Advantages of Infrared Detection over the Electrochemical
Principle when Measuring Carbon Dioxide in Manholes
Author Dipl.-Ing. Bernhard Kleine, GfG

Before entering sewage plant drains, landfills, overflow reservoirs, or revision works in containers, multi-gas detectors must be used to protect personnel against combustible and toxic gases. Nowadays, in sewage plants and landfills a 4 to 6-gas detector like the G750 is commonly used.
G750

The following hazards must be monitored:

  • Combustible gas-vapor-air mixtures, to prevent an explosion.
  • Oxygen deficiency, to make sure that entering without a breathing mask is possible. Oxygen surplus, to avoid explosion hazards.
  • Hydrogen sulfide, which builds up if organic processes are carried out without oxygen. Even small concentrations of hydrogen sulfide are toxic and act as a neurotoxin.
  • Carbon dioxide (CO2), a gas that builds up in all organic processes and, under certain geologic circumstances, even diffuses into a drain from the soil or from a balance reaction with water. The carbon dioxide (CO2) is, contrary to most other gases, 1.5 times heavier than air and therefore builds up in badly ventilated rooms in the ground. Once it is there it is hard to remove without actively ventilating the room. Its danger lies in displacing oxygen and in its toxicity, which starts at very low CO2 concentrations. This explains why the TLV (Threshold Limit Value) is set so low. CO2 exists in a concentration of 6% volume in the alveoli (small cells in the lung) and gets breathed in by the alveolus membrane of the lungs. If the CO2 concentration in the ambient air increases, this leads to a change in CO2 concentration in the lungs and an increase of the CO2 amount in the blood. Therefore, ph-changes into the acidic range lead to irritation of the respiratory system. Affected organisms try to reduce the CO2 surplus by increasing respiration. The most sensitive system is the central nervous system. Reactions to high CO2 concentrations include depression, tiredness, and narcotic effects leading to comas.
Carbon Dioxide - CO2
Effect / Toxicity
Threshold limit value / odor threshold
5,000 ppm (= 0.5% volume) odorless

Effects of CO2 in relation to concentration

20 Dead in a few seconds
10 Candle burns out
Convulsions, unconsciousness, death
7.0 Dizziness, nausea, signs of paralysis
Circulatory of sturbance in train, headache
3.0 Exhaled air
Hard respiration, increased pulse
1.0 Short Term Exposure Level (STEL)
0.7 Big crowds in rooms (e.g. cinemas)
0.5 Maximum workplace concentrations (TLV = 0.5% volume)
0.3

0.1		 High concentrations in offices
0.07 Ambient air in cities
0.03 Fresh air

Measuring carbon dioxide via the displacement of oxygen in air is, although often discussed, impossible. Due to the fact that only 1/5 of the ambient air consists of oxygen, the addition of gas only displaces 1/5 of the amount of oxygen. Most gas detectors give a first alarm for oxygen deficiency at 19% volume. This means that only at a presence of 1.9 * 5 = 9.5% volume CO2 the first alarm would sound, and only at 19.5% volume oxygen would the main alarm be activated. At these concentrations you already face the danger of unconsciousness, death through suffocation, and paralysis of the nervous system within a few seconds.

While the detection principles for EX-, OX-, and hydrogen sulfide measurements in portable gas detectors have been in use for decades, the detection of CO2 with portable detectors has only recently become possible. You have to differentiate between two different sensor techniques for carbon dioxide measurement: electrochemical and infrared.

 
Electrochemical Sensor
Infrared Sensor
Sensor Lifetime
1 year
More than 5 years

The electrolyte of an electrochemical CO2 sensor decomposes after a certain time span. The lifetime of the sensor can dramatically deteriorate in the presence of CO2 or other gases.

Manufacturer’s notice: Lifetime: 1 year, reduced by CO2 exposure

Costs of sensor replacement (once a year): After the first year of operation, the electrochemical sensor is already more expensive than the IR sensor.
The infrared sensor has no mechanical parts that can wear out. Poisoning, as with electrochemical sensors, is impossible even at high gas concentrations.
Cross-sensitivity

Electrochemical CO2 sensors show a cross-sensitivity to many different gases: ammonia, chlorine, carbon monoxide, methanol, sulfur dioxide, nitrogen oxide, hydrogen, phosphine, and hydrogen sulfide.

In drains hydrogen sulfide is usually present.
Infrared sensors measure selectively. No other gas interferes with the CO2 measurement. Faulty alarms due to other gases are impossible. IR sensors have a comparably fast response time.
H2S Selective Filter

100 ppm of H2S decreases the CO2 measurement by 0.6% volume. A manufacturer of electrochemical sensors therefore stipulates that an H2S filter must be used. The filter only has a limited lifetime, which is reduced even more in the presence of H2S.

Manufacturer’s notice:
At 10 ppm H2S a filter has a lifetime of 15 days.

Cost per filter:
DM 71.00
The filter gets stuck to the sensor and this means that the sensor has to be torn apart to replace the filter.
No filter needed, due to the fact that H2S does not interfere with or disturb the IR sensor. The CO2 concentration is correctly displayed even with high amounts of H2S.
Long-term Sensitivity Shift
15% of the measurement value per month. According to the T031 information sheet of the BG-Chemie a calibration interval has to be chosen so as not to exceed a deviation of 5% of the measurement value. Therefore, an electrochemical CO2 sensor needs to be calibrated every 10 days. The customer will not recognize the lack of sensitivity without checking the
sensor.
Less than 0.4% of the measurement value per month. This means longer maintenance intervals and, as a result, reduced service costs.

 

Conclusion:
Nowadays, infrared sensing technology is the most appropriate for monitoring carbon dioxide (CO2) with portable instruments.
The short lifetime of electrochemical sensors, the cross-sensitivity, the high follow-up costs, and the extreme drift make using
an electrochemical sensor impossible not only for safety reasons, but also for economical reasons. As there is an adequate
supply of multi-gas detectors that use the infrared technique, this detection principle is preferred.