Using the dewpoint to precisely calculate Relative Humidity

[Mario Proulx]

Knowing, and controlling, the relative humidity(RH) of our workshops is imperative, and all mechanical and digital hygrometers must be checked against a "known" RH often, as all hygrometers are prone to drift out of calibration. Measuring RH requires a very delicate device, because we're measuring a gas. Small amounts of dust can cause even the finest hygrometers to read incorrectly, for example, but even if kept spotlessly clean, their delicate nature is such that they must be checked often if we are to completely trust them.

In a recent thread, I asked that we brainstorm together to find a simpler, more foolproof and ultimately precise and repeatable method of determining the precise RH, so that we can then check and re-calibrate our hygrometers correctly, and quickly. Until now, I, like many others, relied on the dry bulb/wet bulb measurement system, but it always left me wanting. What we came up with was a method that we later discovered already existed! <lol> But it works, works well, and is as precise and reliable as anything I have ever seen, so here is the method, and how I go about in conducting the test. All in all, it takes me roughly 5 minutes.

[Mario Proulx]

We will seek to find the true dewpoint in order to calculate the precise RH from that figure. In order to find the true dewpoint, I tried various containers, and ultimately decided that either a plain, -unlined- tin can(soup can here) or a thin-walled glass works perfectly. We want a vessel that has very little insulation(thus, no lining in the can, or thick glass). In this vessel, we will add cold water, and some ice cubes. 3 or 4 cubes works for me, but if your water is warmer, you may need more. use as many as necessary. We will also require a quality, reliable thermometer. Mine is a lab thermometer, but any good one will suffice, though the better, longer, easier to read it is, the better and more precise your findings will be.

In the following photos, we will see the thermometer in a soup can with the ice cubes having been just added. In between that photo and the next one showing the thermometer as around 6(°C), I stirred the water and ice cubes constantly, slowly, using the thermometer, until a fine dew(appears in the form of a light "fog") formed on the surface of the can. I stopped stirring, read the thermometer, jotted-down the reading, and took the photo. The next photo shows the same can, but after taking a "swipe" of the surface to be sure what I was seeing was in fact the dew(it was). The dewpoint in this example was 6.2(°C), and the air temperature, read from the same thermometer before placing it in the can, was 19.2(°C), which calculates to 42.67%RH, but we can round that to 43%.

[Mario Proulx]

In this sequence of photos, the same test was performed using a tall, thin-walled glass. The results were exactly the same, but the exact moment the dew forms is much easier to see on the glass than on the tin can, and for that reason, I prefer the glass, and would suggest that as our preferred vessel, though if all you have is a soup can, fear not! The results are just as precise.

In the first photo, we have the glass, ice cubes and thermometer. Again, I stirred constantly(roughly 60-90 seconds) until the dew formed, then read the thermometer and wrote-down the figure. In the final photo is the thermometer having warmed back up to the air temperature. In this case, I wrote-down the temperature at the start of the tests and at the end(seen here). The difference was one-tenth of a degree, Celsius, which is not enough tom worry about.

[Mario Proulx]

In closing, I'll add that it is very important to have the vessel, be it glass or tin, very clean on the exterior, as any oils or dirt will slow the formation of the dew.

To find your relative humidity from the dewpoint and air temperature, we can search the internet for a chart or table, or find a handy on-line "calculator", such as this one:

http://andrew.rsmas.miami.edu/bmcnoldy/Humidity.html

And of course, we can learn to do all the math and crunch the numbers ourselves...

Myself, I plan on using this test to check my hygrometers twice, maybe 3 times per year. I don't consider taking 5 minutes a few times per year as too much of a bother, especially in light of what working in the improper RH environment can lead to. For my day-day readings, I rely on 3 inexpensive($10-20 each) mechanical hygrometers that are hung on a wall in a tight cluster. I have 3 of them for a good reason; when they are all freshly calibrated and reading well, they all read exactly the same, all the time. As long as they all read within 1 or 2 percent of each other, I know that all is well and that I can trust the reading. But inevitably, the day comes when they begin to no long agree with one another, and this is where having 3 of them comes into play. With 3, I can be pretty certain that the one that has suddenly begun reading differently is the one at fault, such that if I don't have the 5 minutes to perform the calibration test, I can rely on the other two's readings for the time being. If I only had 2 of them, I would have to stop then and there and test and re-calibrate both of them. If I had only one, I would never know for sure if it was reading correctly, at all!

Yes, I do believe that building in the proper environment of 42 to 47%RH is very important. Important enough that I consider the hygrometer as the most important tool in the workshop.

[Miguel Bernardo]

Thanks a lot for the debunked primer Mario! i´m gonna put it to work here (some big humidity fluctuations where i live) - i was keen on buying a sling psychrometer (or even a psycrho-dyne), but will try this first. oh, the joy of simple things!

[Dave Stewart]

Thanks for your perseverance on this Mario, and the resulting straightforward procedure. All that remains is to track down a suitably long thermometer.

[Mario Proulx]

There are many on-line science and/or lab supply places that carry them in various forms and quality levels, for very reasonable prices. Pool and spa places should also have nice thermometers. It really doesn't need to be as precise as I've made it out to be, but the precision does help in assuring you that your results are as precise as possible. In the end, the somewhat vagueness of the dry bulb/wet bulb test results were what frustrated me, and lead to this method.

Also, from the moment that the dew forms along the full length, you have plenty of time to read the thermometer. It's not like the ice is cooling the water rapidly. In other words, it's not a race! <g> Just stir the water slowly, DO NOT breath on the glass/tin, and when the dew forms, wipe the glass and read the thermometer. As an added bonus, the glass and water magnifies the thermometer, making it easier to read....

[Darryl Young]

In case you want a good bang for the buck digital thermometer, here are a couple of options. Most digital thermometers take a long time to settle at a new temperature but these will go from ambient to 0C (32F) within 5 or 6 seconds and they are accurate to 0.5C (0.9F) within the range -10C-100C (14F-212F). These are made for the food industry so have the added benefit of a wider temp range and an IP65 rating so hold up in to extreme conditions (water won't hurt them, these are awesome meat thermometer). The total temperature range is -40C-150C (-40F-302F) but you loose a little accuracy when operated outside the -10C-100C window (no impact for relative humidity calculations in the shop/home). Also, they have a min/max function if you want to test your freezer/smoker and they have a long battery life. Best thing is these digital thermometers are only $19 (even cheaper if you buy 2 or more).

Here are 2 choices with a slightly different design. (the first one is the one I own):

http://thermoworks.com/products/low_cost/rt600c.html

http://thermoworks.com/products/low_cost/rt301wa.html

[Darryl Young]

Oh, thanks for this reminder Mario! I knew this could be done as I learned about it back in school.......but I had totally forgotten this and never thought of it for a simpler method to calculate RH. Thanks again.

[Ron Belanger]

Thanks Mario,
I used this method the other day and it works great.

[Darryl Young]

If anyone is interested, I made an Excel spreadsheet where you type in the 2 temperature readings and it calculates the RH.

[Bob Menzel]

As an added bonus, the glass and water magnifies the thermometer, making it easier to read....

...and you can quench your thirst when you're done.

[Bob Matthews]

Thank you Mario for doing all the legwork and sharing this very important info with us all, much appreciated.