Precision Water Level Instrument
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
41
L =
length of line between instrument and float.
X = Coefficient of expansion (X Factor).
10.3.7 Example
A water level instrument has an 85mm diameter float, a beaded stainless steel
cable and a 160gm counterweight (always above the float). What errors would
occur if the level rose by 2 metres? (In this case we will ignore temperature
changes as there is no information about the supporting structures).
From Formula 1, the error in measurement would be 4.2mm too high. The error
due to Instrument Lag Formula 4 is 0.3mm and the total error 4.6mm. As the
counterweight goes underwater a 2.6mm error occurs.
This example demonstrates the large errors that can result from the use of
unsuitable components. If the recommended 170mm float was used in the system
described in the example, the line shift error would be 0.77mm, the instrument
lag 0.09mm; a total of 0.86mm, and the counterweight submergence error would
be 0.46 mm.
10.4 Reducing Errors
The following hints will assist in reducing errors in float operated level
measurement systems.
•
Use the largest diameter float possible (reduces all errors).
•
Use a lightweight float line (reduces line shift).
•
Make sure the counterweight is always above or always below the float
(eliminates counterweight submergence errors).
•
Mount the level measurement instrument as close to the level being
measured as possible (reduces temperature errors).
•
Use a computer to apply automatic correction for all errors from a known
reference level.
