be a brief time before the molecules will have had enough time to move together and condense. Under either of these conditions (highly purified moist air or rapidly cooled moist air), you briefly have a greater amount of water (by weight) in the vapor phase than would normally saturate the air. Mathematically this would demand that the RH be expressed as a number greater than 100%.

We generally don't look at things from this viewpoint. So your assertion that we "can't" have RH over 100% is understandable even though it is incorrect.

regards--John M.