2.Installation
Before installing the load cell, it should be checked by connecting it to the readout box and taking a no-load reading. This reading, when compared with that given in the calibration data provided with the load cell, will show if the cell is functioning properly. The two readings should agree within about 50 digits (assuming that the same readout box is used for both readings).
When transporting load cells, do not pull on the cable and, in particular, do not carry the load cell by the cable. On the larger load cells, threaded holes are provided in the ends to allow eyebolts to be attached for lifting purposes.
Before installing the load cell, be sure to take the no-load reading. This reading is very important, since it will be subtracted from all subsequent readings to calculate the load. Note that each load cell has a different no-load reading that is not zero. See Section 3 for operation of the GK-502 and Vishay P3 Readout Boxes.
2.2.3Installation on Tie-Backs and Rockbolts
Load cells should be installed between flat steel bearing plates of sufficient thickness; one inch thick where load cell and jack are about the same size, and two to three inches thick where size mismatches are greater. The normal rolled finish on the plates is good. Plates may need to be machined flat if they are warped. Make sure that the bearing plates completely cover the load-bearing surface of the load cell. Centralize the rockbolt or tieback inside the load cell. Where the load cell I.D. is much bigger than the rockbolt or tieback, a centralizer bushing can be used.
Where the anchor block of a multi-tendon tieback bears directly on the load cell, make sure that the load cell bearing surface is completely covered by the anchor block. If the load cell is not completely covered, then make sure that the calibration was performed using the anchor block. If the calibration was performed without the anchor block then for best results consideration should be given to recalibration with the anchor block.
Shield the cable for possible damage from blasting or traffic. Protect the end of the cable or the cable connector from dirt by either using a cap on the connector or by storing the end of the cable and/or connector inside a small box. Figure 4 shows a typical load cell system.
2.3Cable Installation and Splicing
The cable should be routed to minimize the possibility of damage due to moving equipment, debris or other causes. The cable can be protected by the use of flexible conduit, which can be supplied by geokon.
Terminal boxes with sealed cable entries are available from geokon for all types of applications. These allow many gauges to be terminated at one location with complete protection of the lead wires. The interior panel of the terminal box can have built-in jacks or a single connection with a rotary position selector switch. Contact geokon for specific application information.
The cable used for making splices should be a high quality twisted pair type, with 100% shielding and an integral shield drain wire. When splicing, it is very important that the shield drain wires be spliced together. Always maintain polarity by connecting color to color.
Splice kits recommended by geokon employ casts, which are placed around the splice and are then filled with epoxy to waterproof the connections. When properly made, this type of splice is equal or superior to the cable in strength and electrical properties. Contact geokon for splicing materials and additional cable splicing instructions.
Cables may be terminated by stripping and tinning the individual conductors and then connecting them to the patch cord of a readout box. Alternatively, a connector may be used which will plug directly into the readout box or into a receptacle on a special patch cord.
Care should be exercised when installing instrument cables to keep them as far away as possible from sources of electrical interference such as power lines, generators, motors, transformers, arc welders, etc. Cables should never be buried or run alongside AC power lines; they will pick up the noise from the power cable, which will likely cause unstable readings. Contact the factory concerning filtering options available for use with the geokon dataloggers and readouts.
Since the purpose of the load cell installation is to monitor site conditions, factors which may affect these conditions should be observed and recorded. Seemingly minor effects may have a real influence on the behavior of the structure being monitored and may give an early indication of potential problems. Some of these factors include, but are not limited to: blasting, rainfall, tidal or reservoir levels, excavation and fill levels and sequences, traffic, temperature and barometric changes, changes in personnel, nearby construction activities, seasonal changes, etc.
Unlike other types of instrumentation available from geokon, load cells do not have any integral lightning protection components, such as transorbs or plasma surge arrestors. Usually this is not a problem, however, if the instrument cable is exposed, it may be appropriate to install lightning protection components, as the transient could travel down the cable to the gauge and possibly destroy it.
Recommended lightning protection is as follows:
■If the instrument is connected to a terminal box or multiplexer, components such as plasma surge arrestors (spark gaps) may be installed in the terminal box/multiplexer to provide a measure of transient protection. Terminal boxes and multiplexers available from geokon provide locations for the installation of these components.
■Lighting arrestor boards and enclosures are also available from geokon. These units install where the instrument cable exits the structure being monitored. The enclosure has a removable top to allow the customer to service the components or replace the board in the event that the unit is damaged by a lightning strike. A connection is made between the enclosure and earth ground to facilitate the passing of transients away from the load cell.
■Plasma surge arrestors can be epoxied into the instrument cable, close to the load cell. A ground strap then connects the surge arrestor to an earth ground, such as a grounding stake.
Consult the factory for additional information on available lightning protection.