PTS Flowbench Forum

Tony wrote:

larrycavan wrote:Tony,

Do sensors have a sweet spot?...

Would be interesting to see what happens when an analog manometer is hooked up in parallel with the digital unit on a 1020....

I really don't know the answer to that Larry, you guys are really pushing the limits....

But I will theorise that the biggest sources of initial (straight out of the box) error will be the zero pressure point, and full scale calibration point. The sensor is internally corrected for temperature, and we ourselves can ensure the dc supply voltage to the sensor remains absolutely constant.

After zero and full scale pressures have been tweaked in software, what is left will be some non linearity and hysteresis due to the mechanics of the strain gauge part of the sensor.
We can ignore hysteresis, because the random pressure fluctuations occur evenly in both directions, and after some really brutal averaging, what we are left with will be a simple very slight curvature away from true linearity.

This will be made worse because we are exploiting all of the full maximum possible pressure swing to minimise the effects of other possible errors.

Not certain about the cause of this non linearity, but my guess would be that a cross section of the measurement diaphragm acts something like a beam in bending mode, and the strain gauge part measures the tension/compression changes along top and bottom of the flexing diaphragm.

It is all geometry related, and providing the actual mechanical flexing is kept very small, the linearity should be quite reasonable.
But it would be easy to imagine how, if something like this were made of rubber, how blowing it up into a massive dome shape could introduce some dramatic non linearity into how far the rubber stretched versus the applied pressure.

If you can detect and quantify this non linearity, correcting for it should be possible, and I would expect the correction to remain fairly valid from sensor to sensor, or at least it should get you very close.

This is all armchair speculation on my part, but it is what I would be looking for.

Fluid manometers will be very difficult to read to the required degree of accuracy because of the meniscus effect, surface tension, and how the fluid clings to the wall of the tube.

It may be easier to generate an accurate static pressure rather than trying to read or compare existing pressures by different means of measurement, each of which also have their own set of problems.

How about a header tank full of water and a hose leading down into the pressure transducer.
The header tank could be raised on a lead screw to increase the static pressure by known fixed amounts without any significant fluid movement if the surface area of the reservoir were kept relatively large. It may with care and repetition, be possible to plot this non linearity and then do something about it in software.

Again all just crazy wild thinking on my part.

Tony wrote:These transducers are not recommended to be used directly with water (or corrosive or electrically conductive fluids) but that recommendation is only for very long term permanent installation.

The pressure port is protected with some kind of inert waterproof jelly filler to keep the exposed internal electronics dry.
There should not be any problem using water with the sort of short term testing this experiment will involve.

Brucepts wrote:
A couple of sensors can be sacrificed in the name of R&D