need cheap pressure sensor

Washing machines waterlevel sensors. Taken from old machines they are very cheap. The water level is about what you want.

eSoto/Fluidyne/

I've used Honeywell 24PC and 26PC series sensors's $15 and $23 each.. is that cheap? I'm not sure if they work in fluids.

George H.

Are these sensors that give a continuous indication of water level, or just a switch that senses when water has reached correct level? Seems to me that they would just use a switch.

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You're right, they are just switches. A diaphragm operates a microswitch.

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Like the washing machine sensors, they's work with pressurised a column of air from the pipe down to the bottom of the tub, no?

Grant.

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Yep, But I've never quite understood how they maintain any accuracy... I guess ± a couple of inches is OK ?:-) ...Jim Thompson

Yep.

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On my washing machine there's a position to far clockwise labeled 'Reset', but I've never dismantled one to see how they work :)

Grant.

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Same here. I have no idea, but my curiosity is tweaked :-) ...Jim Thompson

eSoto/Fluidyne/

Do you need to measure pressure, or just need a switch function at a given pressure (+/-)? Try:

We had a similar design a while back and these switches worked fine. Our was air; I can only assume a water column would be even easier.

Good luck. -mpm

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That can be quite painful! ;-)

Mouser and newark have various sensors that look like they would work for $5 to $11 (as well as other ones that run into the hundred$) so it depends what you mean by (very), and whether availability without dicking around with surplus is worth anything to you. Mouser is better at the low end of that price range.

Agony at a very low cost is available by using a manometer tube and a bunch of optical sensors that switch when there's water rather than air in the tube - though by the time you get enough of them on there for resolution, it might not be so cheap after all.

How about a coaxial electrode in the one end of the manometer with a dielectric fluid and measure the change in capacitance as the fluid displaces the air?

Tom

And, it all has to fit down a well?

Best pressure sensing is done by measuring the force that returns a diaphragm to the zero position (or piston, or column of mercury... whatever). So, you could bleed air from a compressor tank until it balances the water pressure, and measure that. It'd be possible to put any number of two-port pressure switches in a system, with air on port #2, and ramp up the air pressure until each one changes state.

A mercury column might be (toxic and) inconvenient to arrange (50 cm water means only a few cm of Hg, though), but it can be capacitively sensed for readout. A water column hasn't got repeatable conductivity, and only a DENSER fluid than water is suitable for a U-tube (which means mercury or CCl4 or ... what else?) Your liquid column would have to be protected against contamination, though.

It's possible to use a piezoresistive sensor (like all those solid-state MAP sensors in automobiles), but the prices scare me.

Bourdon tube gages (the rotary-dial familiar kind) can easily be fitted with variable capacitors or Gray-code scales (one needs low friction or you could use potentiometers), but I'm not familiar with any suppliers of such.

I'm trying to charactorize how the pumps function. In other words: build one to a set of dimensions. Apply a controlled heat/ temperature. Record pressurs and pumping volume. Record all this. Change temperature, repeat.

Now build new pump with different dimensions. Repeat above.

Analyze and try to create algorythm(s) that describe how physical dimensions and energu applied affect pump mechanics.

The PUMP will eventually fit down a well. I need to see how one works "up here" when made at different sizes and shapes, and different amounts of heat are applied.

I was hoping to find something less than $5. I've found what I think will work for about $8. I need less than 10 so I won't be out too terribly much. Self-funding research requires dedication, I'm finding.

Years ago I was working on a project involving digitizing human motion. We needed to record a synchronized low pressure signal along side the video. We ended up making a manometer out of a jar and a glass tube. Height of liquid (red dyed water) gave pressure. Video analysis software converted pixels into inches and actually plotted the data for us along side motion data.

What did you find for less the $8 ??

hamilton