Re: air flow sensor on PCB

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>> >>> Given a PC board in a crate, like PCIe or PXI or VME or something, >>> what would be a good way to check air flow across the board, to verify >>> that the box fans and filters are OK? Assume this board can't access >>> the fan tachs or anything like that. >>> >>> I was thinking that I might stick a small thinfilm RTD in the air >>> stream and measure its temperature at two different voltages, to >>> estimate its self-heating, which would vary with air flow. >>> >>> The classic broken light bulb hot-wire anemometer is a nuisance. >>> >>> Carbon comp resistor? >>> >>> This can't be a new problem. Any other suggestions? >>> >> >> Hot wires are quick and simple. As an extra bonus, if you pick the >> right excitation frequency you can get rid of the poorly-controlled >> sensor-to-board thermal conduction effect. >> >> What's such a nuisance about it? >> >> -- Cutting the bulb? >> -- Soldering the leads? >> -- Calibration? >> >> Or something else? > > Breaking a light bulb tends to be erratic; we've done that. And a > nasty production process. It looks ugly too. > > We sure don't want to calibrate an air flow sensor. The flow sense > would be a selling feature, a fan+filter check, so we'd want decent > accuracy as-built.

The issue is always that the sensor measures its own temperature, which is only obliquely connected to what you care about. Copper lead wires tend to make that track the board temperature much more closely than air temperature.

One approach would be to use a couple of SMT barometric pressure sensors, (MS560702BA03-50 is a fave of ours) to measure the pressure drop across the filter. How easy that is to do will depend fairly sensitively on the physical layout, of course.

For a go/no go measurement, fan current is another possibility.

But an AC measurement with a broken tungsten bulb is surprisingly good.

Cheers

Phil Hobbs

This would go on a new ethernet VME crate controller board. Bullet features would include power supply BIST, ambient temp measurement, and air flow measurement; selling point goodies. Maybe some SMB timing test connectors on the front panel too. And LEDs. Glitz.

It wouldn't have access to the fans, and I'd like the features to be small and cheap and low-hassle in production.

It looks like a TO92 transistor, sticking up a ways off the board, will have a nice theta vs air flow curve. Of course lead thermal conduction to the PCB pads reduces that. With software switched power dissipation, we can measure low-power ambient temp and compensate for that, measure true theta.

The EDN article cited above gives me the theta curve. I need to measure the thermal time constant. Any guesses?

Yes, some people still use VME.

I guess one could put a differential pressure sensor on the module, one side inside and one getting air through a hole in the front panel. Measure crate differential pressure?