Contact Level Measurement System Performance (Part 2 of 3)

By David W. Spitzer

E-Zine August 2007

Click here to review Part 1

A percentage of measured distance statement describes a parameter that is in error by a constant percentage of the actual distance measurement. For capacitance level measurements, the actual distance measurement is the distance from the bottom of the sensor to the material. For radar level measurements, the actual distance measurement is the distance from the base of the sensor to the material. Therefore, capacitance and radar level measurement systems are bottom-up and top-down measurements respectively.

For example, in a 1000 mm level measurement, 250 mm of material covering the sensor represents an equivalent capacitance level measurement of 250 mm. For an accuracy of 1 percent of measured distance, the absolute error can be calculated as:

Level	Probe Coverage	Absolute Error (1% of active length)
0% (empty)	0 mm	1% of 0 = 0.0 mm
25%	250 mm	1% of 250 = 2.5 mm
50%	500 mm	1% of 500 = 5.0 mm
75%	750 mm	1% of 750 = 7.5 mm
100%	1000 mm	1% of 1000 = 10.0 mm

However, a radar level measurement system with 250 mm of material covering the sensor would measure 750 mm from the top of the sensor to the material. For an accuracy of 1 percent of measured distance using a radar level measurement system, the absolute error can be calculated as:

Level	Probe Coverage	Absolute Error (1% of active length)
0% (empty)	0 mm	1% of 1000 = 10.0 mm
25%	250 mm	1% of 750 = 7.5 mm
50%	500 mm	1% of 500 = 5.0 mm
75%	750 mm	1% of 250 = 2.5 mm
100%	1000 mm	1% of 0 = 0.0 mm

A percentage of set span statement describes a parameter that can be in error by a constant percentage of the set span of the transmitter. In a measurement using a transmitter measuring a 1000 mm level with an accuracy of 1 percent of set span, the absolute error can be calculated as:

Level	Probe Coverage	Absolute Error (1% of active length)
0% (empty)	0 mm	1% of 1000 = 10.0 mm
25%	250 mm	1% of 1000 = 10.0 mm
50%	500 mm	1% of 1000 = 10.0 mm
75%	750 mm	1% of 1000 = 10.0 mm
100%	1000 mm	1% of 1000 = 10.0 mm

Click here to review Part 3

Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges

ISSN 1538-5280

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