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Comparison of oxygen measurement technologies

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In this article, Fusetrick specialists compare the main oxygen measurement methods. Luminescent Dissolved Oxygen (LDO); and Electrochemical Technology (EC).

LDO (Optical sensor)

EC (Electrochemical sensor)

Working principle:

LDO sensors work on the principle of fluorescence quenching. They use a fluorescent material that emits light when excited by a light source. Oxygen molecules in the sample quench this fluorescence, and the degree of quenching is proportional to the oxygen concentration.

EC sensors are based on an electrochemical reaction between oxygen and a sensing electrode. Oxygen diffuses through the permeable membrane and reacts with the electrode, generating a small electric current. The current is proportional to the oxygen concentration.

	LDO	EC
Measuring ranges	Liquid: 1) 0 ppb – 2 ppm; 2) 30 ppb – 35 ppm; Gas phase: 1) 0 – 4,2 %O2; 2) 0 – 50 %O2;	Liquid: 1) 0.1ppb – 20ppm; 2) 1ppb – 100ppm; 3) 10ppb – 400ppm Gas phase: 1) 0.25 Pa – 50 kPa; 2) 5 Pa – 200 kPa;3) 20 Pa – 1000 kPa
Response time	t90 < 10 s	1) t90 < 7,2 s; 2) t90 < 38 s ;3) t90 < 2,5 m
Accuracy and Precision	Liquid: 1) +/- 1 ppb; 2) +/- 50 ppb; Gas phase: 1) +/- 0,002%O2; 2) +/- 0,03%O2	1% of the measured value or the lowest value, whichever is greater
Interference	Any type of radiation (including ionising and electromagnetic radiation, etc.)	Chemical interference (effect of chemical composition on measurement)
Lowest detection point	Liquid: 1) 2 ppb; 2) 80 ppb Gas phase: 1) 0.003 %O2; 2) 0.04 %O2	Liquid: 1) 0.2 ppb; 2) 2 ppb; 3) 21 ppb Gas phase: 1) 0.5 Pa; 2) 10 Pa;3) 40 Pa**

1 kPa=1%=10000ppmV

Calibration:

LDO sensors require periodic calibration, the following types of calibrations are possible:

Zero point calibration (requires ultrapure nitrogen 5.0 or better)
Two-point calibration. This involves calibrating a zero point and additionally a point of known concentration. This method is most commonly used when the sesor has a wide measuring range or increased accuracy over the entire range is required. As a second point it is possible to use air at atmospheric pressure or any other gas mixture with a known concentration.

EC sensors are based on an electrochemical reaction between oxygen and a sensing electrode. The main methods of calibration are:

Calibration at ambient atmospheric pressure.
Calibration at a given known concentration. (When calibrating, the sensor may be in a pipeline).

In both cases, oxygen diffuses through the permeable membrane and reacts with the electrode, generating a small electric current. From this the calibration coefficients are calculated.

Maintenance:

LDO sensors generally require less maintenance than EC sensors. They are less susceptible to contamination and drift over time. Typical service intervals are 6-12 months. In some cases it is possible to extend the service interval to 12-18 months.

EC - sensors may require more frequent maintenance due to problems such as contamination of the sensing electrode and calibration drift. It is recommended that maintenance be performed every 1-3 months. In some cases it may be possible to extend the interval to 6 months.

Price:

LDO sensors are comparable in cost to Electrochemical sensors. However, there are also budget versions of analysers available.

EC - have a slightly higher cost, and require more regular maintenance.

Applications:

LDO sensors are often preferred in applications where fast response times and minimal maintenance are important, such as aquaculture, beer and alcohol production, water treatment and others.

EC sensors are suitable for a wide range of applications, including water quality monitoring in wastewater treatment plants, environmental monitoring in natural water bodies and industrial processes where high accuracy is a priority, EC sensors are essential for boiler water monitoring in the power industry, including nuclear power plants in the "dirty" zone as the sensor has no digital electronics inside and is therefore not affected by various radiations, Pharmaceutical industry, especially where a minimum de zone is required. It is also indispensable for monitoring the "purity" of gases, which is especially important for the pharmaceutical and beer and alcohol industry, as it has a direct impact on the quality and stability of the product.

In general, LDO and EC technologies have their strengths and weaknesses and are widely used and the choice between them depends on factors such as application requirements, budget and environmental conditions.

*- In case of correct sample preparation for measurement, namely measurement at atmospheric pressure. The reasons are described in this article. The solution developed by Fusetrick is presented in the link.
**- In case of correct sample preparation for measurement, namely measurement at atmospheric pressure. The reasons are described in this article. The solution developed by Fusetrick is presented at the link.