GWPD 4—Measuring water levels by use of an electric tape
PURPOSE: To measure the depth to the water surface below land-surface datum using the electric tape method.
Materials and Instruments
- An electric tape, double-wired and graduated in feet, tenths and hundredths of feet. Electric tapes commonly are mounted on a hand-cranked and powered supply reel that contains space for the batteries and some device (“indicator”) for signaling when the circuit is closed (fig. 1).
- An older model electric tape, also known as an “M-scope,” marked at 5-foot intervals with clamped-on metal bands (fig. 2) has been replaced by newer, more accurate models. Technical procedures for this device are available from the procedures document archives.
- A steel reference tape for calibration, graduated in feet, tenths and hundredths of feet
- Electric tape calibration and maintenance equipment logbook
- Pencil or pen, blue or black ink. Strikethrough, date, and initial errors; no erasures
- Water-level measurement field form, or handheld computer
- for data entry
- Two wrenches with adjustable jaws or other tools for removing well cap
- Key for well access
- Clean rag
- Cleaning supplies for water-level tapes as described in the National Field Manual (Wilde, 2004)
- Replacement batteries
Data Accuracy and Limitations
- A modern graduated electric tape commonly is accurate to +/– 0.01 foot.
- Most accurate for water levels less than 200 feet below land surface.
- The electric tape should be calibrated against an acceptable
- steel tape. An acceptable steel tape is one that is maintained in the office for use only for calibrating tapes, and this calibration tape never is used in the field.
- If the water in the well has very low specific conductance,
- an electric tape may not give an accurate reading.
- Material on the water surface, such as oil, ice, or debris, may interfere with obtaining consistent readings.
- Corrections are necessary for measurements made from angled well casings.
- When measuring deep water levels, tape expansion and stretch is an additional consideration (Garber and Koopman,