DAVID W SPITZER'S E-ZINE (October 2025)

The most common sensor design incorporates a metal rod with an electrically nonconductive coating such as Teflon. When a conductive material covers the sensor, the transmitter measures the capacitance that is developed between the covered portion of the rod and the material.

A common sensor for non-conductive materials is a metal rod surrounded by a metal cylinder (with passages for the material to enter/exit the annulus). In these applications, the transmitter measures the capacitance developed between the rod and the cylinder as a result of non-conductive material filling of the annular space between the rod and cylinder.

Flexible sensors are available for longer lengths where the shipping and handling of rigid sensors tend to be problematic. The accuracy of flexible sensors is often inferior to that of rigid sensors.

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

Shortly after my investigation, I observed that the Venturi looked a bit "strange" because the inlet and outlet flanges did not appear to be parallel to one another.  Further observation revealed that one of the welds in the Venturi was shinier than the others.  My conclusion was that the Venturi was removed during the shutdown, cut and re-welded before being reinstalled.  More investigation revealed that the piping people had altered the Venturi in order to avoid altering the 8-meter pipe that they installed upstream because small Venturi was easier to alter than the 8-meter pipe. 

Needless to say, cutting and re-welding the Venturi will affect the measurement accuracy of the Venturi flow element, so a new Venturi had to be ordered.  Delivery of this type of Venturi with hardened metal took a few months so it was only 6 months later when the existing Venturi (installed backwards) could be replaced. 

So, we operated our reactor for about 6 months with its catalyst recirculation flowmeter installed backwards.  You may think that we were "nuts", but under the circumstances it made a lot of sense.  Sometimes, the cards you are dealt lead you in directions that may not seem logical to the outside observer.  Has this happened to you?  

 This article originally appeared in Flow Control magazine.

In gas service, the taps should be oriented so that the impulse tubing remains completely filled with gas while ensuring that there is no accumulation of liquid between the taps and the transmitter.  Failure to maintain a gaseous hydraulic connection between the flow element taps and the differential pressure transmitter can adversely affect the accuracy of the flow measurement system. 

Locating the taps on the bottom of the pipe (Answer D) can allow dirt and other solids to enter and potentially plug the impulse tubing, thereby adversely affecting the flow measurement.  Liquids in the pipe can also enter the impulse tubing and cause the flow measurement to be inaccurate.  In some applications, gas can condense and accumulate in the impulse tubing, likewise causing inaccurate flow measurement. 

Locating the taps on the top of the pipe (Answer A) is recommended because liquids that accumulate can drain down into the flow stream thereby ensuring that the impulse tubing remains full of gas.  Answers B and C are also acceptable.

Additional Complicating Factors

Note that simply locating the taps between the side and top of the pipe (Answers A, B and C) does not ensure that the impulse tubing will be full of liquid.  The impulse tubing between the taps and the differential pressure transmitter should be sloped so as to allow liquids to fall and exit the impulse tubing into the flow stream.

This article originally appeared in Flow Control magazine.