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

The absolute error at different levels is:

Level               Distance             Absolute Error (10 mm)

0 % (empty)    1100 mm                    10 mm

25 %                  850 mm                    10 mm

50 %                  600 mm                    10 mm

75 %                  350 mm                    10 mm

100 % (full)       100 mm                    10 mm

A percentage of measured distance statement describes a parameter that is in error by a constant percentage of the actual distance measurement. In the measurement of a 1000 mm high vessel (100% level located 100 mm below the sensor) measured with an accuracy of 1 percent of measured distance, the absolute error can be calculated as:

Level                    Distance      Absolute Error (1% of measured distance)

0 % (empty)      1% of 1100 mm                 11.0 mm

25 %                  1% of 850 mm                    8.5 mm

50 %                  1% of 600 mm                    6.0 mm

75 %                  1% of 350 mm                    3.5 mm

100 % (full)       1% of 100 mm                    1.0 mm

A percentage of set span statement describes the error in terms of the full scale range. 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 set span. The set span is 1100-100 or 1000mm, so the absolute error at different levels is:

Level                 Distance           Absolute Error (1% of set span)

0 % (empty)  1% of 1000 mm                   10 mm

25 %              1% of 1000 mm                   10 mm

50 %              1% of 1000 mm                   10 mm

75 %              1% of 1000 mm                   10 mm

100 % (full)    1% of 1000 mm                  10 mm

Many of the above terms do not have clear meanings. In addition, discussions with suppliers during investigation for this report revealed different meanings for specifications that otherwise seemed to be clear and well defined. Regardless of the terminology used by the supplier, the reader is advised to confirm exactly what the meaning of the terms used in the specification in order to correctly relate them to the terms used in this report and to correctly evaluate performance.

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

The transmitters are currently calibrated in the instrument shop at a comfortable temperature using a dead weight tester. This requires that the transmitter be physically removed from the process and transported to the instrument shop.  In addition, the transmitter should remain in the shop for a few hours such that its temperature approximates that in the shop before calibration. After calibration in the shop, the transmitter is physically reinstalled in the process and the zero is trimmed.

Not only can this process is take approximately 6-8 hours per transmitter, but it can potentially result in calibration shifts due to the movement and handling of the transmitter. In addition, the calibration is performed manually so it is subject to human error. Approximately 1-2 days later, operating steam pressure is applied to both sides of the transmitter. The transmitter is trimmed under these conditions to compensate for the transmitter pressure effect (zero shift) at elevated pressure. The calibration process is repeated a few months later and some transmitters routinely fail their calibrations. This has created some acrimony due to potential billing errors.

The problem appears to be that the transmitter calibrations are not documented after they are (potentially twice) trimmed in the field. Therefore, after the calibration interval, trimming the transmitter in the field can cause the shop calibration to not agree with the documented shop calibration prior to trimming --- even if the transmitter calibration may not have change since it was (correctly) trimmed.

This problem can be resolved by calibrating the transmitters in the field with a portable calibrator. This should reduce calibration time to approximately one hour per transmitter, include more calibration points, automatically document calibrations, eliminate the zero trim at reinstallation, reduce the probability of damaging the transmitter, and reduce human error.  The transmitter pressure effect trim adjustment could likely be performed immediately after the calibration. Documenting the "As Left" transmitter performance should eliminate billing issues when the transmitter has not shifted its calibration since being trimmed.

This problem has multiple symptoms to include excessive labor requirements, multiple adjustments, increased potential for transmitter damage, and the potential for legal action. The proposed solution is straightforward, yet it effectively addresses the most of the symptoms at hand.

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

1. Coriolis
2. Magnetic
3. Ultrasonic
4. Vortex shedding

It is often difficult to generalize this type of question across an entire technology.  For example, most Coriolis mass flowmeters do not have upstream or downstream straight run requirements. However, the installation instructions for one Coriolis mass flowmeter require that the flowmeter be installed with 20 and 10 diameters of straight pipe upstream and downstream of the flowmeter respectively. Another Coriolis mass flowmeter requires symmetrical supports 6-20 diameters upstream and downstream of the flowmeter.

Magnetic flowmeters generally require 3-5 diameters upstream and 2-3 diameters downstream whereas ultrasonic flowmeters typically require 10-15 diameters upstream and 5 diameters downstream of the flowmeter. Vortex shedding flowmeters have straight run requirements that are similar to those of ultrasonic flowmeters.

Given this analysis, there can be no definitive determination of which flowmeter technology has the shortest straight run requirements. However, a number of Coriolis mass flowmeter technology designs can exhibit the shortest straight run requirements of the listed technologies. Answer A is a practical answer.

 Additional Complicating Factors

There are other flowmeters and variants of these technologies. For example, some smaller ultrasonic flowmeters (typically under 1-inch) do not require straight run. The same is true for some small vortex shedding flowmeters. Small magnetic flowmeters often have their entire straight run requirements satisfied within the face-to-face length of the flowmeter.

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