EQUIPMENT AND SUPPLIES--Modification of the field calibration chamber for the YSI dissolved oxygen meter March 7, 1979 QUALITY OF WATER BRANCH TECHICAL MEMORANDUM NO. 79.08 (B) Subject: EQUIPMENT AND SUPPLIES--Modification of the field calibration chamber for the YSI dissolved oxygen meter This is to call your attention to a potentially significant source of error in measurement of dissolved oxygen with the YSI model 5739 dissolved oxygen probe used in conjunction with the YSI model 5075 field calibration chamber. The problem occurs because the pressure compensation diaphragm located on the side of the probe body in this model will not fit inside the field calibration chamber. As a result, when the chamber is submerged in water for calibration, the pressure transmitted to the inside of the membrane is hydrostatic pressure and the pressure on the outside of the membrane is atmospheric pressure. (Fig. 1). This pressure imbalance generally results in a high reading on the meter and leads to incorrect meter calibration. Figure 2 shows the potential error that can be encountered by using the probe with an unmodified calibration chamber. To remove this potential error, a modification kit for the field calibration chamber has been designed and is available at no cost from the Operations Section, Reston, VA. The kit extends the air chamber 2 inches so that the pressure diaphragm is enclosed within the chamber during calibration (Fig. 3). The pressure is then equalized and no calibration error will occur. No other modification is needed except the rubber stopper is moved up the probe housing to the smooth area above the manufacturer's logo. Modified calibration chambers can still be used in conjunction with the older style dissolved oxygen probes that do not have the pressure compensation diaphragm feature. However, it is recommended that all calibration chambers be modified even though no new probes are in use at the present time. The modification kit consists of three pieces: a 2 inch extension for the chamber, an adapter sleeve for connecting the extension to the original section, and a retainer ring that mounts above the first ring. The components should be silver soldered to the original chamber as diagrammed in Figure 3. The adapter sleeve soldering must be watertight. To order modification kits, write to Wayne Hammond, Operations Section, MS 405, Reston, VA, or telephone, FTS 928-6861. There is no charge for the kits but the districts must do their own modification work. The YSI corporation has been notified of this calibration problem and hopefully new calibration chambers will be of sufficient length to enclose the pressure diaphragm within the chamber during calibration. R. J. Pickering Chief, Quality of Water Branch WRD Distribution: A, B, S, FO, PO