COMPARISON OF XCTD CTD DATA

XCTD (eXpendable Conductivity Temperature Depth) probes, developed rec ently by SIPPICAN Inc., have been used simultaneously with a CTD sonde in order to test, in the field, their performance and accuracy (inter preted as +/-2 standard deviations pf the XCTD-CTD differences). We ha ve taken advantage, during the THETIS-I experiment in March 1992, of b oth the homogeneous and the stratified areas encountered in winter in the northern part of the western Mediterranean Sea to differentiate th e errors due to the experimental conditions from those effectively due to the sensors. Although some intrinsic problems are evident, so that only seven out of the nine probes considered for comparison are usabl e, the accuracy specified by the manufacturer for the temperature (AT= +/-0.03 degrees C) is reached after standard processing, while the acc uracies in conductivity, salinity and potential density are AC similar to+/-0.06 mS/cm (the specified value is AC=+/-0.03 mS/cm), AS similar to+/-0.04 and A sigma(theta)+/-0.03 kg/m(3). However, when the experi mental errors (in situ natural variability, relatively rough estimatio n of the XCTD depth) are considered, it appears that the effective acc uracies of the XCTD sensors are better than +/-0.02 degrees C and +/-0 .04 mS/cm, that is to say better than and close to the specified value s of +/-0.03 degrees C and +/-0.03 mS/cm. Occasional offsets in conduc tivity can further be well corrected for by using a temperature-salini ty relation in some limited depth range and area where this relation i s known to hold well; the conductivity-sensor accuracy then significan tly improves to AC similar to 0.02 mS/cm resulting, for our study area , in corresponding salinity and potential density accuracies of AS sim ilar to+/-0.03 and A sigma(theta)similar to+/-0.02 kg/m(3). Thus, such instruments promise to be useful tools for many experimental studies. Complementary comparisons, performed with new versions of the XCTD pr obes under less convenient experimental conditions, are also presented . Copyright (C) 1996 Elsevier Science Ltd.