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2. INSTALLATION
2.1 Preliminary Tests
It is always wise, before installation commences, to check the cells for proper functioning. Each
cell is supplied with a calibration report which shows the relationship between readout digits and
pressure and shows the initial no load zero reading. The cell electrical leads (usually the red and
black leads) are connected to a readout box and the zero reading given on the sheet is now
compared to a current zero reading. (See Section 3 for readout instructions.) The two readings
should not differ by more than ≈50 digits after due regard to corrections made for different
temperatures, barometric pressures and height above sea level and actual cell position (whether
standing up or laying down).
By pressing on the cell, it should be possible to change the readout digits, causing them to fall as
the pressure is increased.
Checks of electrical continuity can also be made using an ohmmeter. Resistance between the
gauge leads should be approximately 180 ohms, ±10 ohms. Remember to add cable resistance
when checking (22 AWG stranded copper leads are approximately 14.7Ω/1000' or 48.5Ω/km,
multiply by two for both directions). Resistance between the green and white conductors varies
with the temperature. Compare the measured resistance with the values given in Table 2 of
Appendix B. Resistance between any conductor and the shield should exceed 20 megohm.
2.2 Stress Cell Installation
Cells are positioned on the wall of the tunnel in two ways, one way to measure tangential stresses
and the other to measure radial.
2.2.1 Installing the Model 4850-1
The Model 4850-1 is designed to measure tangential stresses in the lining. Figure 3 shows
one method of installation using short pieces of steel rebar grouted inside short boreholes
and protruding into the area where the lining will be placed.
The pressure cells are tied to these rebars using soft iron wire connected to the lugs at the
corners of the cell. The cable is fixed firmly to other pieces of rebar or to the reinforcing
mesh, if one is used, and is strung out to the readout location which typically is made of a
metal junction box with removable hinged cover. The cable is terminated inside this box.
Sufficient cable is coiled inside the box to allow it to be pulled out and connected to a
portable readout box.
Note: It is very important that the concrete makes intimate contact with the
pressure cell. Therefore, the concrete should be sprayed on, first from below and
then, after removing any rebound material, from above. The person spraying the
concrete should receive special instructions so that no open shadow zones are
created next to the cell.
3
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