Dissolved Oxygen Technical Education

DO Probes

Dissolved Oxygen (DO) is the term used for the measurement of the amount of oxygen dissolved in a unit volume of water. In water quality applications, such as aquaculture (including fish farming) and waste water treatment, the level of DO must be kept high. For aquaculture if the DO level falls too low the fish will suffocate. In sewage treatment, bacteria decompose the solids. If the DO level is too low, the bacteria will die and decomposition ceases; if the DO level is too high, energy is wasted in the aeration of the water. With industrial applications including boilers, the make-up water must have low DO levels to prevent corrosion and boiler scale build-up which inhibits heat transfer.

Although dissolved oxygen (DO) is usually displayed as mg/L or ppm,  DO sensors do not measure the actual amount of oxygen in water, but instead measure partial pressure of oxygen in water. Oxygen pressure is dependent on both salinity and temperature.

There are two fundamental techniques for measuring DO— galvanic and polarographic. Both probes use an electrode system where the DO reacts with the cathode to produce a current. If the electrode materials are selected so that the difference in potential is -.5 volts or greater between the cathode and anode, an external potential is not required and the system is called galvanic. If an external voltage is applied, the system is called polarographic.

• Galvanic probes are more stable and more accurate at low dissolved oxygen levels than polarographic probes.
• Galvanic probes often operate several months without electrolyte or membrane replacement, resulting in lower maintenance cost.
• Polarographic probes need to be recharged every several weeks of heavy use.

Galvanic DO sensors consist of two electrodes, an anode and cathode which are both immersed in electrolyte (inside the sensor body). An oxygen permeable membrane separates the anode and cathode from the water being measured.  Oxygen diffuses across the membrane.   It interacts with the probe internals to produce an electrical current (more detail is shown below the DO sensor graphic).    Higher pressure allows more oxygen to diffuse across the membrane and more current to be produced.  The actual output from the sensor is in millivolts.  This is achieved by passing the current across a thermistor (a resistor that changes output with temperature). 

V = i * R,

V is output in Volts, i = current

R is resistance from thermistor in ohms

The thermistor corrects for membrane permeability errors due to temperature change.  In other words, increasing permeability at higher temperature allows more oxygen to diffuse into the sensor, even though the oxygen pressure has not changed.  This would give falsely high DO if the thermistor were not used.

To represent sensor output in ppm or mg/L, the temperature of the water must be known.   A separate temperature sensor can be used or one can built into the sensor. This is independent from the thermistor connected between the anode and cathode to compensate for membrane permeability changes due to temperature change. 

Some characteristics of membrane DO probes are:

• The pH of the solution does not affect the performance of membrane probes.
• Chlorine and hydrogen sulfide(H2S) cause erroneous readings in DO probes.
• Atmospheric pressure (altitude above sea level) affects the saturation of oxygen. DO probes must be calibrated for the barometric pressure when reading in mg/l (ppm).
• Membrane thickness determines the output level of the probe and the speed of response to change in DO levels.
• Salinity correction must be made

The cathode is a hydrogen electrode and carries a negative potential with respect to the anode. Electrolyte surrounds the electrode pair and is contained by the membrane. With no oxygen, the cathode becomes polarized with hydrogen and resists the flow of current. When oxygen passes through the membrane, the cathode is depolarized and electrons are consumed. The cathode electrochemically reduces the oxygen to hydroxyl ions:

O2 + 2 H2O + 4 e- = 4 OH-

The anode reacts with the product of the depolarization with a corresponding release of electrons.

Zn + 4 OH- = Zn(OH)42- + 2e-

The electrode pair permits current to flow in direct proportion to the amount of oxygen entering the system. The magnitude of the current gives us a direct measure of the amount of oxygen entering the probe.

Because all of the oxygen entering the probe is chemically consumed, the partial pressure of oxygen in the electrolyte is zero. Therefore, a partial pressure gradient exists across the membrane and the rate of oxygen entering the probe is a function of the partial pressure of oxygen in the air or water being measured.

Since the partial pressure of dissolved oxygen is a function of temperature of the sample, the probe must be calibrated at the sample temperature or the probe’s meter must automatically compensate for varying sample temperature. Note that this thermal effect is different from the thermal response of the membrane discussed above.

The reading of a DO probe must be corrected for the amount of salt in the sample. As seen in the chart below, the salt in solution will reduce the actual concentration of oxygen.

In all DO Probes, the membrane/sample interface should have a few cm/sec flow of the sample for precision performance. Without flow at the interface, the surrounding oxygen will be consumed and the local reading drops. The output of the probe increases(up to a point) with relative movement between the probe and sample.

Dissolved Oxygen Measurement

The amount of oxygen that a given volume of water can hold is a function of the atmospheric pressure at the water-air interface; the temperature of the water; and the amount of other dissolved substances (such as salts or other gases) in the water. Recall seeing bubbles in a pot of water just before it starts to boil. These bubbles are the air which was dissolved in the water at room temperature. When the water boils, the dissolved oxygen is ejected—warmer water contains less DO. When other substances, such as salts, are dissolved in a unit volume of water, there is less room for oxygen to dissolve—oxygen is less soluble than most salts

The following table shows the relationship of dissolved oxygen (mg/L) to temperature and salinity:

The relationship between temperature, salinity, and dissolved oxygen is approximated with the following exponential equation:

ln( C ) = -139.34 + (1.5757 x 105/T) - (6.6432 x 107/T2) + (1.2438 x 1010/T3)

- (8.6219 x 1011/T4) - S [1.7674 x 10-2 - (10.754/T) + (2.1407 x 103/T2)]

T = Temperature in degree Kelvin

S = Salinity in parts per thousand (ppt)

C = Concentration in mg/L

As the pressure of the air above the water is increased, more oxygen will be dissolved in the water. This increases the concentration of the dissolved oxygen. The solubility of a gas in a liquid is directly proportional to the pressure of that gas above the liquid—Henry’s law. This is often expressed as:

p = k C (C = concentration of DO)

If different gases are mixed in a confined space of constant volume and at a definite temperature, each gas exerts the same pressure as if it alone occupied the space. The pressure of the mixture as a whole is the total of the individual or partial pressure of the gases composing the mixture—Dalton’s law of partial pressures. The partial pressure of each gas is proportional to the number of molecules of that gas in the mixture. Air is 20.948% oxygen. When air bubbled through water, only 20% as much oxygen dissolves as would dissolve if pure oxygen were used instead of air, at the same pressure.

Concentration of dissolved oxygen is also measured in units of % saturation. % saturation is simply the ratio of the measured mg/L of dissolved oxygen divided by the mg/L of dissolved oxygen at saturation—as given in the above tables, saturation levels is dependent upon the temperature, salinity, and pressure. Since % saturation is a ratio, it is not affected by these conditions if the calibration at 100% saturation was performed under the same conditions.

Solubility of solutes as a function of temperature

(mg of solutes per liter of water):

With stationary, continuously monitoring Dissolved Oxygen Probes, the source of the oxygen being measured is air. Thus, Dissolved Oxygen in air or saturated water (mg/l or ppm) as a function of temperature is determined by:

Solubility (ml/L) x Density (mg/ml) x % in air = saturated DO in mg/L (ppm)

Solubility (mg/L) x % in air = saturated DO in mg/L (ppm)

Increasing temperature usually increases the solubility of solids and liquids whereas it reduces the solubility of gases. Also keep your units straight--mg/L, ppm, ml/L, % saturation.