RST Instruments VW2100, Instruction Manual, Page: 11 / 34

VW2100 Vibrating Wire Piezometer
Instruction Manual
ELM0005P RST Instruments Ltd. Page 11
stone to allow direct atmospheric connection with the transducer diaphragm if
there is any question regarding the adequate saturation of the filter stone.
4.4 P
RESSURE
E
QUATION
(U
SING THE
VW2106 R
EADOUT
)
The VW2106 Readout Unit displays vibrating wire piezometer readings in frequency
units called B-Units, which equal Frequency
2
x 10
-3
, where frequency is in Hertz.
The B-Unit values represent absolute pressure and must be corrected for changes in
temperature and barometric pressure. B-Unit changes from the initial zero reading
are converted to the actual pressure changes using equations in Sections 4.4.1 and
4.4.2, which include corrections for temperature and barometric pressure changes.
4.4.1 Linear Equation
𝑷 = 𝑪𝑭(𝑳
𝟎
− 𝑳) − 𝑻
𝑲
(𝑻
𝟎
− 𝑻) + 𝑭(𝑺
𝟎
− 𝑺)
E
QUATION
1 L
INEAR
E
QUATION
Where:
P = Corrected Pressure in kPa
CF = Calibration Factor in kPa/B-Unit (From the VW Piezometer Calibration
Record sheet for each individual sensor)
L
0
, L = Initial and Current B-Unit reading (Frequency
2
x 10
-3
)
T
K
= Temperature Correction Factor in kPa/ºC (From the VW Piezometer
Calibration Record sheet in each individual sensor)
T
0
, T = Initial and current temperature readings in (ºC)
F = Barometric Pressure Constant = 0.1 kPa/mbar
S
0
, S = Initial and Current Barometric pressure readings in mbar
Example for a 350 kPa Range Piezometer
CF = 0.11594 kPa/B-Unit
L
i
= 8776 B-Unit
L = 7200 B-Unit
T
K
= - 0.03413 kPa/ºC
T
i
= 22.9 ºC
T = 5.0 ºC
F = 0.1 kPa/mbar