Humidity 2

Humidity is one of those subjects that most people have a cursory knowledge of, but in fact is far more complex than it appears at first glance.

Relative Humidity Colloquially when we talk about ‘humidity’ we probably mean Relative Humidity (RH). This is a comparison of how much water vapour is in the air compared to how much could theoretically be held by the air.

 

So, for example, at 100% RH, the air is saturated and no more vapour can be held by the air, at that temperature. I added ‘at that temperature’ as humidity and temperature are inexorably linked. As temperature rises, the air can hold more vapour and conversely, as it falls, it can hold less, as seen in this graph.

 

Thus, on a cold morning when we breath out, we see our breath because that air we’ve exhaled is warm (perhaps 30℃) and full of water (perhaps 90% RH). As our breath cools, it can no longer hold as much moisture and tiny water droplets are seen as mist or steam.

 

Dew Point Dew Point tells us at what temperature condensation would start to form. But it is actually a measure of the quantity of water vapor in the air, but it is independent of temperature. In a domestic environment it will tell us the temperature that ones windows will have moisture, and will tend to be within ‘normal’ temperature ranges from -30 ℃ to +30℃ (always less than the ambient). In industry there may be a very wide range of gases that are dryer or wetter and might range from -100 ℃ to +100 ℃. Some years ago, when selling a Dew point transmitter to a customer, I could make no sense of his compressed air having a -60 ℃ range! At least I know better now!

Other terms include Frostpoint, Absolute Humidity g/m3, Mixing Ratio, Wet Bulb temperature and others but this article will become too long if we look at them all!
Homershams are an IANZ accredited calibration lab, and part of our scope includes calibrating RH. It’s one area where many manufacturers specifications have a degree of ‘wiggle room’. It’s not uncommon to see instruments with specifications of 2 or 3% of reading, actually achieving only 5% accuracy.
At Homershams, products that don’t meet our quality standards are rejected. We supply instruments from Center, Delta Ohm and Vaisala. They are all good instruments, but all achieve different levels of accuracy and price.
One instrument I’ve seen, quotes 3% accuracy from 20 to 80 RH, but 5% outside this range. This may be perfectly acceptable for certain uses. Our Center 317 hand-held is a very good basic instrument and has a quoted accuracy of +/-2.5% RH.

Again, it comes down to ‘horses for courses’. If we compare any of these to the more expensive Vaisala handheld HM40, we see that the actual calibration certificate results are ruler-flat. 1.5% quoted, actual, 0%! Here’s an actual portion of a calibration certificate for an HM40




So it becomes clear why there is a price difference.
Here in New Zealand, summer is almost upon us, and in case you’re wondering MBIE recommend 19 to 24 ℃ for office/sedentary work in Summer, and 40 to 70% RH. (16 to 21 ℃ for physical work or big boys like me!)