Apera Instruments DO803 Luminescent Optical DO Probe with 1o-ft length is the replacement optical DO probe for Apera DO850 and DO8500 Portable Optical DO meters. The probe adopts state-of-the-art optical DO sensor, which has significant advantages compared to the traditional galvanic/polarography probes. The Optical DO probe does not consume oxygen in the liquid, hence there is no requirement for water flowing speed. Just insert the probe in the liquid, and get the readings when it's stable -- as simple as measuring pH. It takes only 1 to 3 minutes for reading stabilization. Frequent calibration is not necessary. Calibration only takes 1-2 minutes.