Research for The Consumer Guide to Non-Contact Level Gauges involved extracting information from the level gauge specifications from about 60 suppliers. These suppliers used a total of about 30 different terms to express performance. Let's think about this for a minute --- on the average, there was one term used to express performance for every two suppliers. This would seem to be far from consistent.

In the early going, the well-defined published specifications were tabulated and used for calculations. As research progressed, conversations with suppliers made it apparent that the published specifications could not be trusted --- even if they were technically clear and well understood. As a result, I had to contact the suppliers again to verify that their (supposedly) well-defined published specifications reflected the supplier's intentions.

For example, after speaking with the suppliers, 0.25% of measuring range, 0.25% of span, and 0.25% all meant 0.25% of maximum sensor range. Think about this for a minute. Measuring range and span are well defined --- or are they? The percentage is not well defined and begs the question, "Percentage of what?"

On a positive note, despite the discrepancies noted in the published specifications, suppliers were forthcoming in explaining their intentions. Wouldn't it be nice if they would update/correct their specifications to use standard terms that describe level gauge performance?

This question is of interest personally because many or most of us cook and heat our homes with natural gas. Natural gas is delivered to customers at a nominal pressure set and controlled by the local gas company. Natural gas consumption in most houses and apartments is typically measured using a positive displacement diaphragm flowmeter designed and approved for natural gas custody transfer service. These flowmeters will measure the volume of natural gas accurately when the nominal natural gas pressure is maintained within tolerances and its temperature remains within a specified range.

However, varying composition can and will affect the heating value of a given volume of natural gas. Therefore, the heating value of actual natural gas at a given location could be different from a typical NAESB natural gas heating value by as much as a few percent. A higher (lower) heating value than typical would deliver more (less) thermal energy to the user in a given volume — decreasing (increasing) billing revenue for the same amount of thermal energy delivered. Extreme variations could potentially adversely affect combustion. To ensure that the natural gas provided to the customer contains the proper amount of energy per unit volume, natural gas companies typically monitor the heating value of their natural gas.

This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.

A. 1 inch upstream and 1 inch downstream of the orifice plate

B. 0 diameters upstream and 0 diameters downstream of the orifice plate

C. 1 diameters upstream and 0.5 diameters downstream of the orifice plate

D. 8 diameters upstream and 2.5 diameters downstream of the orifice plate

Commentary

Any of these tap locations could be correct --- depending upon the application and its design. The mechanical location of flange taps (Answer A) and corner taps (Answer B) is typically maintained by purchasing special flanges that are pre-drilled to accurately locate these taps relative to the orifice plate. Radius taps (Answer C) and full flow taps (Answer D) are typically located in the pipe itself. Vena contracta taps (not included as an answer) are located 1 diameter upstream and at the vena contracta --- determined by orifice geometry --- downstream.

Additional Complicating Factors

Selection between flange/corner taps and pipe taps is typically related to cost and/or the ability to accurately locate pipe taps in small lines. Selection between flange taps and corner taps is largely determined by personal and/or geographic preference.

This article originally appeared in Flow Control magazine at www.flowcontrolnetwork.com.