Page 5-1
Rev. 0
5. Installation of vibrating w ire system
The typical layout for the uplift pressure pipe with its connecting fixtures and vibrating wire uplift pres-
sure meter is illustrated in figures 1.1 and figure 3.1 respectively.
A system of piping is installed at several stations in the dam in the first lift just above the contact be-
tween the foundation rock and the base of the dam. Drill holes with installed drainpipes extend below
the dam, both upstream and downstream of the grout curtain. The pipes extend into one or more of
the lower galleries in the dam.
The pipes are fitted with a tee-section and a vibrating wire uplift pressure meter for observing water
pressure. A shutoff valve is included on one leg of the tee. Although readings may be taken at any
time, it is common practice to leave the shut-off valve open and to close the system the day before
readings are to be taken. Then after readings are taken, the valve is reopened. Therefore in addition
to the existing uplift pressures, readings taken in this manner indicate the uplift pressure that may be
achieved if one or more systems of drains should become inoperative through blockage. The shut-off
valve may also be kept closed and opened only when the uplift pressure builds up.
5.1.
Prep arat io n o f t h e sen so r b ef o re in st allat io n
5.1.1
Remove the cable joint housing from the sensor. This gives access to the four pin terminal.
Two of the terminals are marked with red and black colors. These are internally wired to the
coil of the magnet assembly inside the sensor. The other two terminals are utilized in case a
thermistor is used for temperature measurement. Clean the terminals with a tooth brush.
NOTE:
Do not use any acetone for cleaning the terminals as it may damage the glass to metal
seal. Acetone should be used to clean the other portions of the sensor.
5.1.2 Check the working of the sensor as follows:
The coil resistance measured by a digital multimeter between the red and black pins, should lie
between 120-140 Ohm. In case thermistor is used, determine resistance at the room temperature
from thermistor temperature resistance chart in § 3.5. This resistance should be equal to that be-
tween pins marked green and white. For example, in case the room temperature is 25
o
C, this re-
sistance would be 3,000 Ohm.
The resistance between any lead and the protective armour should be > 500 m Ohm.
Connect the sensor to the TRITECH model TES-DI-51V portable readout unit and switch it on. The
display will show something like:
Freq: 2629.8 Hz
where the actual figure will vary depending on the transducer connected to the indicator.
For the uplift pressure meter, the initial reading (offset) in frequency should lie between 2,250 -
2,650 Hz. This initial reading on the portable readout unit should be stable.
Check whether sensor is responding to changes in pressure. A crude but a simple and very effec-
tive method of checking whether the sensor is responding to changes in pressure is as follows:
•
Connect sensor to the portable readout unit and remove ½” BSP adapter from the meter.
•
Press diaphragm with thumb and verify that frequency reading on the indicator decreases.
•
This change in reading ensures that the deformation produced by the pressure of the
thumb on the diaphragm is transmitted to the vibrating wire sensing element.
Shift the read-out unit display to the frequency
2
mode. Each sensor is provided with a test certifi-
cate giving relationship between applied pressure and output. The zero reading in frequency
2
given
in the test certificate should not differ from the current zero reading by more than 50 (x 10
3
) divi-
