LABORATORY EVALUATION OF CONDUCTIVTY SENSOR ACCURACY AND TEMPORAL CONSISTENCY

The focus on specific conductivity (EC 25ºC ) in the Appalachian Coal Belt Region of the U.S. has highlighted the need to obtain accurate EC 25ºC measurements, particularly in light of the U.S. Environmental Protection Agency (USEPA) guidance that water discharged from mine sites in this region should have EC 25ºC levels less than 300-500 µS cm -1 .Being able to accurately determine the EC 25ºC levels of mine discharged waters has significant implications for the USEPA as well as mine operators particularly when EC 25ºC levels approach this designated threshold.Presently, a number of sensors are available on the market for recording EC 25ºC measurements; however, a detailed study comparing sensor performance under controlled conditions (e.g.temperature and EC 25ºC levels) has not been performed.The objectives of this paper were to 1) evaluate sensor measurement stability over time (i.e.consistency) and 2) evaluate sensor accuracy of four commonly used sensors YSI 6600 V2-4 data sonde, HOBO U-24-001, Solinst Model 3001 LTC Levelogger Junior, and In-situ Aqua TROLL 100 at seven temperatures, ranging from 0 to 35°C, for six NIST traceable EC 25ºC standards, ranging from about 5 to 10,000 µS cm -1 .Results indicated that three of the four sensors recorded consistent EC 25ºC values over time for the majority of the given temperatures while the Onset HOBO U24-001 displayed temporal fluctuations for most of the temperatures.Pair-wise comparisons demonstrated that these temporal fluctuations were present most often at the highest EC 25ºC tested, 10,000 µS cm -1 .With regards to accuracy, the Onset HOBO U24-001 consistently overestimated EC 25ºC values while the other sensors tended to underestimate EC 25ºC values.Examination of the individual sensors within each sensor types revealed that in many instances at least one sensor performed quite differently than the others of the same type.As such, careful attention should be paid to individual sensor performance, particularly when the sensor is used for regulatory enforcement.