How do you measure zero pressure simply???

There are cheap differential pressure sensors around that work in this range.

John

A vane? Essentially a small plate that sits in a square profile, with a position sensor that signals when it's in the middle (meaning no flow in either direction). Can be centered by gravity and then balanced around the pivot, but not completely. How much you balance it determines its sensitivity, to some extent.

Of course it'll react to pressure from the wind but so will just about anything else.

--
Regards, Joerg

http://www.analogconsultants.com/

I don't think that there is anything simple that would work. When I was a graduate student the best way of doing it seemed to be with a stretched conductive (and grounded) diaphragm between two closely spaced flat electrodes.

You set up a capacitance bridge, with the capacitance from the electrodes to ground as one side of the bridge, and a centre tapped transformer as the other. The net excursion at the centre-tap was proportional to bridge imbalance - zero at balance, in-phase on one side and anti-phase on the other.

For added extra capacity you could apply high DC voltages to each of the electrodes and adjust them up and down to apply a compensating electrostatic force to the diaphragm. If you over-did it, the diaphragm distorted itself into hills and valleys.

Baratron was the industry leader back then, and still seems to be in business. I don't recall them doing electrostatic pressure compensation - it only works for very low differential pressures.

-- Bill Sloman, Nijmegen

A diaphragm is simple, doesn't require a flow to maintain it's position and is easy to dampen mechanically. Position sensing can be as simple as a pair of point contacts or as complex as you want to make it. Art

We use a honeywell differential pressure gizmo. I think it sells for ~ $15. (Newark carries them.)

George H.

How much closer than 0.01" WG do you want? And do you realise the impact of opening or closing a door on any hypersensitive gauge/transducer?

Right, like that. Motorola used to make similar things once, but they spun that off to somebody.

I've seen both linear sensor and switch types. The switches are even cheaper.

John

Thanks, but the low pressure versions are $72 and digital, so I need a microprocessor and a circuit board and power supply and box to put it in and code to run it and some kind of display....And I've already got that: Omega PCL-200.

I was hoping for a creative option similar in complexity to what I described.

I've got a UEI EM150. Gonna see what happens when I try to increase the gain X10.

Freescale AFAIK

--

John Devereux

Dwyer has all sorts of stuff for this.

< >

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Many thanks,

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So, you want a manometer that indicates 'positive', 'negative' , and 'near zero'?

Make a manometer of the diaphragm type, and have the center of the diaphragm operate a pair of (over-under) switches. To protect from gusts and surges, just use a narrow tube to 'outdoors' and let air viscosity work for you.

With a light-colored diaphragm, the switches could operate on light reflection.

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Hi Mike, Looks like the last time we bought these they came from digikey. part number 480-2516-ND. (The price has gone up a bit since the first purchase.) They make different sensitivities. I stuck an instrument amp across it. I=92m not sure how much gain... I could look up the circuit. I would guess maybe something like 10V full scale.

George H.

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Just plugged one off the self and into a 'scope. 10V =3D 1 atm. about 10mV of DC offset, (No attempt made to zero this.) and 2 mV of crud. The crud is mostly 100MHz stuff that seems to be all over my lab space. So that could be easily filtered.

George H.

metalized mylar perhaps (a space blanket)?

Thanks for the inputs. I've failed at trying to focus the discussion on simple/elegant solutions. So far, the thermistor bridge seems to be the optimum match to the requirements. Think I'll build a more physically robust one in a ball point pen tube.

I don't think there's any chance I can build a diaphragm system that works any better.

0.001 inches of water is an incredibly small number compared to the stiffness of diaphragm materials I could easily come by and mount.

I did manage to get 10x more gain out of the UEI EM150 before the sensor and op amp offsets used up too much of the dynamic range. I thought about replacing the LM324 with something lower offset, but the thing is already rather sensitive to flexing the input port on the plastic case.

I think I'm a happy camper. Thanks, mike

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A capacitance bridge with a centre-tapped transformer as it's other arm can be remarkably stable, and remarkably sensitive to small displacements.

Getting the two electrodes properly parallel to the diaphragm - and symmetrical - typically relies on the services of a machine shop, and you do need to stretch the diaphragm to get it flat enough, but that kind of gauge can go a long way below 0.001 inches of water.

-- Bill Sloman, Nijmegen

Using the thermistors themselves as the heat source would save considerable power. (i.e., wire in series, feed constant current (or power), feedback from the tap) That avoids relying on air-coupling to the heat source, which is lossy.

-- Cheers, James Arthur

I used a self-heated thermistor in my Bachelor's Thesis... measuring blood flow-rate in the heart (joint MIT/Harvard Med, Peter Bent Brigham Hospital). Don't know how well that'll work with just differential pressure. If there was a flow difference, thus a cooling difference, then it'd work/ ...Jim Thompson

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Are the low cost automotive sensor lacking in resolution for your purpose?

I'm more interested in what you intend to do with the information. You know that the differential pressure will vary a lot from the basement to the top floor, it will be negative in the basement and positive at the upper floors.

Are you going to run a blower to try to equalize the pressure? At what location in the home? To what end?

I have a dryer type vent installed allowing outside air into the basement near the furnace. Cold outside air flows in via the vent. My theory is that this cold air WILL enter the house one way or the other. By allowing it to flow in __at the location of my choice__ (into the furnace) I reduce the inflow in other parts of the house.

Also you should try to seal the upper floors as best as possible, any outflow up there MUST be replaced by inflow in the lower floors.

I'm curious about your strategy for using this pressure info?

Mark