Making the Numbers Add Up

Understanding the specifications and performance of IAQ test instruments is essential for taking accurate measurements that will stand up to scrutiny.

AIR quality test instruments must deliver accurate and verifiable performance, both to ensure precise and reliable air quality diagnosis and to provide credible answers if results or procedures are challenged. The air quality professional's reputation depends on the quality and performance of the test tools in use, as well as on the user's understanding of instrument specifications, technologies, applications, and maintenance.

Issues in Air Quality Measurement
The measurement of indoor air quality is influenced by a variety of factors, including the characteristics air exhibits as a fluid and gas, the standards of accuracy we must meet when making measurements, the performance characteristics of air quality test instruments, and the way we use and maintain those instruments.

The Fluid Nature of Air. The subtleties of air quality measurement begin with the very nature of air. Measuring air quality is not like measuring a two by four.

Because it is a gas, air is compressible. Its density varies with changes in altitude, temperature, and barometric pressure. Unless compensation is dialed in when instruments are set up or compensation circuits are built into the instruments, changes in air density can affect the accuracy of some air quality measurements. Unless it carries large quantities of pollutants, air is invisible. Instruments are needed to determine the levels of chemicals or particles borne along by the air.

The principal indoor air quality characteristics we measure include:

  • Air temperature
  • Humidity
  • Airborne particle sizes and numbers. In sensitive environments such as medical facilities, the type of particles (such as the species of mold or bacteria) is important.
  • Gases, such as carbon dioxide (CO2), a byproduct of respiration that can indicate the rate of fresh air exchange into an indoor space, and carbon monoxide, a poison.

Limited and Imprecise Standards. Outside the industrial workplace, absolute standards for most indoor air conditions and air pollutants do not exist. Guidelines, not specific limits, are the rule. Government has been slow to establish specific standards to control levels of indoor air pollutants.

With few exceptions, science has yet to convince lawmakers that indoor air contaminants cause specific health problems that demand legislation. In 1994, OSHA filed a notice of proposed rulemaking for indoor air quality in non-industrial workplaces, but it withdrew the proposal in 2001. The management of indoor air quality remains primarily a private, not public, concern.

Test Instrument Performance. Another important consideration is the accuracy of test instruments over time and in varying environmental conditions. Instrument standards and performance specifications, together with appropriate testing and calibration, are the keys to making accurate, repeatable, defensible measurements.

Accurate performance over time is essential for valid, repeatable results and effective remediation. Yet the technology used to measure some air quality parameters, such as CO2 and CO, is inherently subject to drift and change as days and weeks go by. It's important for practitioners to understand these characteristics and know what they can expect from their instruments--and what steps to take to achieve high-quality results.

Calibration and Traceability. Without documented proof that a test instrument has been calibrated against a known standard, called traceability, test results may be difficult to defend against a challenge. In a court case, failure to prove valid measurement, made by instruments that were calibrated and employed as specified by the manufacturer, could result in liability for the organization and the air quality professional.

Instrument Usage and Operator Error. Measuring the characteristics of an invisible, changeable gas, using instruments that may perform within spec for a limited time or in a narrow range of conditions, puts the onus on the operator to understand each test tool's limitations and to maintain and use those instruments correctly.

What is avoidable operator error? Consider what could happen if a technician drove to a job site to assess indoor air quality. In the middle of winter, this tech left his instruments in his unheated garage overnight. Carrying the chilled instruments into the job site, he ran a quick temperature check. The results are way off. The temperature tester's electronic circuits are accurate only within a specified temperature range, and the instrument is still very cold. Only by letting the tool warm up to room temperature will this tech get the accurate result he's after.

Performance Characteristics of IAQ Test Technologies
Each characteristic of indoor air quality is measured using a specific type of sensor. In some instances, instrument engineers can choose from several alternative sensor technologies. In every case, their decisions about which sensor technology to use and how to employ it must take into account a number of performance parameters:

  • Accuracy,
  • Response time,
  • Stability over time and in varying environmental conditions,
  • Strength, durability, and longevity,
  • Portability and ease of use,
  • Ease of adjustment and calibration,
  • And, always, cost.

In most cases, product designers must make tradeoffs to deliver an affordable end product that meets user needs and expectations. A "perfect" instrument is something few would be willing to pay for.

In the following section, we review the technologies used to measure the various air quality parameters, the nature and limitations of those technologies, and what users need to know to get the best results.

Air Temperature
Sensor technologies
. Though household thermometers may employ expanding liquid (alcohol or mercury) or a bimetallic strip attached to a pointer, professional instruments generally use one of two sensor technologies: the thermocouple or the bead thermistor.

Most common in the service industry is the thermocouple, which looks like a bead on the end of two wires. The sensor is a combination of two metals that, when joined together and presented with a temperature, create a voltage differential across the connected wires. Thermocouples have little mass and therefore respond quickly to temperature changes. This is significant when measuring air, a gas with relatively low density and limited ability to quickly heat or cool a material.

The thermistor uses a different technology. A small resistor in the device receives a voltage or current. Resistance in the device varies as temperature changes, causing output current or voltage to change, as well.

A third electronic technology is infrared, which is used in non-contact thermometers. Infrared thermometers do not measure air temperature, but measure the infrared radiation emitted from surfaces. Infrared thermometers provide greatest accuracy at short distances and provide at best an indirect indication of air temperature.

Technology characteristics. Comparing thermistor and thermocouple technologies reveals several performance differences. Because simpler circuitry is needed to convert thermistor signals into temperature readings, instruments using thermistors are likely to cost less than those using thermocouples. Thermocouples can perform from near absolute zero to thousands of degrees. Thermistors operate over a limited temperature range (approximately -30F to 180F). This range is likely adequate for indoor air quality applications.

In terms of response time, both technologies perform well. But response time is also affected by the overall design of the test instrument. If the temperature sensor is enclosed in a massive housing, the surrounding material is liable to affect the speed with which air heats or cools the sensor, thus slowing response time. Minimal response time requires minimizing mass, and that means shrinking components. If carried too far, this could affect instrument durability.

Stability and drift. The thermistor temperature sensor is very stable and does not drift as some other sensors do. However, the temperature sensing function can drift over time because of other components used and the layout of the circuit design. In design and manufacture, care must be taken in the layout of the printed circuit board that connects with the thermistor. Ionic contamination from soldering flux residues can cause performance degradation. This increases as the impedance of the circuit increases. Good analog design is essential to achieving optimal temperature performance.

User guidance. Allow for instrument "settling time" when measuring temperature. The mass of a temperature tester may slow response time in rapidly changing conditions. In simple terms, it takes a while for the instrument to reach ambient temperature. In addition, the instrument's electronic circuitry will perform differently as temperatures vary, so the circuitry, too, must settle and stabilize.

Humidity
Sensor technologies
. The device traditionally used to measure humidity is the sling psychrometer. The instrument consists of two bulb thermometers, one of which is surrounded by a wet cloth. When the device is slung through the air, moisture evaporates from the wet bulb and cools the thermometer. The dryer the air, the greater the temperature difference between the wet and dry thermometers.

A more convenient alternative is a hand-held humidity meter that uses a capacitance sensor technology. A semi-permeable membrane in the sensor becomes more conductive as humidity increases and moisture penetrates the membrane. The meter interprets this change in terms of humidity level.

Technology characteristics. The sling psychrometer is highly accurate when used correctly. It is also relatively slow and cumbersome to use. The hand-held meter is much faster and easier to use, but over time, the sensor membrane will be affected by airborne contaminants that reduce its ability to absorb moisture. The sensors cannot be cleaned, so if they are accidentally contaminated or simply "get old," they must be replaced.

Stability and drift. Users should be aware that the performance of humidity meters will decline over time. The rate of change will depend on how and where they are used.

User guidance. Testing to determine accuracy and calibrate the instrument requires sophisticated and expensive test equipment beyond the means of the individual user. Instruments can be returned to the manufacturer or sent to an independent testing laboratory to verify their performance. If out of spec, the sensor of an expensive instrument may be replaced. In the case of a less costly instrument, the user will probably choose to replace the entire tool.

Airborne Particles
Sensor technologies
. The particle counter uses a pump to pull a sample of air into a space called the view volume, where particles intersect with a laser beam. The particles reflect differing amounts of light based upon their size. Photo detectors "see" these light flashes and convert each one to a millivolt signal. Larger particles reflect more light and create a stronger signal. Signals within a certain millivolt range are counted in one size "bin," particles in another range are put into another bin, and so on.

Technology characteristics. Accuracy can be affected when the counter is used where extremely high levels of particles are present. Particles may collect within the intake passage and measurement chamber. If these particles are dislodged during a subsequent test, they can cause a misleading spike in the particle count.

Stability and drift. Laser particle counter technology is generally stable over time, but the manufacturer's recommended calibration interval should be observed to maintain optimal performance.

User guidance. Operating the air pump for a period of time before taking a reading will help to flush particles out of the instrument. Users can calibrate the instrument for a zero particle count by applying a HEPA (high-efficiency particulate air) filter over the air intake port.

Carbon Dioxide
Sensor technologies
. CO2 sensors use a non-dispersive infrared technology. Incandescent light is projected through a small sample cell called the "bench link." The CO2 present in the test sample will absorb a specific wavelength of the projected light. A filtered infrared detector at the other end of the chamber measures the amount of light at that wavelength that passes through the chamber. As CO2 levels increase, the gas absorbs more light, which reduces the strength of the electric signal emitted by the detector.

Technology characteristics. Design affects both the performance and accuracy of a CO2 tester. The length of the test chamber or "bench" is important, because a longer chamber enables the light to pass through a larger air sample and more CO2 molecules before reaching the detector, making greater accuracy possible. A test instrument that pumps air through the test chamber will respond faster than a dispersion unit that does not pump the air.

Changes in air temperature, pressure, and density all will affect the accuracy of CO2 test results, so CO2 meters must be adjusted before use to compensate for changes in air pressure and temperature. Some instruments are built to compensate automatically for these changing conditions.

Stability and drift. The CO2 sensor will degrade and drift over time because of loss of sensitivity and declining bulb performance. Airborne containments will pass through the sensor filter (assuming there is one) and accumulate on the interior walls, emitter, and detector of the sensor. This contamination will affect the intensity of the light source, as well as the signal strength of the optical filter/detector. The more contaminants (smoke, dust, etc.) the sensor sees, the faster the degradation of the signal strength.

Another factor affecting long-term stability of the sensor is degradation of the light source. Like any bulb, it will burn out.

User guidance. A CO2 sensor should be calibrated about once a year to compensate for the reduced output of the bulb and collection of contaminants. CO2 meters can be tested and user-calibrated using a standard span gas that contains a known percentage of CO2. A rough calibration can be achieved using outdoor air, which should contain 350 to 450 parts per million of CO2. Dry nitrogen, which contains no CO2, may be used as a zeroing gas.

Carbon Monoxide
Sensor technologies
. Carbon monoxide testers use an electrochemical gel sensor technology.

Technology characteristics. The gel sensor has a limited life span (two years), and its accuracy can be affected by changes in both temperature and ambient humidity levels.

Stability and drift. Prolonged exposure to humidity levels below the ideal 50 percent RH level can dry the gel sensor and cause the readings the instrument delivers to drift out of spec. The changeable nature of CO sensor technology means that users must calibrate their CO meters frequently (monthly) to ensure accuracy. Most CO meters can be user-calibrated using a span gas containing a known percentage of CO. They can be zero calibrated (zeroed) in free air.

User guidance. Calibrate the instrument when you receive it to ensure it is set for your environment. Then, recalibrate if your environment changes (for instance, if humidity increases during the summer). Simply transporting the instrument from a humid outdoor environment into a dryer, air-conditioned space should not cause problems. Environmental changes over a longer period are the issue.

Conclusion
Accuracy is fundamental for those who measure, monitor, and control air quality in workspaces. By choosing instruments carefully based on an in-depth understanding of their specifications and performance characteristics, using them properly, and maintaining and calibrating them as recommended, IAQ professionals can ensure themselves and their clients of accurate measurements and effective guidance to improve indoor air quality.

This article appeared in the October 2006 issue of Occupational Health & Safety.

This article originally appeared in the October 2006 issue of Occupational Health & Safety.

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