The process of gas calibration involves the testing and comparison of your gas detection instruments against some fixed reference value. The process includes using an accurate gas mixture as a reference and comparing it with the values provided by the instrument being tested. The cylinder mixture or device that produces the assigned correct value is known as the standard and it plays a vital role in all gas calibration.
Calibration is a must when using any gas measurement instrument but it is particularly important when dealing with extremely toxic and corrosive gases such as chlorine, phosphine, hydrogen cyanide, hydrogen sulphide and many others.
In fact, timely, periodic calibration is a must for any facility dealing with gas measurement. Ignoring regular calibration may lead to inaccurate readings, quality issues, production downtime and even legal infringements.
What is Chlorine?
- Chlorine is an element used in industry and somtimes found in household products.
- Chlorine is sometimes in the form of a poisonous gas. Chlorine gas can be pressurised and cooled to change it into a liquid so that it can be shipped and stored. When liquid chlorine is released, it quickly turns into a gas that stays close to the ground and spreads rapidly.
- Chlorine gas can be recognised by its pungent, irritating odour, which is like the odour of bleach. The strong smell may provide adequate warning to people that they are exposed .
- Chlorine gas appears to be yellow-green in colour.
- Chlorine itself is not flammable, but it can react explosively or form explosive compounds with other chemicals such as turpentine and ammonia.
Where is Chlorine Found and How is it Used?
- Chlorine is one of the most commonly manufactured chemicals in the world. Its most important use is as a bleach in the manufacture of paper and cloth, but it is also used to make pesticides, rubber and solvents.
- Chlorine is used in drinking water and swimming pool water to kill harmful bacteria. It is also used as part of the sanitation process for industrial waste and sewage
- Household chlorine bleach can release chlorine gas if it is mixed with certain other cleanig agents.
How People Can Be Exposed To Chlorine?
- Peoples’ risk for exposure depends on how close they are to the place where the chlorine was released.
- If chlorine gas is released into the air, people may be exposed through skin contact or eye contact. They also may be exposed by breathing air that contains chlorine.
- If chlorine liquid is released into water, people may be exposed by touching or drinking water that contains chlorine.
- If chlorine liquid comes into contact with food, people may be exposed by eating the contaminated food.
- Chlorine gas is heavier than air, so it would settle in low-lying areas.
Airborne Toxicity Standards
Globally chlorine toxicity standards are all measured in parts per million (ppm).
Safe Work Australia: Workplace Exposure Standard: 1ppm Peak(ceiling)
ACGIH: TWA: 0.5ppm ; STEL: 1ppm
NIOSH: 0.5ppm Ceiling
OSHA: 1 ppm Ceiling
Challenges in Chlorine Calibration
The challenges faced with chlorine measurement and chlorine calibration are becoming more difficult as permissible levels (TWA, STEL, PEAK or CEILING) are lowered.
With occupational health standards now at or below 1ppm, obtaining accurate and repeatable measurements becomes a challenge even with the best instruments and near impossible if they are not calibrated correctly.
Selecting calibration gas that is as low as possible is critical if instruments are going to be meeting accuracy requirements. Low ppm chlorine calibration gas may require high pressure/large volume cylinders to permit the lower levels. Typically 3 or 5ppm is the best you will get in a disposable cylinder. You should determine what is achievable from both your instrument company and gas supplier.
If calibration gas cannot be used, then possibly a gas generator is the answer. Gas generators are available for chlorine in levels below 1ppm providing the lower gas concentrations required for low level measurement. Look for a generator which is traceable to standards such as NIST.
Whether you use calibration gas cylinders or a gas generator, always select the gas concentration which reflects your monitoring objectives and desired outcome.
Zeroing the instrument in ambient air is not sufficient. At low ppm concentrations, you must zero the instrument with zero air or nitrogen (sensor dependent) in order to be sure you are getting true zero. Background gases in plant sites could shift your zero measurement slightly creating errors in readings.
Instrument sensor response data (characteristics) needs to be understood before calibrating a chlorine instrument, particularly if the instrument calibration is not controlled by a microprocessor. Even if the calibration is controlled by a microprocessor understanding sensor characteristics can provide you valuable information which can be used to reduce gas consumption and calibration time.
- How long will it take for the sensor to respond?
- Does the sensor require pre-conditioning?
- Does the sensor response time increase with longer time periods between gas exposures?
These are all key pieces of information you need to know to determine how the sensor will respond and the impact on calibration gas consumption, time and frequency.
Stainless steel regulators and “Teflon” tubing are a must. Using incorrect materials can cause some or all of the gas to be adsorbed onto the regulator or tubing. Even a short piece of incorrect tubing will create incorrect readings and failed calibrations. See: Selecting Compatible Materials For Calibrating Gas Detectors
Furthermore, using poor quality materials could potentially add contaminants to the gas mixture stream.
So, to effectively calibrate your chlorine senor ;
- Know your sensor specifications
- Know your gas specifications
- Use the correct procedures
- Use the correct materials
While chlorine calibration does pose a number of challenges, most can be overcome with the correct gas, materials and procedures. It is important to carry out the process regularly to ensure high quality processes and accuracy of your instrumentation.