DAVID W SPITZER'S E-ZINE (November 2023)

The maximum span is 8000-400 or 7600 mm, so the absolute error at different levels is:

Level                 Distance           Absolute Error (1% of set span)

0 % (empty)  1% of 7600 mm                   76 mm

25 %              1% of 7600 mm                   76 mm

50 %              1% of 7600 mm                   76 mm

75 %              1% of 7600 mm                   76 mm

100 % (full)   1% of 7600 mm                   76 mm

A percentage of empty distance statement describes a parameter that is in error by a constant percentage of the farthest measurement distance in the span. In the measurement of 100-1100 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of empty distance, the empty distance is 1100 mm, so the absolute error can be calculated as:

Level                 Distance           Absolute Error (1% of empty distance)

0 % (empty)  1% of 1100 mm                   11 mm

25 %              1% of 1100 mm                   11 mm

50 %              1% of 1100 mm                   11 mm

75 %              1% of 1100 mm                   11 mm

100 % (full)   1% of 1100 mm                   11 mm

The percentage of maximum sensor distance statement is similar to an absolute (fixed) distance error statement in that its absolute error is constant. However, the error is related to the maximum distance that can be measured by the sensor. For example, the stated accuracy of a level measurement system in a 1000 mm high vessel (100% level located 100 mm below the sensor) might be +/-1 percent of maximum sensor distance of 8000 mm. The absolute error at different levels is:

Level                 Distance           Absolute Error (1% of maximum distance)

0 % (empty)  1% of 8000 mm                   80 mm

25 %              1% of 8000 mm                   80 mm

50 %              1% of 8000 mm                   80 mm

75 %              1% of 8000 mm                   80 mm

100 % (full)   1% of 8000 mm                   80 mm

Excerpted from The Consumer Guide to Non-Contact Level Gauges.

It would seem logical that this was once a true statement.  Early multivariable designs used technology that was significantly better than that incorporated into the then-available single-variable transmitters. However, improvements have been made to single-variable transmitters over the years to the extent that it is now questionable as to whether multivariable transmitters offer better performance than single-variable transmitters.

Part of this issue arises from the flexibility of single-variable transmitters. For example, a multivariable transmitter may be capable of measuring pressure up to 1000 psia. However, measuring the flow of 25# steam with this transmitter will likely introduce more measurement error than a similar pressure transmitter with an upper range limit of (say) 100 psia. This is because some performance specifications are related to the upper range limit of the transmitter. Typically, the pressure error associated with a transmitter with a high upper range limit is typically higher than the pressure error associated with a transmitter with a lower upper range limit.

An additional aspect of this issue leads one to wonder whether existing multivariable transmitters have better performance specifications than the best pressure, temperature and differential pressure transmitters currently available. A preliminary (and cursory) examination of some offerings indicates that the best single-variable transmitters can offer better performance than multivariable transmitters.

This discussion is not intended to imply that that single-variable transmitters are superior to multivariable transmitters so you should have a preference for single-variable transmitters. The discussion is intended to alert you that the validity of this paradigm should be examined in the context of the application at hand.

Best-in-class performance may not be the prime objective of your measurement, so a multivariable transmitter may be the proper selection because it offers fewer parts with fewer taps at a lower cost while meeting the requirements of the application. However, the paradigm that multivariable transmitters are superior to single-measurement transmitters should be questioned when best-in-class performance is the objective of your flow measurement system. You may be surprised by the results of your investigation.

This article originally appeared in P. I. Process Instrumentation magazine. 

1. Coriolis
2. Target
3. Thermal
4. Ultrasonic
5. Vortex shedding

Flowmeters that restrict the flow typically tend to exhibit more pressure drop than those that do not restrict flow. Target and ultrasonic flowmeters both contain restrictions to flow so Answers B and E are not correct.

Ultrasonic flowmeters tend to be full-bore flowmeters with no restrictions to flow so Answer D is a candidate to exhibit the lowest pressure drop.

Small thermal flowmeters (Answer C) can present a restriction to the flow, but larger thermal flowmeters tend to have internal probe(s). The pressure drop across the probe(s) in generally small, but it will be larger than that of a full-bore ultrasonic flowmeter with no restrictions.

Coriolis mass flowmeters (Answer A) are available with straight single-tube designs that are full-bore flowmeters and do not restrict flow. Using this analysis, one might conclude that Coriolis mass flowmeters also exhibit the lowest pressure drop. However, Coriolis mass flowmeter installations should be considered in their entirety. In order to operate accurately, Coriolis mass flowmeters should not be operated at relatively low mass flow rates. Maintaining sufficiently high mass flow rates typically entails increasing the gas velocity. This is typically accomplished by reducing the size of the flowmeter --- often by as many as 3 pipe sizes. While a straight single-tube flowmeter may appear to exhibit almost no loss of pressure, pipe reductions can cause significant pressure drop.

Therefore, full-bore ultrasonic flowmeters with no restrictions (Answer D) typically exhibit the lowest pressure drop of the listed flowmeters.

Additional Complicating Factors

Full-bore thermal flowmeters can exhibit almost the same (low) pressure drop as full-bore ultrasonic flowmeters.

This article originally appeared in P. I. Process Instrumentation magazine.