8.Troubleshooting

Maintenance and troubleshooting of Model 4000 Vibrating Wire Strain Gauges is confined to periodic checks of cable connections and maintenance of terminals. Once installed, the gauges are usually inaccessible and remedial action is limited. Should difficulties arise, consult the following list of problems and possible solutions. Return any faulty gauges to the factory. Gauges should not be opened in the field. For additional troubleshooting and support, contact geokon.

Symptom: Thermistor resistance is too high

oIt is likely that there is an open circuit. Check all connections, terminals, and plugs. If a cut is located in the cable, splice according to instructions in Section 4.4.

Symptom: Thermistor resistance is too low

oCheck all connections, terminals, and plugs for a short-circuit. If a short is located in the cable, splice according to instructions in Section 4.4.

oWater may have penetrated the interior of the strain gauge. There is no remedial action.

Symptom: Strain Gauge Readings are Unstable

oIs the readout box position set correctly? If using a datalogger to record readings automatically, are the swept frequency excitation settings correct?

oIs the strain reading outside the specified compressive or tensile range of the instrument? The gauge may have become too slack or too tight; inspect the data to determine whether this is a possibility. Loosen the two oval point setscrews in one of the mounting blocks. This will permit the internal spring to re-tension the gauge and the gauge will read again. Set the gauge to some new datum and re-tighten the setscrews. If the gauge does not respond to resetting, and if the old plucking coil will pluck a new gauge, replace the gauge.

oIs there a source of electrical noise nearby? Likely candidates are generators, motors, arc welding equipment, high voltage lines, etc. If possible, move the instrument cable away from power lines and electrical equipment or install electronic filtering.

oMake sure the shield drain wire is connected to ground.

oDoes the readout or datalogger work with another gauge? If not, it may have a low battery or possibly be malfunctioning.

Symptom: Strain Gauge Fails to Read

oDoes the readout or datalogger work with another gauge? If not, it may have a low battery or possibly be malfunctioning.

oIs the cable cut or crushed? Check the resistance of the cable by connecting an ohmmeter to the sensor leads. Table 2 below shows the expected resistance for the various wire combinations; you can use Table 3 below to fill in the actual resistance found. Cable resistance is approximately 14.7W per 1000 ft. (48.5W per km) of 22 AWG wire.

If the resistance is very high or infinite, the cable is probably broken. If the resistance is very low, the gauge conductors may be shorted. If a break or a short is present, splice according to the instructions in Section 4.4.

 

table 2: Sample Resistance

table 3: Resistance Work Sheet