RST Instruments VW2100, Инструкция по эксплуатации, Страница: 32 / 34

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P RST Instruments Ltd. Page 32
: U
SING THE
S
ECOND
O
RDER
P
OLYNOMIAL
TO
I
MPROVE THE
A
CCURACY OF THE
C
ALCULATED
P
RESSURE
Most Vibrating Wire Pressure Transducers are sufficiently linear (<0.2% FS) that the
use of a Linear Equation and a Linear Calibration Factor will satisfy most normal
output requirements. However, it must be noted that the accuracy of the calibration
data used to establish the Linear Calibration Factor is dictated by the accuracy of the
calibration procedure and apparatus, which is always <0.1% FS.
The level of accuracy for a Vibrating Wire Pressure Transducer can be improved,
especially when the transducer output is non-linear, by using the Second Order
Polynomial Expression, which is better suited to the real pressures than the Linear
Equation.
The Second Order Polynomial Expression has the following form:
P (pressure) = A(L)
2
+ B(L) + C
Where, L is the current Vibrating Wire reading (in B-Units) and A, B, and C are the
polynomial coefficients determined by the individual instrument calibration procedure.
Appendix A shows a sample calibration sheet for a Vibrating Wire pressure
transducer which has a comparatively low non-linearity. In this case, there will only
be a very small difference between the pressure value calculated by the Linear
Equation and by the Second Order Polynomial Expression.
In contrast, it is noted that the Second Order Polynomial Expression method will
provide more accurate pressure values for VW transducers which have a high non-
linearity (greater than 0.2% FS). The vibrating wire calibration sheet contains a
column labeled “Linearity Error (% Full Scale)”. This column displays the calculated
linear error percentage for the calibration steps. If the average of these percentage
values (usually 6) exceeds 0.2%, it would be advisable to carry out all pressure
calculations using the Second Order Polynomial Expression.
The Linearity Error (% Full Scale) is calculated as follows:
LE = [(Calculated Pressure
– Applied Pressure) / Full Scale Pressure] * 100%
The Second Order Polynomial Expression will provide a calculated pressure which is
more accurate to the actual pressure monitored and will contain less error. However,
it should be noted that where the accuracy of absolute pressure measurement is not
required, such as monitoring relative water level changes, it makes little difference
whether the Linear Equation or the Second Order Polynomial Expression is used.