What's humidity got to do with it?

Only having read a little about climate controlled environments for precise, repeatable gaging, I was wondering if you guys could run down for me the effects of humidity on part and equipment accuracy. Forum search brought up a few posts with exhortations not to forget about it when installing these types of facilities, but no insight as to the mechanism by which it has an effect. Or am I totally off base, and the warnings had more to do with the possible corrosive effect of airborne moisture on sensitive metrology equipment?

Alot of common gauge materials(yes steel,iron, and especially granite) are porous and can absorb or "soak" a certain amount of moisture in.Time is a factor as to saturation. But the most important thing is gradiant. If part of your gauge (gage) ,whatever it may be is hotter than another part or has sponged up more of anything than another part ,that is where warpage and inherent inaccuracies develop.as long as temperature differance or humidity is uniform and you allow enough soak time than distortion from original geometry should be absolutely minimal but you can't allow anything to fluctuate too rapidly.

yzfr1pwr said:

Alot of common gauge materials(yes steel,iron, and especially granite) are porous and can absorb or "soak" a certain amount of moisture in...

Click to expand...

Iron, steel? Porous? Can absorb a certain amount of moisture in????

I'm sorry, I don't normally get quite so blunt but this is absolute nonsense.

On possible reason for controlling humidity in climate controlled environments is that when you cool air, which is going to be the case in many locations during summer months, the relative humidity goes up. If the relative humidity is allowed to get close to 100% then you have the possibility of condensation on anything, anywhere. Keeping the relative humidity down in the 60% region removes any chance of condensation.

A/C = dampness?

I need to unhook that blasted A/C compressor so that it doesn't come on when I run the defroster!

( I don't use it in the summer)


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***What's humidity got to with it?***
Ox

I would do a search on humidity effects on laser interferometers and capacitive sensors if they play a part in your measurement gear (CMM). Just saw some mention, that's all.

humidity affects the speed of light

This might fall in the category of worthless trivia but if you have a CMM that depends on a laser interferometer air temperature, pressure and humidity all affect the speed of light. Under normal conditions the speed of light in air is about 300 ppm slower than that in a vacuum. Over the range of 0-100% humidity the affect is less than 2 ppm. The air density affected by pressure and temperature causes most of the total velocity change. Some laser sysems have sensors to correct these issues. A properly corrected laser can do 0.1ppm.

Impress you friends with this tidbit.

Humidity has little effect on metrology connected to metals unless its warm enough and humid enough to accelerate corrosion. I've neasured plenty of stuff in very humid environments and found my data reliable when the items were inspected in a climate controlled inspection room. Granite does indeed have some porousity and gain weight slightly. I've heard credible colleagues declare using water based cleaners on their reference surfaces causes a subtle defomation of the plane. Some other kinds of stone will also take up water to some extent but I have no idea how that affects dimeensional stability in a shop setting.

As for permeability of common metals to water and gasses etc I've never seen it. If metals absorbed water to any extent at all it would change the weight. If metals absorbed water how would that effect precision scales capable of weighting items to accuracies and resolutions to six significant figures.

So temperature? Yes. Sweat even small temperature differences. Humidity and moisture not otherwise affecting part and apparatus temperature, no. Humidity is no a factor in metals.

Wood and some plastics, yes but there are tables for that.

Getting back to the original question, metrology rooms are climate controlled.

What this means is, the temperature is held at a constant value all the time.

During the winter the air is heated, and during the summer the air must be cooled.
The air in question is not only the air *in* the room, but air that is brought in,
either deliberately through a ventillation system, or accidentally through the
opening and closing of doors.

The point being, during the time that cooling is in effect, the air brought in
is cooled, and moisture in that air (unavoidably present) can drop out of
the air if the incoming air is brought below its dew point. This causes
condensation on colder surfaces, and in some instances (and yes I have
personally seen this) it can actually rain inside the lab.

So any discussion of climate control for a metrology lab must invariably
include a discussion of humidity control.

One common way to do this, is to bring in outside air, and cool it well below the
dew point in the air handling system. Moisture then drops out as condensate
and the very cold air then gets 're-heat' to bring it to the desired temperature.

The flip side during the heating season is, cold dry air is brought in as make-up
air, and then is heated to bring it to correct temperature. This drives the
relative humidity of the air down to about five percent or so. At this point
humidty tanks are used to add moisture in.

The sytems in question are run closed loop, with temperature and humidty sensors
on the outgoing air to maintain the parameters in the acceptable range.

In the case where it actually rained inside a laser lab, the engineer who designed
the system made the humidification part three times too large, and the re-heat
coils half the size they should have been.

The maintenence technician at work who was called in to fix the screwup figured it
out in about a day, disconnected one of the humidifier tanks and set the setpoint
on the other one about half full. Then he added another re-heat coil of a slightly
larger size. After that, and a bit of tuning on the PID parameters for the control
circuits, no more rain!

Jim

You'd typically spec a metrology room to hold 68 +/-XX NON CONDENSING. Using an undersized system might produce the 68* but with 90 to 100% relative humidity. A good example of this is a house or commercial building with an undersized system. It doesn't feel hot, but it feels sorta clammy.

To control the humidity you have to run the cooling coil at some temperature substantially less than the space temperature. Chilled water systems typically hold the water temp at 45*F and the discharge air temperature coming off the cooling coil will be in the 50* range. At 50 the air is saturated, but as it warms by mixing with air in the space its capacity to hold moisture increases and its relative humidity decreases. Direct expansion type cooling coils run in the 40* range and the discharge air is still around 50* for normal applications.

Obviously you can't just keep driving the temp down to control humidity, so its normal to have either reheat capability downstream or have the ability to mix warm and cold air in either a hot deck / cold deck air handler or in mixing boxes that are fed with both hot and cold air. Either of these modulate to provide more cool air and less warm as the temp moves above the setpoint, and further modulate to maintain a balance of hot/cold that maintains the space conditions when the temp is at the setpoint.

We had a couple large package heat pumps on the shop at Michelin. The units had an economizer on them, which is a damper setup that will pump outside air into the building for cooling purposes if the outdoor temp is cool enough. The cooling would run all day and then at sometime near dark the outdoor temp would drop enough to kick in the economizer. Problem was, the outdoor air was near saturation as it typically is about the time things begin to cool down in the evening. Moisture would condense on the surface plate in particular, and if anything was left sitting on it we'd have to bump it before we could move it due to rust starting to form. Supposedly an economizer has an enthalpy based control which is reading both temperature and humidity, but we found the ones on our units were evidently illiterate The only solution we found that really worked reliably was to disable the economizers and let the mechanical cooling run as necessary.

Note to OX... The a/c compressor runs with the defroster to do the same thing as I described above. The cooling coil knocks the water out of the air and then the heater core heats it back up so you're blowing warm dry air onto the windshield. If its real cold outside the humidity will be low enough that just heating the air is a plenty, but if its about 50 to 60 outside and raining and dew wants to form on the inside of the windshield, the defroster will have a hard time getting the dew to evaporate without the a/c being able to run. I seldom turn the cooling on in my truck in the summer, but when the low pressure switch went bad and locked out the compressor I had to replace it to allow the compressor to run because the windshield would fog up and the defroster wouldn't do a thing but make it worse.

Yep, in the lab I was discussing the glycol unit would
provide the cold air via heat exchanger, and there was
electric re-heat to bring it back to temperature once
the water dropped out and went down the condensate drain.

In my pickup without AC, it was possible to defog the
windshield by running the defrost on full heat, with the
windows cracked open a bit to let the heat out of the
cab a bit. But as you say, cool moist rainy days, those
were times when the entire system was running close to
the ragged edge.

Hard to imagine there was a time when cars simply never
had AC. And the problem of fogged windows, was always
possible, in any of them. I consider defrost, defog,
winshield wipers to be critical safety items, and won't
drive a car if they're not working.

At one time my truck had the thermostat fail open. During
the winter this was a bit problem before I realized it.
I recall driving home from work during a bad sleet storm,
and I the wipers began to ice up. I had to peridically
reach out and flip the wiper so it would knock the ice
loose. Until, at one point, I fliped it too hard, and
the wiper tore off the arm.

Fortunately we were quite near home when that happened.

I commented to my buddy at work, the temperature gage
was not reading right on the truck, even after having
replaced the sensor. He asked "hows the heat been on
it, lately....?"

Ah. Lightbulb.

Jim