4
| PRIOR TO INSTALLATION | GEOKON
1.
Place the assembled piezometer, filter down, in a vacuum chamber that has
an inlet port at the bottom for de-aired water.
2.
Close off the water inlet and evacuate the chamber. The transducer should
be monitored while the chamber is being evacuated.
3.
When maximum vacuum has been achieved, allow de-aired water to enter
the chamber until it reaches an elevation a few inches above the piezometer
filter.
4.
Close off the inlet port.
5.
Release the vacuum.
6.
Observe the transducer output. It may take up to 24 hours for the filter to
completely saturate and the pressure to rise to zero.
7.
After saturation, the transducer should be kept in a container of de-aired
water until installation. If de-aired at the factory a special cap is applied to
the piezometer to maintain saturation.
3.1.3 SATURATING MODEL 4500C FILTER TIPS
WARNING!
The filter housing is not removable on the 4500C. Any attempt to
remove the filter stone or the housing will destroy the transducer!
If the pressure to be measured is less than 5 psi the filter stone must be
saturated. A hand operated vacuum pump and short length of half inch surgical
tubing is required. Hand pumps and tubing are available from the factory. (A
hand pump that has been used successfully is the MityvacII® by Lincoln
Industries Corp. of St. Louis, MO.)
The saturation procedure is as follows:
1.
Attach the tube to the transducer as shown in Figure 2.
2.
Fill the tubing with approximately two inches (five centimeters) of water.
3.
Attach the other end of the tube to the hand vacuum pump.
4.
While holding the transducer so that the water rests on the filter, but does
not enter the pump, squeeze the hand pump to initiate a vacuum inside the
tubing. This will draw the air out of the filter and the area behind it, replacing
it with water. A vacuum of 20 to 25" Hg. (50 to 65 cm Hg.) is enough for
proper air evacuation.
FIGURE 2:
4500C Saturation
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.
Note:
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. It will not matter if the filter stone is saturated when using a standard
stainless steel filter. However, if the piezometer is equipped with a ceramic high
air entry filter stone, then it must be saturated while taking the zero readings.
