Posts Tagged deadband

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?

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Why is pressure switch deadband such a crapshoot?

Pressure switch deadband isn’t an unknown, but neither is it a known value when you take a new switch out of its shipping box. 

Experienced users will tell you that three identical units of the same pressure switch can have three different deadbands.  All three switches will trip at the same pressure setpoint, but reset at different points.   To complicate matters, the deadband reset point changes with the pressure setpoint.  And, they’ll also say, there’s no way to know, out of the box, what the deadband is without applying pressure and checking.  

Who cares about deadband?  Not everyone.  The exact deadband reset point is critical in some applications, and inconsequential in others.  

Why is getting a known, repeatable deadband such a challenge with conventional electromechanical pressure switches, and is there anything to be done about it?

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