Issues & Solutions for Calibrating a Six Gas Detector in the Wastewater Industry
Continuing improvements in workplace safety and the ongoing protection of workers within Industry is creating a new wave in the water/wastewater sector. Many organizations are moving towards measuring six (6) gases in sewage treatment and sewer lines throughout Australia.
Traditionally hydrogen sulphide, carbon monoxide, methane and oxygen were measured with many gas detection companies providing such instruments. In recent years more and more wastewater organizations began to measure volatile organic vapours (VOC) in ppm concentrations and in the past year ammonia in ppm.
Instrument manufacturers are rushing to meet the demand for this unique six sensor gas detection instrument of H2S/CO/NH3/CH4/VOC/O2.
What challenges does this create and how do water/wastewater organizations manage the calibration and bump testing of such an instrument?
Non-Corrosive and Corrosive Gas Mixtures:
There are very few gas mixtures which can be combined. Non-corrosive (reactive) gas mixtures such as methane, other hydrocarbons, organics, carbon monoxide, carbon dioxide and oxygen mix very well together. Adding hydrogen sulphide creates issues with cylinder materials changing from steel to aluminium but over the past twenty years H2S/CO/CH4/O2 has been a common stable gas mixture.
Most other corrosive/reactive gases cannot be mixed together. Ammonia must be stored in it’s own cylinder separate from the hydrogen sulphide.
PID & CH4:
The desire to measure low concentrations of VOC in ppm concentrations creates the need for a Photo Ionisation Detector (PID). Typically isobutylene is used as the calibration gas for the Photo Ionisation Detector/VOC sensor in concentrations of 100ppm. Although gas manufacturers can mix isobutylene with all the other gases and remain stable, the combination of methane with isobutylene creates a separate issue.
Methane in concentrations above 1% will reduce the sensitivity of the PID by as much as 60%.
This means a concentration of 100ppm isobutylene will show a response of 40ppm in the presence of 2.5% methane
As a result any gas mixture with CH4 and isobutylene cannot be used for calibration.
A mixture of H2S/CO/CH4/Isobutylene/O2 could be used for “Bump Testing” an instrument but it cannot be used for calibration. This is extremely important to understand so that there isn’t confusion in the work place. Any organization considering this type of mixture must make their users aware of the difference between calibrations and bump testing and that the response of the PID will be low when bump testing.
Based upon the reaction of gases and sensors the six gas detector will require:
- One cylinder of H2S/CO/CH4/O2
- One cylinder of Ammonia
- One cylinder of Isobutylene
Regulators and Tubing Materials
Corrosive/reactive gases create other issues which must be addressed correctly for effective calibration of the gas detection instrument.
Ammonia is highly corrosive and reacts with many materials. Therefore there are important best practice procedures you should/must follow.
It is highly recommended that only stainless steel regulators be used with ammonia. Although a nickel plated brass regulator will work initially, eventually the plating will corrode from the ammonia and expose the brass. This will create an opportunity for the ammonia to react with the brass potentially destroying the gas mixture.
You MUST use Teflon or Teflon lined tygon tubing. Ammonia adsorbs onto standard tygon tubing so you will have zero response from your detector with only a short length of tygon tubing. Use only Teflon based tubing.
Do not use the same regulator for the three different gas cylinders. Each gas mixture should have it’s own regulator and tubing. Using a regulator from one cylinder to the next will contaminate the gas mixture and potential cause the mixture to fail.
Be sure to practice the correct method of connecting a regulator to a gas cylinder. This will ensure the gas mixture remain stable.
Each of these issues can easily be managed by the correct selection of gas mixtures, regulators and tubing.
This applies whether you are manually calibrating the instrument or using one of the new docking stations on the market.
In either case, ask your gas detector supplier if there is a specific order to which you need to calibrate the instrument. In some cases instrument manufactures may want the gas applied in a specific order such as Ammonia, Isobutylene then 4gas (H2S/CO/CH4/O2).
Following these best practice tips is the only method to ensure your calibration program is effective and your new six (6) gas instrument works correctly.
