Nova-Tech pH-Dissolved Oxygen Meter - Manual (Page 34)

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14. Appendix 1: Meter Theory
Dissolved oxygen levels in natural and waste waters depend on the physical,
chemical, and biochemical activities in the water body.
This meter uses a galvanic probe. It consists of a cell that contains electrolyte and
that is enclosed by a selective membrane, and two metallic electrodes. The mem-
brane is practically impermeable to water and ionic dissolved matter, but is perme-
able to oxygen and a few other gases.
The cathode consumes the oxygen passed through the membrane, and produces an
electric current in the probe. This current is proportional to the partial pressure of
oxygen in the sample.
Since the cathode consumes the oxygen in the sample, it is essential that the fluid
must flow past the sensor to maintain accurate readings.
The solubility of oxygen in water varies with barometric pressure and temperature,
and decreases as salinity increases. For the most accurate DO readings, you need
to compensate for these factors. This meter automatically compensates for tempera-
ture readings. It also allows you to enter a salinity correction factor and the baromet-
ric pressure to correct for this variability.
15. Appendix 2: Pressure vs. Altitude table
Barometric pressure affects DO readings, therefore this meter lets you enter the cor-
rect barometric pressure at your altitude. If you do not have equipment that lets you
measure the exact barometric pressure at your altitude, you can estimate it using the
chart below.
If you change the barometric pressure setting from its factory setting (760 mm Hg),
the % saturation calibration value in air will automatically adjust to a value other
than 100% (see “corrected % saturation value” column below). The adjusted value is
correct for the new barometric pressure setting.
See page 39 or 47 for information on how to adjust the barometric pressure.
Pressure vs Altitude
Altitude (ft) Pressure (mm Hg) Corrected % saturation value
0 (Sea level) 760 100
500 746 98.1
1000 732 96.3
1500 720 94.7
2000 707 93.0
2500 694 91.3
3000 681 89.6
3500 668 87.8
4000 656 86.2
4500 644 84.6
5000 632 83.0
5500 621 81.6
6000 609 80.0
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