Advance PID technology gives resistance to humidity effects

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1 FOR RELEASE FEBRUARY 2016 Advance PID technology gives resistance to humidity effects Photo-Ionization Detectors (PIDs) Handheld Photo-Ionization Detectors or PIDs are convenient tools for airborne pollution detection in such uses as 1) fence-line monitoring, 2) industrial hygiene worker exposure measurements, 3) confined space entry, and 4) screening excavated soils for contamination. They are an efficient and inexpensive way to continuously detect and measure volatile organic compounds Dr. Werner Haag, Senior Applications (VOCs) and will give instantaneous readings. PIDs normally Chemist at Ion Science, Inc. measure in concentrations from low parts per billion (ppb) to >5000 parts per million (ppm), can data-log results, and give visual and audio alarms when concentrations exceed predefined set-points including time-weighted average (TWA) and short-term exposure limit (STEL). PIDs are non-specific, broad band detectors and give a total VOC reading for the atmosphere examined. Thus they are useful for warning of possible toxic chemicals present when the compounds are unknown or poorly characterized, such as in a hazardous waste spill or historical dumping site. In such cases, PIDs give only a semi-quantitative readings, indicating relatively high or low levels. However, after a pollution site or event has been characterized, the specific chemical or chemicals are known and true concentration readings can be obtained by calling up the response factor for the chemical that is built into the instrument software. Ion Science PIDs have over 800 factors covering the most common compounds encountered and many less common and even fairly exotic ones. Humidity Effects In all these applications high humidity can be present, which has long caused difficulties for PID measurements. The biggest humidity problem had been when a PID was used for a few days or weeks in a dirty or dusty environment and then encountered high humidity, resulting in a drifting high reading (false positive), even when no VOC was actually present, as shown in Figure 1.

2 - 2- Figure 1. Drifting high readings at high humidity with conventional PIDs. Such situations can occur when a PID is calibrated in an air-conditioned building and then brought out into the field where it is hot and humid. Soil Headspace Sampling Another common case of drifting readings was when PIDs were used for screening excavated soils by headspace analysis. Such screening can save an enormous amount of time and money, by quickly characterizing which dug up soil needs to be treated for hazardous waste and which can safely be returned to the ground. A jar is half-filled with the soil sample, covered by some type of film, foil or other penetrable lid, and then warmed gently to drive the VOCs into the headspace, such as by setting near an indoor heater or in the sun for an hour or so. Teflon tubes are attached to the inlet and outlet of the PID, and these inserted through the jar cover to measure and recirculate the headspace vapors, as shown in Figure 2. In this procedure it is easy to accidentally draw in dust from the soil, and the heating of the soil drives moisture from the soil to the air, thus resulting in the drifting high readings described above. Figure 2. Soil headspace measurement by PID

3 - 3- Formerly it was necessary to undergo frequent sensor cleanings to avoid this problem. Alternatively one could attach humidity filtering tubes, which added cost and complexity, and sometimes inadvertently absorbed VOCs in addition to the moisture. Ion Science has developed Fence Electrode PIDs that are almost completely unaffected by humidity, allowing much easier operation and less down time. To understand how this is accomplished, we need to review more detail about how a PID works. How a PID Works PIDs use high-energy photons in the ultraviolet (UV) range to break VOC molecules (A) into positively charged ions (A+) and negatively charged electrons (e-), as shown in the inset in Figure 3. The A+ ions travel to the cathode and the electrons are collected at the anode, resulting in a current proportional to the concentration of VOC. This is amplified and output as a ppb or ppm concentration on the instrument display. Figure 3. PID sensor design (Fence Electrode). The inlet system allows only about 1% of the sampled gas to actually enter the ionization chamber, resulting in far less contamination issues than conventional PIDs where 100% of the sample gas passes through the sensor. Figure 4 is an exaggerated depiction of the microscopic dust build-up on the sensor walls, resulting in leak current at high humidity and drifting high readings. The combination of anti-contamination design and fence electrode results in eliminating this humidity problem. Cont../4

4 - 4- Figure 4. Leakage current at high humidity is trapped in the Fence Electrode system. Fence Electrode System Also Eliminates Humidity Quenching The triangles in Figure 5 show that 2-electrode PIDs also have a different humidity problem, namely a decrease in response to VOCs due to a quenching effect as humidity increased (false negative), at least when the sensor is clean and not drifting. To combat this effect, a rapid RH sensor and microprocessor compensation is sometimes used to calculate a corrected reading. However, Figure 5 (square data points) shows that such correction is difficult and in this case greatly over compensates and gives false high readings when the compensation algorithm is turned on. By contrast, the Fence Electrode PID is essentially unaffected by humidity at all RH levels. This is accomplished largely by precision manufacturing that allows a thinner light pathway in the sensor to supress the loss of photons by water adsorption. The state-of-the-art Fence Electrode PIDs with anticontamination inlet and thinner gas path thus solve both the quenching effects and drifting problems, and therefore are much simpler and more accurate to use than 2-electrode PIDs, without the need for filter tubes and software corrections, etc. Cont../5 Figure 5. Effect of humidity on the response of a conventional 2-electrode PID and a humidity resistant PID.

5 - 5- ENDS For product information please contact: Sam Holson, Ion Science, The Way, Fowlmere, SG8 7UJ, UK tel: + 44 (0) marketing@ionscience.com Ion Science on Social Media: on Twitter Join us on Facebook at facebook.com/ionscienceltd Join us on Linked In at linkedin.com/ionscienceltd The Ion Science blog can be found at /blog