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3.2 Establishing an Initial Zero Reading
Vibrating Wire Piezometers differ from other types of pressure sensors in that they indicate a
reading when no pressure is exerted on the sensor.
It is imperative that an accurate initial zero
reading be obtained for each piezometer, as this reading will be used for all subsequent
data reduction.
Generally, the initial zero reading is obtained by reading the instrument prior to installation.
There are several different ways of taking an initial zero reading. The essential element in all
methods is that the piezometer needs to thermally stabilize in a constant temperature
environment while the pressure on the piezometer is barometric only. Because of the way the
piezometer is constructed, it usually takes 15 to 20 minutes for the temperature of all the
different elements to equalize.
A question may arise as to what to do with the filter stone while taking zero readings. If a
standard stainless steel filter is being used, it will not matter if the filter stone is saturated or not.
However, if the piezometer is equipped with a ceramic high air entry filter stone, then it must be
saturated while taking the zero readings.
It will be necessary to measure the barometric pressure only if the piezometer is unvented, and it
will be installed in a location that is subject to barometric pressure changes which require
correction, such as in an open well.
A piezometer sealed in place at depth could be recording
pressures in groundwater that is not hydraulically connected to the atmosphere, for which
barometric pressure compensation would be inappropriate. See Section 6.3 for more information
on Barometric corrections.
Calibration data is supplied with each gage, a factory zero reading taken at a specific temperature
and absolute barometric pressure is included. (See Figure 12 in Section 6 for a sample calibration
sheet.) Zero readings at the site should coincide with the factory readings within 50 digits, after
barometric and temperature corrections are made. Barometric pressures change with elevation at
a rate of approximately 3.45kPa (½ psi) per 300meters (1,000 ft.)
The factory elevation is +580
feet. All stated barometric readings represent absolute pressure uncorrected for height above sea
level. A thermistor is included inside the body of the piezometer for the measurement of
temperature.
NOTE REGARDING THE 4500C: The construction of this very slender vibrating wire
transducer requires a miniaturization of the internal parts, which consequently are somewhat
delicate.
Handle the transducer with care during the installation procedure.
Despite taking
every precaution to ensure that the transducer arrives unharmed, it is possible for the zero to shift
during shipment due to rough handling. However, tests have shown that though the zero may
shift, the calibration factors do not change. Therefore it is doubly important that an initial no load
zero reading be taken prior to installation.
Содержание 4500AL
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