The performance of a level measurement system is quantified by means of its accuracy statements. The reader must understand not only which parameter is being described, but also the manner in which the statement is expressed. In level measurement, parameters are commonly described in terms of a:

absolute (fixed) distance error
percentage of the empty distance (farthest measurement in span)
percentage of the maximum sensor distance
percentage of measured distance
percentage of set span
a percentage of maximum span

An absolute (fixed) distance error statement describes an error that is constant. This error is independent of the calibration range and the actual level in the vessel. 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 +/-10 mm. The absolute error at different levels is:

Level	Distance	Absolute Error (10mm)
0% (empty)	1100mm	10mm
25%	850mm	10mm
50%	600mm	10mm
75%	350mm	10mm
100% (full)	100mm	10mm

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	Absolute Error (1% of empty distance)
0% (empty)	1% of 1100 = 11mm
25%	1% of 1100 = 11mm
50%	1% of 1100 = 11mm
75%	1% of 1100 = 11mm
100% (full)	1% of 1100 = 11mm

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 6000 mm. The absolute error at different levels is:

Level	Absolute Error (1% of maximum sensor distance)
0% (empty)	1% of 6000 = 60mm
25%	1% of 6000 = 60mm
50%	1% of 6000 = 60mm
75%	1% of 6000 = 60mm
100% (full)	1% of 6000 = 60mm

Note the significant variation in absolute errors associated with the different error statements above.

Additional performance statements will be described in Part 2.

This article was excerpted from The Consumer Guide to Non-Contact Level Gauges

Natural gas is typically transported over long distances in high-pressure natural gas transmission pipelines that often operate outdoors and/or underground at temperatures that are not controlled. Further, the operating pressure --- aside from being high --- varies along the pipeline as the natural gas is compressed at multiple locations before succumbing to downstream pressure losses. As such, accurate flow measurement entails a larger magnitude of compensation for pressure and/or temperature than would occur in a residential diaphragm flowmeter located downstream of a low-pressure pressure regulator installed in a conditioned residential space.

The selection of a particular flowmeter technology is often dependent upon the application at hand. Coriolis mass, differential pressure, positive displacement and turbine flowmeters are often specified for custody transfer applications in natural gas service. These and other technologies can be applied to flow measurements used for operational purposes such as the addition of odorant and combustion control. Insertion turbine, thermal and vortex shedding flowmeters are also available.

The measurement of energy flow will be discussed next month.

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

A. Above the flow element with the impulse tubes sloping upward to the transmitter

B. Above the flow element with the impulse tubes sloping upward then downward to the transmitter

C. Below the flow element with the impulse tubes sloping upward then downward to the transmitter

D. Below the flow element with the impulse tubes sloping downward to the transmitter

Commentary

Accurately transmitting the differential pressure generated by the flow element to the transmitter involves keeping the impulse tubing full of liquid. Locating the transmitter above the flow element enables gas to be trapped and potentially compressed, whereby it can adversely affect accurate transmission of the differential pressure signal to the transmitter --- eliminating Answer A and Answer B. Upward sloping impulse tubing also eliminates Answer C.

Answer D is correct.

Additional Complicating Factors

In some installations, it is not possible to install impulse tubes that constantly slope downward to the transmitter, so other means may be used to effect accurate transmission of the differential pressure signal. These include diaphragm seals or pots where gas can accumulate without affecting the measurement.

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