Tank Level Measurement: Draw-Down Calibration Column vs. Poly Tank Markings
CheckPoint recommends using a draw down calibration column for flow rate reading and calibration, because the device has a very low dimensional tolerance and allows for high accuracy measurements. Such columns also act as great aids to field-verify and calibrate flow meters of varying technology types. CheckPoint considers using poly tank draw-down only for very rough overview tank level status, because they are designed for inexpensive storage and not precise measurement. As such, they should never be considered as tools to determine accurate pump flow rate. Some of the reasons are as follows:
- The industry standard allows for up to 3% tolerance in any dimension which equates to a maximum 27% volume variance. The tanks are almost never installed at 100% level, which reduces the accuracy of the volume marks, especially in horizontal tanks, and different operators record their measurement from different positions on the tank, compounding the issue.
- The walls are translucent, and sometimes discolored or stained, making it hard to see the chemical line exactly. The sidewall volume marks are typically in very large graduations, so it leaves the operator guessing the value in between marks with a very high margin of error.
- The tank and chemical expand and contract depending upon temperature throughout the day, which changes the liquid level relative to the sidewall marks.