UNIDATA
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
40
This error (caused by friction in the measuring apparatus) is not cumulative and is
usually not very large.
Formula 4
(on the following page) details the error
caused by instrument lag.
10.3.4 Temperature
Change
Changes in temperature will cause the float line to expand and contract. The
amount of expansion (contraction) is usually small and is often compensated by
the simultaneous expansion (contraction) of the level instrument supporting
structures. The floatline length change can be calculated from
Formula 5
below,
using the following factors.
10.3.5
Coefficients of Expansion
Float Line
Coefficient X Factor
1.2mm SS Beaded Cable
0.0000009 0.0009mm/°C/metre
0.4mm Stainless Steel
0.0000009 0.0009mm/°C/metre
Supporting concrete
0.0000065 0.0065mm/°C/metre
Therefore the error expected from a 10
°
change in ambient temperature for a 10
metre cable will be 0.9mm (The float line expands/contracts by
0.009mm/
°
C/metre).
10.3.6 Correction
Formulae
Formula 1 E = 2560 * (W/D^2) * S
Formula 2 E = 2350 * (W/D^2) * S
Formula 3 E = 117 * (C/D^2)
Formula 4 E = 2560 * (F/D^2)
Formula 5 E = X * T * L
where:
E =
measurement error in millimetres.
W = weight of float line in grams/metre. (6.0 for beaded cable, 1.2 for
0.4mm cable).
D = diameter of float in millimetres. (D^2 means diameter squared).
C = weight of lead counterweight in grams.
F =
frictional force within the instrument in grams (F = 1.0 for Model
6541A Water Level Instrument.)
T =
ambient temperature change in degrees Celsius.
S =
shift of line level in metres.
