Flowmeter analog totalizer vs. pulse input totalizer: A side-by-side comparison

When it comes to flowmeters, there are some pretty hot opinions in the control and instrumentation world about the inaccuracies of an analog totalizer, and why using a pulse-driven counter/totalizer is the way to go.

After reading some really vicious web debates on the topic, I had the opportunity to test for myself, and I was surprised at how close the resulting totals were, and how insignificant the error was.

My test involved comparing a magnetic flowmeter’s internal totalizer to the totals calculated by a recorder from an analog output and a pulse output from the magmeter.

Siemens SITRANS FM Magmeter Display and Honeywell eZTrend QXe paperless recorderMy test equipment and conditions

I generated the flow using a Siemens SITRANS Mag3100 flow tube, and attached the Mag6000 converter/transmitter for local display of the flow rate and total. The flow rate signal was output as a 4-20mA signal corresponding to a 0-10 GPM range. I configured the pulse output to trip every 0.25 gallons. The magmeter’s damping was set to the lowest value, 0.1, so the flow rate value and its 4-20mA jittered considerably with changing flow, as expected.  The flow rate bounced from about 1.0 to about 2.9 gpm, throughout the test which lasted more than four hours.

Then, I connected a Honeywell eZTrend QXe paperless recorder. I took the flow rate signal and totalized it, tagging it on the screen as Analog Total. The magmeter pulses wired to the recorder’s digital input were counted and scaled, and tagged on the screen as Pulse Total. I configured the recorder’s analog input to sample at the 5GHz sample rate, on a scale of 0-10 GPM, with no averaging or damping applied to the flow rate signal.

Comparing the results

As I expected, the pulse total on the magmeter’s display and the Pulse Total on the recorder matched exactly.

Then, I looked at the differences between the pulse total and the analog total. I took screen shots almost every 100 gallons (My apologies: I missed the 300 gallon screen shot because I was on a tech support call with a customer.)

Screen shots compare analog total and pulse totals for flowmeter
The results were far closer than I’d been expecting, after reading all the bad press the analog flow totals had been getting.

Compare analog total and pulse total and display percent errorThe table here summarizes the error as a percentage of the pulse total value.

The error, which never exceeded 1%, might be due to a minor calibration offset at the recorder’s 4-20mA input (or shunt resistor), and correctable with an analog calibration.  The recorder was brand new, out-of-the-box, and I did not do a cal check on it prior to the test.  If I had, I might have been able to calibrate out the minimal differences.

Next time, I want to do the same test with a couple seconds damping on the magmeter to see how that affects the error, given that factory default settings usually have damping enabled.

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  1. #1 by Kyle Chase on April 19, 2012 - 10:48 pm

    That is a huge error, and will cost your client a lot of money. For example, we have a plant that runs roughly 400000 m3 of oil a year. If you convert that to barrels, that is 2515924.31 barrels. With an overage error of 0.46% (from your numbers), that would be an error of 11573.25 barrels, in your clients favour. With the current price of oil being $102.59/bbl, that would put your error as $1,187,299.90, which might make someone freak out, either your client, or their client. Not a risk anyone should be taking. Mind you, if its not for custody transfer, this might be fine.

    • #2 by danstips on April 20, 2012 - 2:28 pm

      You make a good point, Kyle. It’s always important to consider the media and what level of accuracy is required for the application. Custody transfer apps tend to run closer to the 0.1% range, and in that case, the difference would be substantial. In my experiment, I wasn’t looking at a particular application, like the one you described, and my flowing water was significantly less costly than oil.

      I was just curious as to exactly what kind of error I’d see, and whether that’s really enough to tell customers they had to use pulse input totalization, or avoid analog totalization at all costs. In some applications, that accuracy level would be sufficient. But it’s a question to be asked on the individual application level.

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