YSI 58 - Manual (Page 19)

come in contact with the sample include FEP Teflon, acrylic plastic, ABS plastic, EPR rubber,
stainless steel, epoxy and the polyurethane cable covering.
¾ Long-term use, as for monitoring, in certain applications can magnify the effect of factors that
impair probe accuracy.
4.4.2 MEASURING OXYGEN IN FLUIDS OTHER THAN WATER
The YSI 58 is normally used for measuring the oxygen content of naturally occurring waters and
wastewaters. The % air saturation feature of the instrument additionally permits 02 measurement in
some non-water fluids including air, most gases, foods and some non-aqueous liquids.
Suitable fluids for measurement are those that do not attack the sensor materials and are of
sufficiently low viscosity to permit sample stirring across the probe’s membrane. Strong acids and
solvents capable of swelling or dissolving the probe’s ABS plastic body or EPR o-rings must be
avoided. See Section 3, Preparing the Probe, for a list of interfering gases.
The percent air saturation of any fluid not excluded in the description above may be measured
directly. The instrument is calibrated by the customary air calibration technique and measurement is
carried out just as in natural waters.
In measuring non-aqueous liquids, the mg/L mode should not be used. Such samples may have an
oxygen solubility or Bunsen coefficient significantly different from that automatically programmed in
the mg/L mode for water.
4.4.3 CALIBRATING TO DISPLAY OXYGEN PARTIAL PRESSURE IN mm HG
For some liquid measurements, and for most gas phase measurements, it may be desirable to read the
meter in oxygen partial pressure units such as millimeters of mercury (mm Hg). To calibrate the % air
saturation mode to mm Hg units (0 to 200.00 mm Hg range), proceed as follows:
1.
Remove the back cover and move the membrane selector switch to the 2 MIL position, BUT
DO NOT CHANGE THE MEMBRANE. The standard 1 MIL membrane is used in this
measurement. Replace the back cover.
2.
Place the probe in a constant room temperature air environment. Determine the room’s
oxygen partial pressure (Dry air at 760 mm Hg total pressure has an 0
2 partial pressure of
20.94% of the total pressure, or 159.1 mm Hg.)
∗
Example: 760 mm Hg x 0.2094 = 159.1 mm Hg.
3.
Adjust the 02 CALIB control until the meter reading (read as mm Hg instead of the marked
%) matches the oxygen partial pressure of the room air. Once calibrated, oxygen partial
pressure may be measured in any gas environment between 0 and 45°C, and at any pressure
from atmospheric to 100 psi (7 atmospheres). Vacuum conditions should be avoided because
the probe’s internal electrolyte can outgas and cause membrane distortion.
CAUTION: When making gas phase measurements, the operator must avoid rapid temperature
fluctuations. The thermal sensor located in the stainless steel tube on the oxygen sensor operated to
∗
L. Machts, “Atmospheric Oxygen in 1967 to 1970, “Science, Volume 168, June 26, 1970, pp. 1582-1594.
YSI Incorporated YSI 58 DO Meter 15