Accuracy testing of steel and electric groundwater-level measuring tapes: Test method and in-service tape accuracy

The accuracy of groundwater-level tapes was investigated by developing a tape calibration method and device and testing the accuracy of a sample of groundwater-level tapes with the calibration method and device. The sample of tapes included in-service U.S. Geological Survey (USGS) Water Science Center steel and electric groundwater-level tapes.

The tape calibration method developed during the study is based on a National Institute for Standards and Technology (NIST) protocol, and compares the tape under calibration (test tape) with a NIST-traceable reference tape. The calibration method can be used to determine tape accuracy and tape corrections. The tape calibration device consists of an anchor and tensioner. The device allows individual tensioning of the test and reference tapes. The Vernier scale on the tensioner allows measurement of the difference (or displacement) between the test and reference tapes graduations with a resolution of 0.0005 foot (ft). The calibration method used with the calibration device has a repeatability (standard deviation) of 0.0011 ft.

The calibration device and proposed method were used to calibrate a sample of in-service USGS steel and electric groundwater tapes. The sample of in-service groundwater steel tapes were in relatively good condition. All steel tapes, except one, were accurate to ±0.01 ft per 100 ft over their entire length. One steel tape, which had obvious damage in the first hundred feet, was marginally outside the accuracy of ±0.01 ft per 100 ft by 0.001 ft. The sample of in-service groundwater-level electric tapes were in a range of conditions—from like new, with cosmetic damage, to nonfunctional. The in-service electric tapes did not meet the USGS accuracy recommendation of ±0.01 ft. In-service electric tapes, except for the nonfunctional tape, were accurate to about ±0.03 ft per 100 ft. A comparison of new with in-service electric tapes found that steel-core electric tapes maintained their length and accuracy better than electric tapes without a steel core. The in-service steel tapes could be used as is and achieve USGS accuracy recommendations for groundwater-level measurements. The in-service electric tapes require tape corrections to achieve USGS accuracy recommendations for groundwater-level measurement.