YSI 1003 [605103] pH/ORP (ISE) Sensor.The YSI 1003 combination pH/ORP sensor features a 'long-life' sealed gel reference, eliminating the need to refill. The ORP sensor features a platinum electrode for optimum performance. The YSI 1003 pH sensors have been carefully designed to perform under all ionic strength conditions, from seawater with a conductivity of 53,000 uS/cm, to "average" freshwater lakes and rivers with conductivities of 200 to 1500 uS/cm, to pure mountain streams with conductivities as low as 15 uS/cm, which has historically been the most difficult medium with respect to accuracy, quick response to pH changes, and minimal flow dependence.