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.
My 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.)
The results were far closer than I’d been expecting, after reading all the bad press the analog flow totals had been getting.
The 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.
- How many credits do I need for my Honeywell paperless recorder?
(Note: I used 10 credits to run this test: 6 for Events to do pulse, and 4 for analog totals.)