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Old January 11th, 2018, 12:05 PM   #16
daba
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Quote:
Originally Posted by Austin1985 View Post
Lots of input here. Thanks everyone!

Also, this is a quench, so I'm only looking at the signal for about 10 seconds. The first second of that is crazy noisy because the water shifts from a bypass valve into the tool, and it takes a bit to settle down. It sounds to me like the current meters and mechanical setup may not be well fitted for totaling.

Again, Thanks!
The highlighted will inevitably be your downfall. Not even an expensive flowmeter can cope with what you describe.

Do you want to know how much water was used in the quench ?

Or,

Do you want to control how much water is used in the quench ?
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Old January 11th, 2018, 01:17 PM   #17
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Originally Posted by Austin1985 View Post
Also, this is a quench, so I'm only looking at the signal for about 10 seconds. The first second of that is crazy noisy because the water shifts from a bypass valve into the tool, and it takes a bit to settle down.
If I were faced with this measurement task, which sounds like total volume for a short-duration flow event, I would go ahead and try the existing setup with a "field calibration" if practical. By this, I mean setting up a method to capture the total volume of dispensed fluid for a typical quench event, and then correlate actual quench volume with the PLC-computed volume. Then use this relationship to correct the PLC result after the quench is complete.

This would likely give decent results assuming there is nothing terribly wrong with the existing setup (e.g., flowmeter type, flowmeter installation, valve positions, wiring), and other process variables (e.g., fluid pressure, restrictions, nozzles) are reasonably consistent over time.

If taking this approach, capture the fluid using quench periods in the range of typical operation. For example, if it is typically 9 or 10 seconds, capture the fluid for 8-, 9-, 10-, and 11-second quench events. Carefully measure the captured volume and record the PLC-computed quench volume for each test. If possible -- not too expensive or time consuming -- do a couple repeats.

Then use the a spreadsheet to plot the actual volume against computed volume to get a correction equation. MS-Excel is very convenient for this.

Finally program the correction equation in the PLC to correct the flowmeter-measured volume after each quench event. As long as the events are not too far out of the tested time range (and other assumptions hold), the corrected result should be somewhat accurate.

I am assuming quench time is the variable and others stay constant. If time is constant and, say, pressure varies, you would do the same but with different pressure settings.
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Old January 12th, 2018, 08:51 AM   #18
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When integrating rate readings as suggested in the OP, it is common to lose accuracy due to rounding error at each division point (div by 60 in the OP). This may not be an issue if floating point is available (more on that below) but even when using integer arithmetic, you don't have to tolerate this rounding error:

At each division, save the remainder (modulo) term, and add that to the next total before dividing. Eventually the errors will add up to another count.

If your PLC supports floating point (real) data type, then you mostly avoid the rounding issue above, but there is a trap waiting for you there too:

If you have a "lifetime" total function that doesn't get periodically reset then eventually this will grow to the point that the mantissa (fractional part) is only representing (say) 1/10 or 1, or 10 gallon increments. When it reaches this point, you will start having the problem that it no longer totalizes at low flow rates.

Suppose your real data type only supports 3 significant decimal digits (not a real example)

If your total is 3.97 x 10**5 gallons, and you flowed 10 gallons the previous minute, then 3.97 x 10**5 +1.0 x 10**1 = 3.97 x 10**5 gallons....your accumulator is "stuck" because it no longer has enough resolution to count by 10's much less by single gallons.

The last trap to avoid is that many flow meters will have a small offset at zero flow...maybe .25% of full scale or some such. If you are integrating this 24/7 then it can add up to a big error. If there is a detectable zero flow condition (perhaps a valve is off) then you can lock the integration function when you know flow must be zero. Otherwise you will need to set a minimum flow threshold and call anything below that zero.

Last edited by Kevbo; January 12th, 2018 at 09:02 AM.
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