Saturday, March 2, 2013

Household Dashboard Instrumentation

[Every so often, I like to tackle some little project that really, truly needn't be tackled. This post sheds some light on why that is.]

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The other day, it came to mind that I have this gorgeous old Smiths oil temperature gauge that's been sitting idle in a box for decades. Here's a view of it.

It's an item I acquired in my adolescence that I never got around to installing in a vehicle. (As for why I ever thought I needed an oil temperature gauge, let's just say that it seemed like a good idea at the time, and leave it at that.)

The gauge is a proper Bourdon tube type with a sensing bulb and capillary tube. I could try to sell it -- it seems these classic gauges are worth a few bucks -- but I'd rather enjoy the use of the thing. I could install it in my truck, but I really don't feel like doing that. So, what to do with it? How about installing it as a furnace hot air plenum temperature gauge? I think that would be a fine thing for a house to have.

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Things I'll Need

  • A mounting panel for the gauge.
  • A flange to fit the sensing bulb to the sheet metal wall of the plenum.
  • A power source for the gauge's light bulb.
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The Mounting Panel

I have a piece of brushed aluminum salvaged from an old impact printer that will serve nicely for panel material. Let's see what I can do with this.

This part is a pretty straightforward thing to fabricate. I should point out something about the use of a fly cutter, though. Here's a view of the setup I used for cutting the 2 1/32" diameter gauge hole.

Whatever it takes, the work must be absolutely, positively secured. Your hands are not to go anywhere near the work while the fly cutter is in operation. Use your machine's lowest spindle speed. Slow and easy feed of the cutter gets the job done.

Here's the panel off the drill press and trial-fitted to the gauge.

Fly cutters are a devil of a thing to adjust for an exact hole size, and right off the drill press the hole was 1/64" undersize. I had to enlarge it with a 1 1/2" diameter sanding drum with a coarse sleeve on it. For a critical job that has to come out exactly right, one would be wise to do a trial cut on scrap material.

Note that I did the layout marking for the panel's semicircular lower perimeter prior to cutting the hole. The centre-point for the hole is also the centre-point for the perimeter, so the complete layout had to be done prior to obliterating the centre-point. That sort of work-sequence consideration arises often. It pays to be mindful of it.

Here's the finished panel, all shaped and deburred and polished and ready for installation.

I did the shaping on the new belt sander/grinder I acquired recently. I'm quite pleased with the machine. This little shaping job was a breeze with it.

Next up is the flange for installing the sensing bulb in the furnace plenum. That will be a little more challenging than the mounting panel was.

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The Flange w/Pipe Fitting -- SUNDAY, MARCH 3, 2013

Here it is.

At the left is one if the 2" square sill-plate washers I started with. I added the four 3/16" mounting fastener holes, and bored out the 9/16" bore to 5/8" to accept the pipe fitting that accepts the gauge's sensing bulb, then soldered[1] it together. I've cut a roughly 3/4" diameter hole in the wall of the plenum with a fly cutter.[2] That'll do for this weekend.

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Installed -- SATURDAY, MARCH 9, 2013

Here's the gauge attached to the underside of the main hot air duct, in a spot where I can see it easily, but it's out of harm's way.

And here's the sensing bulb end of the capillary tube mounted in its pipe-fitting flange. (I thought the flange deserved a paint job, so I gave it one.)

All that's left is to provide power for the gauge's light bulb.

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Gauge Illumination Power -- SUNDAY, MARCH 17, 2013

12.6 volt filament transformers from vacuum tube days are a near-ideal power source for 12 volt automotive light bulbs, and I happen to have one on hand that's suitable for this. Here it is fully installed and operational.

The transformer's primary winding line cord is plugged into a switch-controlled outlet, so the transformer is only energized when I need/want it to be.

One end of the transformer's secondary winding is tied to the same air duct's sheet metal (i.e. earth ground) that the gauge's frame is attached to, so this gauge illumination installation is no different from that in a vehicle -- only one discrete electrical conductor is needed to feed the gauge's light bulb, like so.[3]

The lighting effect is adequate, but dim and unspectacular -- it doesn't photograph well.

As the furnace runs, the gauge's reading peaks at about 40 degrees Celsius (104 degrees Fahrenheit). It's not great utilization of the gauge's scale, but it's better than having the gauge sit idly taking up space in a box, and it does give me some information.

I suppose that a rising peak plenum temperature would be indicative of the filters becoming clogged and diminishing the air-stream.

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Now, where to from here?

A plenum air-velocity gauge comes to mind. A plenum air relative-humidity gauge would be nice. A plenum air particulate matter counter wouldn't hurt. (Did I mention that I really like instrumentation?)

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[1] Mild steel solders quite readily with tin/lead solder and ordinary paste flux, but the steel must be absolutely, positively free of oxides. In preparation for soldering the pipe fitting to the sill-plate washer, I sanded the washer with medium aluminum oxide paper on an orbital sander. That was the one sure way I could be certain of getting down to bright, clean steel that would accept tin/lead solder. Steel-wooling and/or wire-brushing would likely have been inadequate.

[2] The same fly cutter I used for the 2 1/32" diameter hole in the mounting panel. Using a fly cutter with a portable drill is not a good practice, but this was a case where it was a reasonably safe and sure thing to do. The work (the plenum's wall) was absolutely immobile. The material was thin. The fly cutter was sharp. As with the larger diameter hole done on the drill press, slow rotation speed and a gentle, easy rate of feed get the job done.

[3] This is the great thing about a DeVry Institute of Technology education in Electronics Engineering Technology -- you know how to wire up stuff like dashboard gauge lighting in unorthodox applications.

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