Accuracy Testing of New and in-Use Electric Groundwater-Level Measurement Tapes
Electric water-level measurement tapes are generally assumed to be accurate to ±0.01 foot (ft), but little information is available from the manufacturers and USGS field office calibration and initial calibration studies by the USGS Hydrologic Instrumentation Facility (HIF) indicate that this accuracy is seldom attained. This study was designed initially to determine the accuracy of six models of new, unused electric groundwater tapes often used by the USGS for routine water-level measurements and groundwater studies. A tape-calibration system was designed and built at the HIF. The tape-calibration system compares the length of each electric tape to a calibrated-steel reference tape and measures each probe’s activation accuracy. The tape-length accuracy combined with the probe-activation accuracy gave the overall measurement accuracy of the tape. Results demonstrated that none of the electric-tape models tested consistently met the suggested accuracy of ±0.01 ft, and that most tapes were accurate to approximately ±0.05 ft per 100 ft without additional calibration. Precise calibration of in-use tapes is necessary to gain the required accuracy. Comparisons between results at established metrology laboratories and the tape-calibration system at the HIF indicate that the HIF system is adequate for calibration of USGS electric tapes, and a program has been established to retrieve all USGS field electric water-level tapes for evaluation and calibration. Results of this ongoing evaluation and calibration of in-use USGS electric tapes will be presented.