Testing Transducers

Alan Case

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Join Date
Apr 2002
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Wagga Wagga
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I use screw in pressure transducers quite regularly (0 - 1 Bar or 10 metres) To check the calibration I usually carry a 5 metre section of clear hose, a funnel, food colouring, tape measure and a valve and tap arrangrement. This involves climbing the nearest stucture or tree and hnging the pipe so I can fill it with coloured water. Very accurate but a pain.
I dont think a pump up system and a gauge will give the accuracy I require.
This now is a physics questions, is there a way to get say 5 metres of water pressure if you have a system only 1 metre high. ( like a pulley arrangement where the number of ropes is the mechanical advantage)

Regards Alan Case
PS Or another accurate way to test in the field
 
The hydraulic equivalent of a block and tackle pulley system is an intensifier. An intensifier is a hydraulic piston which has a large area on one side, and a small area on the other side. If you have a 5:1 area ratio, and place 1 meter of water on the large area, then you will have 5 meters of head pressure on the small side.

intenstifier2109q.GIF



In practicallity however, friction will play a major role at such small pressures. And of course, the piston weight is a component of the overall equation, another factor at low pressures.

Which brings us to a better way. What if you replaced the piston and the water on the top with a calibrated weight? Selecting a piston with the right weight/area ratio, and spinning the piston to overcome static friction, it will create the desired pressure in the cylinder below the piston. This is how a deadweight pressure tester works. Deadweight testers are commercially available from a number of suppliers.

2000dwt.jpg
 
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I don't have any specific proof but...

I think if you serpentine a single length of tube horizontally and fill it with only enough liquid to fill part way up the loops, the additive change in height of all the loops combined would be the same as the total change in height of a single column.

Again, I'm not a physicist. I just play one on TV. But I think that works out.

Keith
 
Alaric said:
The hydraulic equivalent of a block and tackle pulley system is an intensifier. An intensifier is a hydraulic piston which has a large area on one side, and a small area on the other side. If you have a 5:1 area ratio, and place 1 meter of water on the large area, then you will have 5 meters of head pressure on the small side.

intenstifier2109q.GIF


Huh? You are saying that simply because there is more volume at the top there will be more head? Me thinks not. Vertical height and density are the two factors related to hydrostatic pressure...not volume. Unless of course you actually have something "pushing" on the top- either additional pressure in the form of a sealed system or sometime of mechanical piston.
Just trying to clarify things!

Solve the tube length issue by using a more dense liquid- such as Mercury with a specific gravity of more than 13 I believe. That way the accuracy is just as precise, with less tube height required. (There are other calibrated fluids lighter and cheaper than Hg.)
Mike
 
Ooops. My miss Alaric. You were showing a piston and not just water volume! Shoulda looked closer!
I still maintain that liquid of a higher density is the easiest solution and allows Alan to use the same configuration he is used to.
Mike
 
Or something of an even higher density than a liquid, like maybe a solid metal object, pressing on a liquid (like a dead weight calibrator 🍺 ) The big disadvantage there however is friction.

processfig142.gif


The problem with high density liquids is contamination of the transducer. I sure wouldn't want someone using a heavy oil, or worse, mercury, to test a transducer in a beverage processing plant.


Explanatory note on the intensifier in case someone doesnt understand how it works: It is a force apparatus. Force = pressure * area. Apply a fixed pressure at one end of the piston, generationg a force equal to the pressure * the area, and the force on the other end must equal it. If the area at the other end is less, then it follows that the pressure will be higher. Hydraulics 101.
 
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Keith, m' daddy tole me thet water always seeks its own level. Thet's true, and regardless of the shape of the container or how many loops ya put in, the pressure will be dictated by the height of the fluid.

A dead weight tester would work. Using mercury will also work.

The easist solution, though, is a pump up tester. If you are worried about accuracy, get a very high quality gauge and tee it into the line with transmitter in question. Then you can verify the pressure.

Or, you can get a very tall fish tank. Decorative as well as functional.
 
kamenges said:
I think if you serpentine a single length of tube horizontally and fill it with only enough liquid to fill part way up the loops, the additive change in height of all the loops combined would be the same as the total change in height of a single column.
<snip>

I have to admit that I'm not sure exactly what head pressure partially-filled loops in a serpentine tube will exert, other than it could NOT exceed the difference in elevation between the highest point and the lowest point. I don't know whether the air fill at the tops of partially filled tubes would "transmit" water pressure accurately or not.

But I am certain that a tube totally filled with liquid, arranged in a serpentine (up and down; up and down) manner will exert a head pressure only equal to the difference in elevation between the highest level of liquid and the low end. The total head pressure is a cumulative sum of partial loops pressures, but the individual loops "zero" each other out, resulting in a pressure difference representing elevation difference.

This is seen routinely in "capillary filled remote seals" on process pressure transmitters.

Sometimes a seal is too long and is coiled, rather than extended, and the resulting pressure effect of the coiling or serpentines is zeroed out. It doesn't matter if the coil is horizontal or vertical, the total effect on measured pressure is the same.

The elevation difference between the seal and the port on the transmitter is still present (and affected by the specfic gravity of the fill fluid).

In answer to the original question about an alternative, the dead weight tester is a primary calibration standard of very high accuracy and there are "portable" models (need a sturdy tripod or bench).

One might consider using mercury (Hg) with its specific gravity of 13.546 if health and safety regulations permit. One only needs a column 1/13.546 as tall with Hg.

Dan
 
Alan, a dead weight tester is the best but a cheap one will cost you $20000 + and they are best suited to clean room / work bench.


The two best options for you is a hand pump and fluke process calibrator with module or carry a small bottle of high pressure nitrogen around and make up a connection manifold that allows you to bleed pressure off. I prefer the second option, I find hand pumps are too easy to break. Nitrogen is stored at 7000kPa and is inert. Just dont vent it off in a closed space as you will aspyxiate if there is enough nitrogen.
 
A good quality large gauge will at best have a 0 - 5 metre readout of say 150 mm, whereas a water column will have a readout of 5 metres.
To accurately calibrate it is very hard to look at a readout of 150 mm and get the accuracy that you can with a read out if 5000mm.
Have decided to find a surveyors staff (collapsible), tape the clear tubing to the staff and set up a hand pump to pump water into the column from the bottom. Can then open the tap and drain the water out as I check the accuracy and linearity of the transducer. Also saves climbing the nearest tree. Mercury is not an option as these transducers are for drinking water. A bit of cochineal won't hurt anyone but I will get shot if I use mercury.
Thanks to all.
Alan
 
Alan
As accuracy is your prime concern I would be interested to hear how you correct for temperature. This might not be a big problem in Australia but in the UK where temperatures could be above or below 4C applying the right correction to the height of water in your tube could be tricky.
 
TimeFlux & JohnW are correct. Most gauge (Gage) manufactures sell a hand pump and a meter with built in pressure transducers. These meters a calibrated and come with certificates that can be traced back to a know standard. Most of these meters display the pressure in any reading you choose (PSI, PSIG, KPA, BAR, etc). The meters will also do a single pressure or Delta Pressure test. If you still prefer a water column method see Dwyer Instruments for a roll up slack tube manometer that is ready to go.

 

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