Images credits: TBD; Kin-Tek; Matheson Gas

 

Laboratory and calibration gases are used as standards (for reference purposes), as well as for detection, sample preparation, environmental monitoring, and analysis applications. According to Kin-Tek, the purpose of a gas standard is to establish accurate reference points for the verification of an analysis. Analytical or laboratory applications may include gas chromatography (GC, GC-MS, LC-MS), spectrometry (ICP, ICP-MS, Flame A.A., GFAA, NMR), specialty analysis techniques (thermal and elemental analysis, total organic carbon analyzers), as well as sample or materials processing. Some analytical measurement instruments require specialty laboratory and calibration gases for instrument operation and/or calibration.
 

The purity, quantity, and type of laboratory and calibration gases used in an analytical instrument must be properly assessed based on the technique being used. For example, laboratory gases used in gas chromatography must be pure gases that adhere to specific industry purity standards. Helium, nitrogen, argon, hydrogen, oxygen, carbon dioxide, and acetylene must have a determined purity to function properly in an analyzer. Nitrogen is often used as a calibration gas in spectroscopic equipment and may need to be determined as pure as 99.9995%. Gas purity can have a great effect on the outcome of an analytical laboratory procedure and must be taken into consideration when using a laboratory gas for sample preparation or measurement. Laboratory and calibration gases are also used in emission and absorption spectroscopy, mass spectroscopy, thermal and surface analyses, and liquid chromatography.

 

Calibration Operations

Calibration is the means to ensure that a device meets a manufacturer's specifications for proper operation. Calibration may include a report or certificate assuring an end user that a device conforms with specifications, and perhaps also with external guidelines, such as those of the International Organization for Standardization (e.g., ISO 9001 standards). A company follows these standards to ensure that its products and/or services gain acceptance among suppliers and customers.

Calibrating a gas sensor

Video credit: Ideal Calibrations via YouTube

 

Over time, gas sensors may "drift" from the calibration specifications and fall outside acceptable tolerance. Reasons for drift may include sensor poisoning, leakage, overexposure, temperature and humidity extremes, or physical damage due to dropping or immersion. Electrochemical sensors, commonly used for carbon monoxide, hydrogen sulfide and oxygen monitoring, are normally stable and degradation is slow. However, normal degradation of electrochemically-based toxic and oxygen sensors is accelerated by low humidity and high temperatures due to the chemical reactions and consumption of the electrolyte. Also, catalytic sensors are susceptible to poisoning when exposed to substances containing silicon, halogenated hydrocarbons, and high concentrations of hydrogen sulfide.

  

Images credits: MSA; Owlstone

 

Quantitative analytical measurements are critical to achieving accuracy in laboratory and manufacturing applications that include petrochemical processes, natural gas, environmental compliance, and health and safety programs. If the measurement or monitoring device is not calibrated, there can be no certainty in the measurements. Accurate, repeatable measurements are possible only when an analytical instrument is calibrated properly. The calibration gas standard establishes a known analyzer response to a certified chemical component concentration. This controlled environment enables sample responses from the device to be converted to an accurate response.

 
A definition for traceability comes from ISO Guide 25 (currently ISO 17025): "...the property of a measurement result, whereby it can be related to appropriate standards, generally international or national standards, through an unbroken chain of comparisons." In the U.S., gas phase national standards are provided by the National Institute of Standards and Technology. However, for gas measurements, NIST traceability is complicated by the fact that a limited number of gases are available as standard reference materials by NIST.

   

Gas Selection

The most important step for proper calibrations of analytical instruments and safety monitors is selecting the correct gas. There are a few important preliminary steps before calibration. First, it is important that the gas has not reached its expiration date. Second, the type and concentration of the gas must match that required by the device manufacturer and/or industry standard. Third, the tubing, regulators, and adapters must be compatible with the device to be calibrated.

 

In some instances, such as combustible gas sensors, the applications are non-specific and may be calibrated to any number of different gases. It is best to choose a calibration gas that closely matches the gas likely to be encountered. If that gas is unknown, or if a mix of gases is suspected (such as may be the case for safety monitors), then calibrating with pentane is recommended. Using pentane as a calibration standard will allow the sensor to detect a larger group of hydrocarbons that are commonly found in confined spaces.

 

To ensure accuracy, calibration gases should be gravimetrically produced and traceable to the U.S. National Institute for Standards and Technology (NIST). NIST traceability comes from certified, traceable weights used in the gravimetric filling process. As such, gas mixtures are produced based on the molecular weight of each gas component in the mixture. The gravimetric process is the most accurate method for producing calibration gases and is not affected by temperature or pressure.

 

Other methods (e.g., dynamic blending, pressure blending) may be dependent on temperature and pressure. Supporting documentation and certificates of analysis should be provided by the gas supplier in order to verify the process used and the results of the gas mixture.

 

Gas Storage and Delivery

Because gases must be contained and readily available at the point of use, the means of delivery is critically important to ensure system compatibility and safe transportation. A variety of storage containers for laboratory and calibration gases include: cryogenic cylinders (Dewars), gas cylinders, and glass bottles (ampules). Additionally, there are more application-specific containers, including permeation tubes and safe delivery systems. Although not covered here, (see Gas Generation Equipment), gases may be produced in an on-site generation system.

 

Gas Cylinder

A gas cylinder or tank is a pressure vessel used to store gases at high pressure.

High-pressure gas cylinders Image credits: Praxair Direct

 

Cryogenic Cylinder/Dewars (Liquid)

An insulated container used especially to store liquefied gases; having a double wall with a vacuum between the walls and silvered surfaces facing the vacuum.

Image credit: Praxair

Image credit: Praxair

  

Glass Bottle or Ampule

Glass bottles or sealed ampules are used for small quantities of gases for R&D, analytical standards, or specialty applications.

 

Permeation Tube

Permeation tubes are small, inert capsules containing a pure chemical molecule. At constant temperature, the device emits the molecule through the permeable tube wall at a constant rate.


Image credit: Kin-Tek

 

Safe Delivery System

SDS (safe delivery system) is a groundbreaking technology designed to reduce the hazards and environmental risks associated with transporting, storing, and delivering highly toxic gases.

In all cases, safe and proper handling of the stored gases is essential.

Compressed gas cylinder training video

Video credit: COnvergence 3D Videos

 

Laboratory and Calibration Gas Safety

The following is an excerpt from OSHA's 2004 safety bulletin "Verification of Calibration for Direct-Reading Portable Gas Monitors." It has been reproduced for instructional purposes.

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Introduction

Gas monitoring instruments are designed to protect personnel from unseen hazards that may exist in workplace environments, including confined spaces. It is vital to worker safety that these instruments are maintained and calibrated properly.

 

Instrument inaccuracy due to improper or irregular calibration can lead to serious accidents. Exposure to excessive levels of toxic gas or an oxygen-deficient environment can cause workers serious illness and even death. Combustible gas explosions are often catastrophic, injuring or killing personnel and destroying property.

 

The International Safety Equipment Association (ISEA), founded in 1933, is a trade association for manufacturers of protective equipment, including environmental monitoring instruments. The ISEA recommends, at a minimum, verification of sensor accuracy before each day's use.

 

The only way to guarantee that an instrument will detect gas accurately and reliably is to test it with a known concentration of gas. Exposing the instrument to a known concentration of test gas will show whether the sensors respond accurately and whether the instrument alarms function properly.

 

Calibration: The Key to Accurate Readings

Gas detection instruments are used to detect the presence of toxic and combustible gases, as well as oxygen deficiency or oxygen enrichment (a fire and explosion hazard). Workers cannot rely on their sense of smell to alert them to odorless hazards, necessitating the use of gas detectors whenever a worker enters an area with potential atmospheric hazards.

 

"Calibration" refers to an instrument's measuring accuracy relative to a known concentration of gas. Gas detectors measure the concentration of a gas in an air sample by comparing the sensor's response to the response generated by a calibration gas of a known concentration. The instrument's response to the calibration gas serves as the measurement scale or reference point.

 

The responsiveness of electrochemical sensors will vary with environmental conditions. Sensor response will be different (lower or higher) depending on the actual environmental conditions. Therefore, as much as possible, the monitors should be calibrated at environmental conditions that are the same as (or similar to) actual field conditions. Calibration at locations where the equipment is to be used is always preferable.

Most instruments are equipped with two levels of alarms - warning and danger. The warning alarm alerts the user that the environment has a detectable concentration of gas and is therefore potentially hazardous. The danger alarm indicates that the gas concentration exceeds the programmed "hazard" threshold, and the area is approaching a hazardous level. Whether an instrument warns and/or alarms at the proper time depends on its detection abilities and its ability to translate its findings into an accurate reading.

 

If the instrument's reference point has shifted, the reading will shift accordingly and be unreliable. This is called "calibration drift" and it happens to all detectors over time. An instrument that experiences calibration drift can still measure the quantity of gas present but it cannot convert this information into an accurate numerical reading. Regular calibration with a certified standard gas concentration will update the instrument's reference point, ensuring that the instrument will produce continued, accurate readings.

 

Causes of Calibration Drift

Over time, the accuracy of gas detection instruments can diverge from their calibration settings in several ways:

Gradual chemical degradation of sensors and drift in electronic components that occur naturally over time.

  • Chronic exposures to, and use in, extreme environmental conditions, such as high/low temperature and humidity, and high levels of airborne particulates.
  • Exposure to high (over-range) concentrations of the target gases and vapors.
  • Chronic or acute exposure of catalytic hot-bead LEL sensors to poisons and inhibitors. These include: volatile silicones, hydride gases, halogenated hydrocarbons, and sulfide gases.
  • Chronic or acute exposure of electrochemical toxic gas sensors to solvent vapors and highly corrosive gases.
  • Harsh storage and operating conditions, such as when an instrument is dropped onto a hard surface or submerged in liquid. Normal handling/jostling of the equipment can create enough vibration or shock over time to affect electronic components & circuitry.

Often, after exposure to the more extreme conditions above, when calibration is attempted, the detector will either display a failure message or it will not allow the user to fully adjust the display reading. At this point, the severely damaged sensor must be replaced and/or the detector serviced by qualified personnel.

 

Worker Safety: The Number One Reason for Proper and Regular Calibration

The primary reason for proper, regular instrument calibration is to prevent inaccurate gas concentration readings that could lead to injury or to death. Correctly calibrating an instrument helps to ensure that the instrument will accurately respond to the gases that it is designed to detect, warning users of hazardous conditions before they reach dangerous levels. In addition to detecting and correcting for calibration drift, regular calibration assures the user that the instrument is functional. Gas detection instruments are often subjected to harsh operating and storage conditions where they can be damaged. Both of these factors can affect instrument performance, leading to inaccurate readings or even instrument failure. While a unit may appear to be sound during a visual inspection, it actually could be damaged internally. Regular calibration is the only way to be certain that a detector is fully functional. Moreover, a standing policy for regular calibration sets the tone for a safety-conscious work environment and indicates to workers that safety is a priority. As a result, workers may be more likely to keep safety principles in mind throughout the workday.

 

A written record of calibration should be kept for the life of each instrument. This record allows users to quickly identify an instrument that has a history of excessive maintenance/repair or is prone to erratic readings.

 

Bump Tests vs. Full Calibration

There are two methods of verifying instrument accuracy: a functional or bump test and a full calibration, each appropriate under certain conditions. A bump test verifies calibration by exposing the instrument to a known concentration of test gas. The instrument reading is compared to the actual quantity of gas present (as indicated on the cylinder). If the instrument's response is within an acceptable tolerance range of the actual concentration, then its calibration is verified. (Note: It is recommended that users check with the detection equipment manufacturer for the acceptable tolerance ranges.) Instruments should be "zeroed" before the bump test in order to give a more accurate picture of the bump test results. When performing a bump test, the test gas concentration should be high enough to trigger the instrument alarm.

Image credit: Ask Dave Blog

 

If the bump test results are not within the acceptable range, a full calibration must be performed. A full calibration is the adjustment of the instrument's reading to coincide with a known concentration (generally a certified standard) of test gas. For verification of accuracy, calibration gas should always be certified by and traceable to the National Institute of Standards and Technology (NIST). In most cases, a full calibration is only necessary when an instrument fails a bump test or after it has been serviced. The full calibration and bump test should be conducted in a clean fresh air environment.

 

When to Bump Test and When to Calibrate

In the past, there often has been confusion regarding proper calibration procedures and frequency. To clarify this issue, the International Safety Equipment Association (ISEA) issued a position statement on instrument calibration that states, "A bump test or full calibration of direct-reading portable gas monitors should be made before each day's use in accordance with manufacturer's instructions, using an appropriate test gas." If the instrument fails a bump test, it must be adjusted through a full calibration before it is used.

 

ISEA recommends more frequent testing if environmental conditions that could affect instrument performance are suspected, such as sensor poisons. The ISEA allows for less frequent calibration verification under certain conditions (see below), but the interval between testing should never exceed 30 days.

 

According to the ISEA, less frequent verification may be appropriate if the following criteria are met:

  • During a period of initial use of at least 10 days in the intended atmosphere, calibration is verified daily to ensure there is nothing in the atmosphere to poison the sensor(s). The period of initial use must be of sufficient duration to ensure that the sensors are exposed to all conditions that might adversely affect the sensors.
  • If the tests demonstrate that no adjustments are necessary, the interval between checks may be lengthened, but it should not exceed 30 days.

  • When calibrating an instrument, always follow the instrument user's manual for the manufacturer's recommended calibration frequency and procedure.

Calibration Rules

The following are a few basic instrument calibration rules to ensure a clear path to health and safety.

  • Follow the manufacturer's guidelines for proper calibration. No job, including instrument calibration, can be performed properly or safely without the right tools. The type and concentration of calibration gas, sample tubing, flow regulators and calibration adapters are key links in the calibration chain. Using equipment provided by the original manufacturer should ensure a proper start to every calibration.
  • Only use certified calibration gas before its expiration date. The most important tool used in calibration is the gas itself. The instrument can only be as accurate as the gas used to calibrate it. Be certain your supplier can provide a traceable certificate of analysis for every calibration gas cylinder. The concentration of calibration gas, particularly the concentration of reactive gases such as hydrogen sulfide or chlorine, will only remain stable for a finite period of time. Never use calibration gas after its expiration date.

  • Train workers on the proper methods of calibration. Most instruments are designed to be field calibrated with instructions detailed in user manuals, training videos or computer-based training modules. Everyone responsible for performing instrument calibration should be trained and tested accordingly. 

Conclusion

There is an inherent risk in many workplaces of injury or illness from respiratory hazards such as oxygen deficiency and the presence of toxic gases. Detection technology and products exist to minimize such risk. Properly verifying the accuracy of gas detection equipment before each day's use will help to ensure that each worker finishes the job safely.

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Resources

 

Matheson Gas

 

MSA Safety

 

OSHA

 

Kin-Tek

Engineering Calculators Related to Laboratory and Calibration Gases