Why is my clamp-on flowmeter’s flow rate stuck at zero?


Siemens ultrasonic clamp-on flowmeter sensors mounted to a pipeI was working with a customer to replace a Siemens clamp-on ultrasonic flowmeter after its electronics had been replaced.  We followed all the connection and startup instructions, but the flow rate was stuck at 0.00 – no dithering, no hunting, not a single flicker in the low-order digits.

Very strange.

Ultrasonic clamp-on flowmeters can be very sensitive to low flows. But even at no flow with the pump turned off or the line isolated, the digits to the right of the decimal point would dither around zero, showing some very small positive values, some very small negative values.  It’s the nature of the beast.

So why were we getting an absolute zero reading? What was going on?

Now, we started the step-by-step hunt for the source of our error.

In this case, the electronics of the Siemens FUx-1010 had been replaced. The customer had been careful beforehand to document all the settings on a printed site configuration datasheet, which is a good thing, since replacing the electronics cleared out any custom configuration he had done in the past. So, during the new install, we duplicated these setting values from the worksheet.

When we powered up the unit, it initialized and came up with a flow value, so we knew it had significant signal strength.  If it didn’t, this model would show a giant F (for Fault) on the display. But there was no fault. In fact, the signal strength was extremely high, and another suspected culprit to our problem – aeration – had a value of zero, which is as good as it gets.

Next, we confirmed that the pump was on. The flowmeter was on a cooling loop, and any lack of flow would have been evident in very short order. But that wasn’t the case. The pump was running and the loop was cooling, but the flowmeter still said Zero.

We dutifully went back through the configuration a second time, checking step by step to make sure the settings matched the last known good setup.

The hard zero was still bothering me. I’d seen it before, but just couldn’t make a connection to the cause. No dithering, just a report of 0.000 flow.

Stuck as we were, the customer and I made a call to Siemens tech support center. He described what we were seeing, paused to hear the answer, and then said one word.

“Deadband.”

Of course. That was it! 

What Siemens calls deadband, most companies call “low flow cutoff”. By either name, it’s a value set in configuration where any flow rate below the selected value is reported as zero (hence, the lack of dither). So our unit was reporting a value of 0.000, exactly as it should.

Solution in hand, we went back through the setup menu to deadband and found a value of 8,000. 8,000 gallons? A bit high for our purposes. It’s safe to say that this system would never flow more than 500 gallons per minute. So, with that low flow setting, all flow would be reported as 0.00.

In our earlier passes, neither of us had caught the fact that the deadband value on the printout was 8-point-000, not 8-comma-000.

As soon as we changed the low flow cutoff value to 8.0 gallons, the actual flow rate showed up on the display, at a logical 40 GPM for our process.

The lesson here?  If your flow rate is a hard zero, think “low flow cutoff” before you call tech support and save yourself the time and frustration.

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Lesman Instrument Company is the authorized Siemens representative in Illinois, Indiana, Missouri, Paducah KY, Wisconsin, and Michigan’s Upper Peninsula. If you are located outside that area, you can find your local sales office or get technical assistance by calling 800-365-8766.

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