Wednesday, April 11, 2007

Drying out a Gauge

Sean writes:

I've noticed something quite annoying: Sometimes, on humid or rainy days or nights, when I turn the headlights on and, consequently, the internal gauge lighting, the gauges fog up from the INSIDE, and eventually the warmth of the internal light defogs them, but it takes a good long time. Why do they do this? Humid day at the factory when they sealed the gauges or what??? It's really really annoying. I don't recall the oil temp gauge I had in the '78 doing this.

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Sean,

The fog inside your instruments may not be water vapor. If could be volatile vapor from plastics or paint used in the gauge. If so, it may dissipate in time. But the odds are, it's water vapor. You are going to have to get the water out of your gauge or it will go bad.

You can 'de-mist' a foggy gauge by making a desiccant 'getter', such as cubes of gypsum board, baked in a warm oven for two or three hours then sealed into a balloon or Mason jar, having a brass nipple soldered thru its lid.

To demist the gauge you must make a hole through its case and couple the dry atmosphere from your 'getter' to the moist atmosphere inside the gauge using a bit of tubing. You can promote an exchange of atmosphere between them by using a bit of heat.

I realize there is some hazard associated with making a hole in your gauge but the risk is often less than the damage that will be caused by the moisture. If the body of your gauge is plastic you should be able to melt a tiny hole in the case. If the case is metal, the best method is to punch a hole in the case using a needle you've made for that purpose.

But before you begin making holes in your instruments you need to figure out how you're going to connect the desiccant bottle and how to seal the hole when you're done. The usual method is make a nipple onto the instrument case by attaching a short length of brass tubing to the body of the instrument with one of the commonly available filled-epoxy resins such as J-B Weld or similar. Such nipples would normally be placed on the rear of the instrument.

To melt the hole in the case you pass the heated wire down through the brass nipple you've created. A paper-clip, straightened and secured to the tip of a soldering iron is handy for melting small holes in plastic. The nipple also serves to guide the needle when punching a hole into a metal case.

The instrument is then plumbed to your desiccant 'getter' using vinyl tubing. Obviously all components are selected for their fit. Hobby shops that cater to model airplane builders carry a variety of fine-gauge brass tubing as well as vinyl tubing of matching diameter. A straight piece of hard 'music wire' about 3/32" diameter by three inches long, sharpened on a stone to a fine needle-point, makes a suitable punch for metal-cased instruments. In the latter case the brass tubing for your nipple should have an ID to accommodate your 'needle'.

Once the moisture has been absorbed by the desiccant 'getter', the hole in the instrument case is sealed with wax or an RTV compound.

But the odds are you needn't go through all that trouble. It is rare to find a sealed automotive gauge (which is how the moisture got in there to begin with). Nearly all inexpensive gauges are vented to the atmosphere for the simple reason they would implode at high altitude of they were not. (Expensive gauges are sealed but fitted with a metal or rubber diaphragm that flexes with changes in atmospheric pressure.) When there is already a hole in the instrument case it may be possible to replace its moist atmosphere with a dryer atmosphere by simply blowing 'canned' air into the thing.

On more complex instrument panels the gauges are often plumbed to a desiccant chamber fitted with a bladder to accommodate changes in atmospheric pressure.

Copyright © 1997 Robert S. Hoover
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