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Electronic control for RV positive air pressure system.. Answered

I am biulding a positive air pressure system for my RV. I would like to include an electronic control system that will allow equal pressure inside and out or positive oressure inside. This will NOT allow any dust to enter the vehicle.

I was thinking using an altimeter/pressure sensor for measuring internal and external air pressures. My concern is that the outside air flow while travelling will create a negative pressure reading.

I also require circuit diagram for sensors cuopled to a 12v air blower.

I look forward to comments and ideas.

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Jack A Lopez
Jack A Lopez

21 days ago

I think the cheapest way to measure the pressure difference between the inside of a big box (like a RV, recreational vehicle) and the outside of that box, is just a length of plastic tubing with some liquid in it.

I know you are excited about these fancy-nancy electronic pressure transducers, but I think it would be instructive, as a first step, to actually try this with an old-fashioned, water-filled manometer.

I used to think it was spelled, "monometer," which means "one-meter," which is confusing because this instrument measures the difference between two pressure sources. But it turns out the spelling is actually "manometer." Man-o-meter? It is somewhat manly, I suppose. Certainly plumbing is a manly art. Most of the time. In any case, a manometer is a U-shaped tube with liquid in it, and it has two ends, and it measures the difference in pressure between those two ends.

Actually, I had to build one recently, because a friend of mine had a leak in the plumbing between her propane tank and her house. I will attach some pictures of this manly, home-built manometer.

I think the Wikipedia article for "Bernouli's principle" had a picture of one also. Note that picture is a Venturi tube and manometer combined. The part on the top is the Venturi tube. The U-shaped tube, in the bottom part of the picture, is the manometer.

https://en.wikipedia.org/wiki/File:VenturiFlow.png

Conveniently, the amount of pressure in a propane service line, is typically, only about 30 cm (or 12 inch) of water column, so this can be measured with a water-filled manometer, that does not have to be taller than that height.

The way the propane-mongers test a line for leaks, is by pressurizing it to 30 cm, or 40 cm, or whatever the regulator is set at, then closing the valve, and then watching the pressure reading for like 10 minutes or so. How fast the pressure drops, gives an indication of how big the leak is.

I am kind of guessing, typical "positive pressure" arrangements, for keeping out dust, or microbes, or radon gas, only use a few inches or centimeters of water column pressure difference, between inside and outside air.

Also I am guessing it will not work at all, when a door or window is open. That would be just too much for your blower, unless it is a, like, a really big blower.

That guess about the magnitude of the pressure difference (just a few inches of water column) is based on a contrivance I saw in somebody's basement one time, for keeping out radon, from the soil surrounding the basement. It was actually a small manometer, a U-shaped tube with fluid in it, strapped to a big white plastic pipe, that ran from floor to ceiling, accompanied by the faint hum of a blower motor. I am guessing the manometer was measuring pressure difference between basement air, and underneath the slab floor (or maybe behind the walls?), where radon was being vacuumed out.

If you ask your favorite search engine to show you pictures of "manometer for radon abatement" or "radon mitigation", it might just show you pictures of that same hardware.

Also, by the way, I think this kind of measurement will be more accurate, if you can measure the pressure difference directly; e.g. using a manometer, or if it is an electronic transducer, it will have two ports, and measure the pressure difference (often called gauge pressure) between those two ports.

The reason why you might run into trouble using absolute pressure sensors, is because in absolute terms, the pressure inside and outside the box, are roughly the same.

The absolute pressure of air, at sea level on your planet, is about 76 cm (or 30 inch) of mercury column, which in terms of water column is about 1028 cm (or 405 inch or 34 feet) of water column. So if you wanted to use two absolute pressure sensors, they would have to be precise enough to tell the difference between 1028 and 1038 (in the case of measuring 10 cm of water column).

I think that necessarily means running some thin (6 mm or 0.25 inch?) plastic tubing to the place, or places, outside the RV, to where you want to measure pressure.

That is, in contrast to, running some wires with a sensor on the end of those wires, to the place where you want to measure pressure.

I am thinking that as long as the air in the tube is not moving, it will reliably transmit pressure through its bulk from one end to the other. Call it a "pressure node." You know, the same way a long piece of wire can be called a "voltage node," and we expect it to have the same voltage on both ends.

water-filled-manometer-01.jpgwater-filled-manometer-02.jpgwater-filled-manometer-03.jpgwater-filled-manometer-04.jpg
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Downunder35m
Downunder35m

22 days ago

I might be overthinking it but:
If the system is pumpin air in then wouldn't that mean positive pressure?
A simple status check for the corresponding fan/pump would do here.
In terms of sensors you only need to shield he outside sensor from wind, like inside a dedicated housing with some fluff inside.