I'm so tired....
I have 5 humidity meters in my getto lab. One that is a data logger, two cheapo models used for general home use, a temp/RH meter that is analog the kids used for their snake cage (sorry strech) and an Omega Temp/Humidity meter which I just spend $48.00 on e-bay last week. All for read different readings. One is reading 40% the other 70%. I jump for joy when 2 of the expensive ones read within 10% of each other. They are pretty close to each other.
I have no idea what my RH is.
Other than boiling water in my lab to raise the RH to 90% is there a way to test them?
I was almost thinking of trying them outside and see which is closest to the US weather service.
I've also grappled with the problem of humidity measurement. One thing I recommend, if you don't have it already, is a modern dehumidifier with digital controls and a hydrostat. (I'm not sure if hydrostat is the right word, but it's something you set using the digital controls, say you set it at 60%, and the dehumidifier holds it close to 60%.) These are not expensive, you can probably get a decent one at Wal-Mart for ~$100 to $150. My lab would be useless without one. My lab is almost always too humid, so far I haven't had to add any moisture. In addition to boiling water, you could try an inexpesive electronic humidifier (~$30 to $40). I haven't used them, but maybe they have some automatic control also. Maybe if you use a good dehumidifier in combination with an electronic humidifier, it might keep the RH adequately stable.
As for measurement, I don't know of a good way to get the precise ("correct") RH. I would just use 2 good (identical) meters that are reasonably close to each other (as far as the kind of readings produced are concerned), and use those as your "absolute" readings. Having 2 will guard against one malfunctioning. All you really need is a reliable "relative" figure, the "true" RH isn't necessary I think.
thank god im not the only one having these kinds of problems.
I got a small room where i can raise the humidity quite fast with a steaming waterbath. Even when theres allready massive amounts of transpiration on the walls, my meter just reads 91%.
I think its just not that accurate.
When i go the other way. I want to get the room dry, i just run the dehumidefier on the setting "continuous", it doesnt manage to go beneath 45% anyway.
Its been raining here for days, thats why i got a second dehumidefier.
If i have no dehumidefier on, the humidity is around 70% in my lab. Thats a problem, the dcg's tend to dissapear immediatly after development.
Well, it's a fundamental principle of measurement theory that the more equipment you have to measure a given variable, the more uncertain you are of that measurement. In fact Quantum Mechanics is built on this principle! But that's another story. Basically, it's unlikely that any two pieces of equipment will give you the same reading unless they're in the same position. In terms of relative humidity, air currents in the room will carry moisture around so the relative humidity is constantly changing. The very fact that you're in the room looking at these meters causes air currents, which disturb the measurement (there's that quantum mechanics again!). Joe Farina's solution is probably the best one, tape two together and take them from point to point in the room, taking the average of the two readings. You'll get a map of humidity in the room. Of course, moving them around causes air currents too, so you should move them for a reading then stay still while everything settles down. Also a lot of labs use Joe's other solution, a humidifier.
Having said all this, it's not that important! I have no idea what the relative humidity is in the lab. We have a humidifier but I have no idea where it is. We have a meter that shows humidity and temperature. I dutifully enter these variables in the coating log every time I coat, but never refer to the numbers. The important factor is the control of the variables and a measurement of the result from the controlled variables - not the absolute knowledge of them.
So, you have the temperature and the humidity; you also have the rate of change of temperature and humidity. In a closed room neither of these variables changes very fast (unless you're boiling water), so it's the humidity and temperature when you coat that's an important parameter. If you suspect temperature and humidity as a problem in your coating, get three boxes A, B and C such that one, say A, can be contained in the another, say B. Also get two dessicant packs. Put some substance at a known temperature in box A - say water or ice, put your plates in box B along with a dessicant and put box B inside A. Now the plates in box B are at a fixed, low humidity and a known temperature. Put some plates in box C with just dessicant. The plates in C are now at a fixed low humidity, but are prone to whatever temperature variations are occurring locally. Leave some plates in the open air with no control. These plates are prone to both temperature and humidity variations. Now shoot one plate from each batch in exactly the same way. If there is no difference, then humidity and temperature are not your problem. If there is a difference, note the difference and the environment causing the difference. Take another plate from each box, shoot it and develop it in a slightly different way. Eventually, after maybe five plates - or 15, one from each environment - or so, you should be able to narrow down the culprit.
It's hard to de-humidify rooms without laboratory conditions i think. In a normal house the rooms aren't really airtight and as you have to open doors to get in and out you always exchange (humid) air with other rooms. You can't do much more than trying to not shower or boil water in that room. Generally speaking the hotter the room temperature (Thus the temperature of plates/objects)the less likely it is for moisture to condensate on plates or other objects where you don't want it. You can also catch some humidity out of the air by having larger amounts of drying agents in the room like salt or dry rice, but unless your room is a dedicated laboratory you might maybe not lay out plates with salt or rice everywhere or the wife and kids will complain when they step into it.
Wow i thought now something like the "Weather Rock" ( http://en.wikipedia.org/wiki/Weather_Rock ) would come up, but the dry bulb/ wet bulb method seems to be super easy and effective. Didn't know that one, thanks for sharing the knowledge, Ed!
