EVALUATION OF ENDECO 1184C DISSOLVED-OXYGEN RECORDERS FOR USE IN TEMPERATE ESTUARIES

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