Total Dissolve Gas Sensors: Water Quality Monitoring

Why would I want to measure Total Dissolved Gas?

Total Dissolved Gas (TDG) is the sum of the partial pressures of all gases in a water sample.  Normally, the sum of gas partial pressures will be about one atmosphere (15 psi) for any water that is exposed to the atmosphere.  The partial pressure of each gas will be the same as that’s gas’s contribution to atmospheric pressure, i.e. about 20% oxygen, 79 % nitrogen, and 1% trace gases.

However, certain natural phenomena can change the sum of partial pressures and the relative constituents of the gas mixture.  For instance, an algae colony can raise the partial pressure of oxygen from 20% of the total pressure to 60% or more of the total as the algae create oxygen during photosynthesis.  Those same algae can reduce the Total Dissolved Gas pressure to dangerously low levels as it consumes oxygen during its respiration phase.

The most common reason for measuring Total Dissolved Gas is detection of abnormal gas pressures caused by turbulent water, for instance that in the spillway of a dam.  Water falling a significant distance before splashing into a lake or river can entrain enough gas to double the Total Dissolved Gas​ in the affected water volume.  And as fish swim into, and out of, the aberrantly high Total Dissolved Gas in water, sudden decreases in Total Dissolved Gas can rupture blood vessels in the gills, and sudden increases in Total Dissolved Gas can burst swim bladders.

Changes in long-term Total Dissolved Gas trends can signal the need for more detailed chemical study of the water and its contamination sources.

How is TDG Total Dissolved Gas measured?

Total Dissolved Gas is measured by creating a small gas pocket adjacent a pressure transducer, and isolating that gas pocket from the water with a polymer membrane.  Because the membrane is permeable to gases, there must be equilibrium between the partial pressures of the gases inside the gas pocket and the gases in the water

If the Total Dissolved Gas sensor is in air, it should produce a reading close to the local barometric pressure (about 760 mm Hg at sea level).  If the sensor is immersed in water in equilibrium with the air, the reading will not change.  If the sensor is immersed in water that is not in equilibrium with the air, the sensor reading will adjust upward or downward as gas travels across the membrane to reach partial-pressure equilibrium with the water.

Total Dissolved Gas sensors are factory-calibrated for slope response, but occasionally must be re-calibrated for zero setting.  This is done by calibrating the sensor to the local barometric pressure in air.

What shoud I know about TDG Total Dissolved Gas measurement in the field?

It is important to remember that the Total Dissolved Gas sensor’s polymer membrane tends to absorb water.  Water absorption makes it difficult for gas molecules to travel through the membrane, thus slowing the response time.  The change can be significant, from 5 minutes for a freshly immersed membrane, to 30 minutes or more for a membrane that has been in water for a few hours.

In some cases, water vapor (which is a gas constituent, just like oxygen and nitrogen) will condense inside the gas pocket.  This will further slow the response time because the trapped water must also be brought into equilibrium any time the Total Dissolved Gas changes outside the membrane.  As a remedy to this problem, soggy Total Dissolved Gas membranes may be removed and dried in air for a day or two, after any condensate inside the membrane has been shaken out.  If the instrument needs to be redeployed immediately, the Total Dissolved Gas membrane may also be swapped out with a fresh dry one.  Easy to replace membrane tips are available at a nominal cost.