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*