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Tech Tips | Gauges

Gauge Tech Tips

The Importance of Initial Zero Readings

Initial zero readings are very important in helping ensure improved accuracy with your measurements.

An initial zero reading establishes a baseline measurement, from which all subsequent data values are calculated.

In general, the initial zero reading is obtained by taking a measurement of the instrument on-site, prior to installation. Use of this field zero reading, instead of using the factory zero reading, will improve the accuracy of your calculated data values. On-site zero readings should closely coincide with the factory zero reading provided on the instrument’s calibration report (after any necessary temperature and barometric pressure corrections have been made). There are several different ways of taking an initial zero reading, and the exact steps involved vary by instrument. Consult the instruction manual provided with the instrument for specific instructions.

Troubleshooting Vibrating Wire Gauges with an Ohmmeter

Has the reading from a GEOKON vibrating wire instrument become unstable?

Has the instrument stopped working altogether?

This may be a sign that the cable has been broken, cut, or crushed. A quick and easy way to determine if the cable has been damaged is to use an ohmmeter to measure the resistance between the gauge leads. The expected resistance for the various wire combinations can be found on the VW Sensor Resistance Worksheet.

  • If the resistance is very high (>1 megohm) or infinite, the cable is probably broken or cut.
  • If the resistance is much lower than the gauge resistance specified in the instruction manual, the gauge conductors may be shorted.

Cable resistance should be taken into account, especially for long cables. Cable resistance for 22 AWG wire is approximately 14.7Ω per 1000 ft.(48.5Ω per km). Multiply this factor by two to account for both directions.

The internal thermistor can be checked in a similar manner by connecting the thermistor leads of the instrument to the ohmmeter. Standard thermistors have a resistance of 3,000Ω at 25 °C. High temperature thermistors have a resistance of 10,000Ω at 25 °C. For other temperatures, consult the thermistor resistance to temperature conversion tables in the instruction manual.

Unstable Gauge Readings

A constantly fluctuating vibrating wire reading may be caused by electrical interference.

Common sources of electrical interference include, 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.

Another common cause of reading instability is a misconnection between the instrument conductors and the readout/datalogger, or incorrect readout/datalogger settings. Consult the instruction manual provided with the instrument to verify that all gauge conductors have been properly connected, and that the readout or datalogger has been set up correctly for the type of gauge being read. (If using a datalogger to record readings automatically, make sure the swept frequency excitation settings are correct.)

If the above suggestions do not correct the problem, the readout or datalogger may be malfunctioning. Try connecting a different gauge to see if the problem persists.