EQUIPMENT AND SUPPLIES--Modification of the field calibration chamber for the YSI dissolved oxygen meter

                                             March 7, 1979


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 

                            R. J. Pickering
                            Chief, Quality of Water Branch

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