FAQ

Loligo^® Systems offers the three main types of oxygen sensors differing in measuring principle, response time, sensor size, maintenance requirement and pricing.

1. Optical oxygen sensors
For many invasive techniques, measurements in tiny volumes (e.g. micro respirometry) or applications which require high temporal and spatial resolution, fibre-optic oxygen sensors are the only solution.

Even for general purposes we recommend optical oxygen sensors featuring low maintenance requirements, high stability and accuracy, no electrical interference, no ground loop problems, and zero O₂ consumption by the sensor.

The main disadvantages is price, single-channel meters start at about €3750. A high temperature sensitivity of this technology and a fragile tip on the <50-140 nm micro sensors might also be a problem in some applications. The macro sensors, on the other hand, are very tough.

2. Galvanic cell oxygen electrodes
Galvanic type oxygen probes are inexpensive and rugged sensors producing a milliVolt signal for easy instrumentation without supplying power. 

We recommend galvanic probes for applications like respirometry and field measurements, and as a low-budget alternative to optical oxygen equipment, e.g. for multi-channel systems. They can be used with relatively long cables (+25 meters). We recommend using a galvanic isolation preamplifier between the probe and any data acquisition system to minimize possible ground loop problems.

The main disadvantage of galvanic probes is a relative long response time and oxygen self-consumption making them unsuitable for measurements in small volumes and in un-mixed samples.

3. Polarographic oxygen electrodes (or Clark type electrodes)

Low oxygen self-consumption and a small size make these sensors suitable for physiological setups like blood gas analysis and low volume respirometry.

However, Clark type electrodes require much maintenance, often membranes and electrolyte fluid should be changed on a daily basis, and polarographic amplifiers for these sensors are quite costly.

Thus, we recommend the E101 as a replacement oxygen electrode for customers with their own polarograhic oxygen meter, e.g. PHM meter from Radiometer Copenhagen, OM200 from Cameron Instr. Co. etc.

CALIBRATION PRINCIPLE
Instrument calibration is an essential first step in analytical and measurement procedures. The rationale for a two-point calibration is to provide two points of reference with which to calibrate the instrument and, therefore, ensuring accurate measurements with your sensor of the unknown concentrations of your samples.  A two-point calibration between two extremes are advocated, e.g. 0 - 100 % air saturation.  For the low calibration point, we recommend using either nitrogen gas or sodium sulphite solution, for the high calibration point, bubble atmospheric air using an aerator.  Please see below for general step-wise instructions on how to carry out a two point calibration.  For specific instructions for a particular oxygen instrument, please click on the specific links.

GENERAL

1. Connect the probe to the oxygen instrument and turn on the instrument.
2. Place the tip of the probe in a mixed air-equilibrated water sample. This can be achived by purging atmospheric air into sample water, e.g. with an air pump and air stone.
3. Wait for the reading to stabilize. Then save this point as the HIGH calibration value (100% air saturation).
4. Place the tip of the probe in an oxygen free water sample (e.g. by purging nitrogen gas into sample water or by dissolving ~10 grams of Na₂SO₃ in 500 mL of distilled water).
5. Wait for the reading to stabilize. Then save this point as the LOW calibration value (0% air saturation).
6. Rinse the probe with distilled water.
7. Place the probe into the sample, and record the sample oxygen saturation when the reading is stable.

See below for a diagrammatic representation of sensor readings at:

1. 100 %  to 0 % air saturation
2. 0 % to 100 % air saturation
3. 100 % to 0 % and back up to 100 % air saturation.

100% to 0% air saturation	0% to 100% air saturation
100% to 0% to 100 % air saturation

The light from the blue-green LED excites the oxygen sensor to emit fluorescence. If the sensor spot encounters an oxygen molecule, the excess energy is transferred to the oxygen molecule in a non-radiative transfer, decreasing or quenching the fluorescence signal. The degree of quenching correlates to the partial pressure of oxygen in the matrix, which is in dynamic equilibrium with oxygen in the sample. The decay time measurement is internally referenced.