Cheap device for measuring pressure

Can you explain a bit more, please. Do you need a pressure switch (volt-free contacts changing at a preset pressure), an electronic gauge (analog output signal proportional to pressure), or just an indicator for manual reading?

You should look into a Magnehelic gauge from Dwyer. They come in many ranges:

http://www.dwyer-inst.com/Products/TOC.cfm?MainCategory_ID=288&iCurrentCategoryID=289

A "map" sensor for a turbocharged fuel injected car, cheap, and available.

that's "manifold absolute pressure"

85-87 Buick Regal 3.8L turbo should work.

if i remember that should be 12vdc input, and 5vdc analog output

range should be 30" vacuum to about 1 atm

An excellent suggestion and the right range, unfortunately I live in Australia. But I will investigate if that device is available here.

Tony

You need a dead weight tester. Here's some links that will explain how it works and also try to convince you to spend several thousand dollars to buy their product.

http://www.omega.com/literature/transactions/volume3/pressure3.html

http://www.ashcroft.com/products.cfm?doc_id=352

http://www.calright.com/ca_59.aspx?gclid=CNyU8-r70JcCFQkiagodD3YlCg

This is a very old but recognized method of calibrating pressure gauges. To calibrate a moderate vacuum such as the minus 10psi you are working with you will need the calibration weight to pull up on the piston rather than push down. This can be done by turning the dead weight tester upside down. You may have to contrive a method to attach the calibration weight to the piston with tape or cord of negligible weight.

If turning the tester upside down is inconvenient rig a pulling setup with the calibration weight, a piece of monofilament, a swivel, and an overhead pulley with good bearings (perhaps the fishing reel that the monofilament came off of). To the calibration weight will have to be added an amount of weight equivalent to twice the exact weight of the dead weight tester piston. You'll need a good triple beam balance to weigh these things to get them right.

If you cannot find a reasonably priced dead weight tester on the used market it is not that much of a machining job to make your own. I made my own once out of brass with my 6" Atlas lathe and a new 1/2" diameter reamer.

The hardest part is machining the piston and reaming the cylinder to a precise matching fit so the light oil you use does not leak appreciably under pressure, especially by letting air in while testing the sub-atmospheric pressure points. This is less of a problem with the low pressures you are working with compared with the pressure ranges a commercial tester has to be able to handle. We're talking a fit close enough that your finger over the end of the cylinder will prevent the piston from falling out. The other part to be made is a screw adjuster with an o-ring seal that varies the oil volume to lift the piston so it can be rotated during the measurement. The rest is pipe, fittings to make the small oil reservoir, a quick connect for the transducer being tested and a stand to hold it upright on the workbench.

Ed Weldon

Sounds like an interesting idea Ed, I have a digital balance which I could calibrate using a known weight. I could zero out the the cylinder weight and feed air though the held piston and measure the cylinder pull for calculating pressure or hold the cylinder and measure the piston pull for vacuum.

Thanks for setting the brain in motion.

If you use a mercury column instead of water you will only need 1 meter U tube to cover your range. I use one myself using 3/16" nylon clear tube very accurate.

1mm Hg= 0.01934 PSI

Its quite safe the merury is fully enclosed and should not contaminate anything.

I agree that would be ideal, but how do I get Mercury without a poisons licence?

Can we take it, then, that you just want a manual indicator (no electrical connection)? You didn't answer my #1.

Yes, I need a basic reference instrument to check my electronic devices and safety pressure releases.

A friend suggested a water manometer with one leg blocked and the test pressure applied to the other. Measure the height difference between atmospheric and test pressure with a small correction for the water height difference and use P₁V₁ = P₂V_2.

Tony

These are cheap'n'cheerful - R1/4 press gauge,50mm dia -1to0bar back - £7.65 UK. This one would cover your vacuum side, there are others in the range for positive pressure.

I haven't looked through the whole range that RS stock - you may need 2 to cover +ve & -ve, or maybe another distributor has one that covers both.

You could try the manufacturer ...

Hi Tonymech,

I did not realise the government had made this a requirement, what a pity our society is becoming so restrictive . You would think that recording name and address for small quantities would be enough.

I must look after the small amount I have .

Use a Manifold Preasure sensor. These are available through industrial and automotive sources. CAT, Detroit Diesel, Cummins etc use them as inputs for electronic diesel injection control.

Tony --

Another approach might be to purchase one or two bourdon tube "test" gauges made by one of the recognized gauge manufacturers. I have purchases some fine examples on ebay for prices in the range of US$50-75. They normally are specified as accurate to 0.25% of full scale; which ought to be close enough for your purposes.

I'm confident that there are in your country one or more commercial laboratories that are dedicated to the periodic calibration and certification of measuring instrumentation for local industry to facilitate ISO quality standard compliance. Your demonstration of the use of certified measuring equipment should be good enough to convince customers of the accuracy you would claim for your transducers.

If you need to go to a higher level of accuracy than what I mentioned above I'd recommend you consider becoming a student of that technology. (I am not) But my library contains two reference copies, 1st(1961) and 3rd(1982)editions, of Buckworth and Beck, Mechanical Measurements, published by Addison-Wesley, who in the 3rd edition list a Sydney, AU location.

I prefer the first edition; the third seems to be more directed at student use. Both devote about 30 pages to the subject of pressure measurement.

Ed Weldon

Hi Tony,

Did a walk-about in the hangar... In Cessna/Beech GA Aircraft there was a unit that was called a Manifold Vacuum/Pressure Gauge. Memory serves that is was calibrated around minus 15 InHg to plus 40 InHg. If you have a local airport with a Fixed Base Operation, you might just find one of these in working order for just the core charge. The below picture is an altogether different instrument, but would be helpful in describing the form factor in your search. The below unit is for the pressure side only (yep, there's something missing in this pack-rat's collection after all!!):

Might be a cheap and easy solution if you don't mind the conversion factors.

Best on 'ya,

GLB

Just wondering... how about using alcohol instead of mercury? Some thermometers use alcohol.

I would suggest a turbo boost guage that can be bought in practically any automotive parts store. It will work in the ranges you are wanting and are not terribly expensive.