To the nonprofessional, measuring groundwater levels may seem to be a process that does not require precise measurement. If you are not a groundwater technician and only want to know if there is water in your well, this may be the case.
Ground-water Measurements to Fractions of an Inch
Many ground-water investigations, however, require extreme accuracy in order to collect results that allow meaningful interpretation. The collection of long-term water level readings is a critical component of most hydrologic and hydrogeologic investigations that require multiple measurements. If accuracy is not maintained consistently, significant error may be introduced over the course of an analysis. For this reason, water tapes are typically demarcated at each one-hundredth of a foot.
Construction of Water Tapes
Many modern water tapes are constructed of a durable thermoplastic polymer with a wire-reinforced interior. The wires, usually of stainless steel or copper, serve two purposes: 1) to lend stiffness to the tape and 2) to carry electric current to and from the transducer at the bottom of the tape, which indicates when water is reached. Care is taken in the design and manufacture of these tapes to ensure that the tape and transducer are snag-resistant, hang vertically and do not cling to well casings.
Some groundwater measurement companies go so far as to claim that their water tapes are “non-stretch” or “stretch-proof,” which would be a desirable feature with respect to accuracy if only it were true.
The outer surface of a water tape is typically a durable polymer that resists curling or kinking, such as PVDF. This type of polymer is also referred to as thermosetting or thermoplastic. They become workable at high temperatures and return to a stable state after cooling.
Due to the long-chain molecules that make up any polymer, all are subject to some elastic stretch. Polymer molecular chains at rest are in a highly entropic state. In other words, they curl and fold in random ways. When tensile load is applied, these chains straighten and eventually form a crystalline organization if enough tension is applied.
Stretch Cannot Be Zero
Though the elastic stretch of many polymers may be negligible, it is not zero. Wires embedded in a polymer water tape are also subject to elastic stretch, though in this case it is due to increasing space between strong molecular bonds, not entropy. In the end, any water tape is subject to some amount of stretch. Furthermore, a common source of permanent stretch is snagging of the tape or transducer upon retrieval of a tape.
Compensating for Water Tape Stretch
Fortunately, except in wells where the ambient temperature is high or the depth is approaching 1,000 feet, the amount of stretch may not be sufficient to impair repeated measurements if the tape has been cared for and is properly calibrated side-by-side with a steel tape of known accuracy that is used solely for this purpose. Another method is to make simultaneous measurements of water level with a field tape and a calibrated steel tape and to compare measurements. Such measurements should be within 0.02 foot of one another.
Consult the Ground-Water Instruments Experts
If a water tape supplier claims they have discovered non-stretch water tape, you should look askance. The experts in groundwater measurement equipment at Heron Instruments, Inc. can explain in detail why stretch-proof tape is physically not possible should you require more information.