PASCO PS-2108 Dissolved Oxygen Sensor User Manual

Page 16

PASPort Dissolved Oxygen Sensor

Model No. PS-2108

2. Overlying Gas Pressure Effects

• Henry's Law states that the solubility of a gas in a liquid is affected by the pressure of the

gas above the liquid-gas interface.

P=gas pressure (or partial pressure)
C=concentration of dissolved gas
k = Henry’s Law Constant (differs with temperature, gas, and solvent)

• Changing pressures, while maintaining the same temperature, gas, and solvent allows one

to draw a relationship that excludes Henry's Law Constant:

• If a gas-liquid system starts at equilibrium and the partial pressure of a gas is increased

(with other factors such as temperature remaining unchanged) the amount of gas that is
dissolved in the liquid at the new equilibrium will increase.

For example:
P1 = 1012 millibars (approximately sea level)
C1 = 9 mg / L
P2 = 790 millibars (approximately 6700 ft or 2050 m above sea level)

• The most significant way in which this effect manifests itself is the reduced maximum

possible dissolved oxygen at higher altitudes. This is the reason why barometric pressure
at the sample site is important. Higher altitudes lead to a decrease in atmospheric pressure
(all atmospheric gases), which leads to a decrease in the partial pressure of oxygen (as well
as all of the other atmospheric gases). Please note the weather services often report
barometric pressure for a location, which is normalized for the same local conditions as if
reported at sea level, that is, the values are corrected for altitude.

• The decrease in atmosphere pressure with increase in altitude is not linear. Please see the

altitude correction table in order to compensate for altitude differences.

3. Hydrostatic Pressure

• Water under significant pressure may hold a higher amount of dissolved gas, compared to

water under less pressure. Plumbing systems are commonly under significant pressure.
The gases dissolved in water that is in a plumbing system may be at equilibrium at that
pressure, but when the pressure is released or decreased by an act such as drawing the
water from a tap into a container at atmospheric pressure, the solution may be
supersaturated. This phenomenon is similar to opening a can of carbonated beverage or
soda pop. However, the dissolved gases may take several seconds to several hours to reach

P=kC

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9mg/L

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

1012 milibars





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790 milibars

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



7 mg/L

