The new Scott Freedom PID fixed gas detector for volatile organic compounds (VOCs) adds yet another technology to the list of available sensing technologies that Scott Health & Safety, a division of Tyco International Ltd.’s Fire & Safety segment, offers as a sole-source provider of fixed gas detection products. Complementing the company’s current product line of combustible, toxic and oxygen detection products, the Freedom PID detector offers a level of sensitivity to hundreds of compounds that has typically only been available with very expensive analytical instrumentation.

Photo Ionization Detectors
Photo Ionization Detectors (PIDs) are capable of measuring organic compounds from as low as parts per billion (ppb) to as much as 10,000 parts per million (ppm), and are nonselective, whereby any gas with an ionization potential below the output of the PID lamp can be detected. Scott Health & Safety’s Freedom PID detector utilizes a common ionization potential to maximize the number of toxic substances that can be detected with a single product. There are literally hundreds of various organic compounds that can be measured with the 10.6 eV lamp in the Freedom PID detector.

Fuels, paints and solvents are probably the three main categories of materials where most PID detectors should be used. Scott has been manufacturing catalytic bead detectors for more than 40 years and infrared lower explosive limit (LEL) detectors for almost a decade to monitor for these compounds in the 100-percent LEL range. PID technology now enables Scott to supply a full line of products to detect various compounds from as low as one ppb to as high as 100-percent LEL, protecting facilities and personnel from toxic exposure and the hazards of flammability.

Materials detected by the PID
One example of a solvent that is both toxic and flammable is toluene. For many years, Scott Health & Safety has provided LEL detection products for industrial applications where toluene is monitored for spills or leaks due to the hazard of flammability. In printing applications — where Scott has provided sample draw systems for detecting toluene levels in oven dryer ducts — there is now the additional capability to monitor toluene in the general workplace for toxic ppm levels to protect the personnel in the facility from exposure to toluene levels that exceed governmental recommendations for both short-term and long-term exposure.

Another material that is typically monitored in the LEL range but is highly toxic is benzene. Now a PID solution can also detect benzene at low toxic levels right alongside an LEL detector monitoring for dangerous flammable levels that could occur from a leak or spill. This is critical where personnel exposure to low levels of benzene can occur without an indication that a low-level leak has occurred if only an LEL instrument is in use. A PID detector in many applications will increase the level of sensitivity and enable actionable procedures to take place sooner, before a spill or leak can escalate and become a greater hazard.

Other materials that can be detected by PID technology are heat transfer fluids, degreasers, plastics and resins. Sulfur compounds, which are known to poison catalytic bead sensors, can be detected with PID technology as can ammonia, which can deplete standard electrochemical sensors with chronic low-level exposures. These are just a couple of examples where PID technology can offer advantages over traditional sensor technologies that are common place in many industrial applications. The list of materials that can be detected is extensive with new compounds added as industrial detection requirements grow. For many toxic materials where electrochemical sensors are not the best technology for an application, PID can now be an alternative or can even complement an existing installation to provide additional protection.

A unique feature of the Scott Freedom PID gas detector is the internal pump, which operates on a duty cycle that maximizes zero stability and response time. By operating the pump on a duty cycle, the ionization detection chamber is able to “self clean” and maximize the time before maintenance is required. Using an internal pump also improves the detector’s response time by drawing a sample stream into the detector and not relying on diffusion for a sample to reach the detector. Scott also provides a “Flow Thru” option of the PID detector, which allows samples to be drawn from locations up to 200 feet away from the instrument.

Calibrating a PID
Because a PID is nonselective, careful consideration must be given to the calibration factor used to calibrate a unit. The Scott Freedom PID detector has an extensive library in the internal firmware that includes most common organic compounds that would be detected, and the calibration factors are applied at calibration to make the units specific for the target gas that is to be detected when the detector is going to be used to monitor for a single compound. If more than one compound will be monitored, consideration must be given to the correction factors for all of the materials that the instrument is intended to detect to select the correct calibration factor.

The decision of how to calibrate a PID detector should be based upon a clear understanding of the potential risk associated with the compounds present as it relates to published exposure limits and the PID’s sensitivity to these particular compounds. The PID sensitivity is referenced to the calibration gas, isobutylene. Just as with catalytic LEL sensors, the correction factors for a PID detector permit calibration using a single gas to allow for a direct reading of the concentration of another. This eliminates the need to have multiple calibration gases to use a PID detector.

Calibrating a PID detector for a single VOC would be the most typical application, but by properly understanding the correction factors for various compounds and the toxic risks associated with these compounds, calibration on mixtures with constant proportional make ups or with varying make ups can be achieved.

When an industrial process involves a number of compounds, calibration and the proper adjustment of the PID detector’s alarm set point requires a careful review and consideration of specific data. If all of the materials can be identified, each individual chemical and its respective level of toxicity can be determined by a review of the material safety data sheet.

If the proportional make up of the materials is known, the toxicity when exposed to a known mixture of these materials can be determined using calculation methods that Scott Health & Safety provides with detailed step-by-step instructions and examples in the Freedom PID user manual.

The proper correction factor to use for calibration is also easily calculated. This too is explained thoroughly in the user manual with detailed examples.

 If the potential make up of a number of materials is unknown, concentrations will vary during processes. In this scenario, predicting the chemicals present or their relative concentrations at any point in time should a leak or spill occur cannot be determined. A PID detector in this situation can still be calibrated properly with an appropriate alarm set point in a conservative manner, to provide the highest level of protection for any mixture of these known materials. By determining the single compound in the mixture that is most toxic, a conservative set point for the entire mixture can be determined. The Freedom PID user manual reviews this process in detail with examples to assist end users in making these determinations.

A powerful tool
The Scott Freedom PID Gas Detector is a powerful tool to use in a variety of industrial applications, and as described can complement existing gas detection installations to provide a more broad range of detection. By applying this technology properly, a higher level of detection and protection can be achieved. Using this technology along with Scott’s line of electrochemical toxic and various LEL detection products will increase the level of safety in a facility and provide critical information necessary to make decisions to protect property and personnel.

For more information, visit www.scotthealthsafety.com or call (704) 291-8492.