Evaluation of ENDECO 1184C dissolved oxygen recorders for use in temperate estuaries

Pulsed Clarke-type electrodes, a relatively new type of dissolved oxygen (DO) sensor, were intended to minimize problems associated with sensitivity to flow and biofouling-problems which affect most oxygen sensors. The commercially available devices (ENDECO/YSI, Inc., Model 1184C), which include temperature and conductivity sensors and a data logger, are designed for coastal environmental monitoring at depths to 30 m. In order to evaluate their performance under field conditions, a series of field deployments and laboratory calibrations was conducted, and the instrument output was compared with precision Winkler titrations. Accuracy of reported DO concentrations varied both between instruments and with time. Errors typically ranged from 0 to 3 mg/l. Thus frequent calibration seems to be a requirement for the accurate use of these units. The recommended 1-point calibration update did not improve accuracy. In the laboratory, the response to varying DO concentrations at constant temperature was highly linear over a range of approx. 5–14 mg/l; both accuracy and response time decreased at low DO concentrations, somewhere between 0 and 1.5 mg/l. Potential sources of inaccuracy in DO measurements include (1) statistical error associated with the estimation of calibration constants, and (2) operation of the instrument near the extremes of the temperature range used to calibrate it. We found support for the claim that the instruments are relatively insensitive to fouling.