*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__ |
__Absolute Error (1% of measured distance)__ |

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

25% |
1% of 850 = 8.5mm |

50% |
1% of 600 = 6.0mm |

75% |
1% of 350 = 3.5mm |

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

*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__ |
__Absolute Error (1% of set span)__ |

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

25% |
1% of 1000 = 10mm |

50% |
1% of 1000 = 10mm |

75% |
1% of 1000 = 10mm |

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

*A percentage of maximum span* statement describes the error in
terms of the maximum sensor distance minus the blocking distance. For example,
the stated accuracy of a level measurement system with a sensor that can measure
from 400 mm to 8000 mm might be +/-1 percent of the maximum span. The maximum span
is 8000-400 or 7600 mm, so the absolute error at different levels is:

__Level__ |
__Absolute Error (1% of maximum span)__ |

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

25% |
1% of 7600 = 76mm |

50% |
1% of 7600 = 76mm |

75% |
1% of 7600 = 76mm |

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

In order to fairly compare performance, the same type of accuracy
statement should be used for each level measurement system. For level measurement, the
best measure of performance is usually the absolute (fixed) level error statement
because it quantifies the amount of error expected to be present. Therefore, in most
cases, statements should be expressed or converted to an absolute (fixed) level error
statement before using the information for comparison purposes.

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

The preponderance of error statements used by suppliers will be
discussed in Part 3.

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