Subminiature Photoionization VOC Sensor Boris Dolgov, Baseline-MOCON, Inc.

Transcription

1 Subminiature Photoionization VOC Sensor Boris Dolgov, Baseline-MOCON, Inc. Lyons, CO 80540, USA (303)

2 1. Objective Monitoring of Volatile Organic Compounds (VOCs) is required for a number of gas analysis applications, such as: - Indoor Air Quality (IAQ) - Industrial Fugitive Emissions - HazMat Events - Environmental Pollution - Others VOC analyzers for those applications are typically based on the Photoionization Detector (PID) principle, which allows for a quick, portable and inexpensive measurement of total VOCs. A number of commercial PID-based instrument are available on the market now (RAE Systems, PhotoVac and others). They are relatively portable (hand held), comparatively inexpensive ($1000 to $4000) and low-maintenance instruments. For a number of applications it is required to simultaneously monitor other gases besides VOCs: e.g. CO, O2, H2S, etc. All those gases can be measured by a set of small, inexpensive electrochemical or catalytic sensors that may be combined in a portable, multi-gas instrument. Photoionization detectors, however, due to their relative complexity, could not be built in the form factor of electrochemical sensor - until now. Therefore, for VOC measurement, a separate PID instrument was required, which adds to complexity, bulk and the cost of monitoring. Development of a PID plug-in sensor would result in the possibility of a total analytical solution in one portable package. Availability of a subminiature PID sensor would also help to address applications, where the small size and low power consumption are critical requirements. Use of hi-tech materials, electronics and technologies would not only help to miniaturize the instrument, but also to reduce the cost and make it a true mass-production device, which would help to expand the application tremendously and penetrate such markets as the Residential Indoor Air Quality monitoring. 2

3 2. Project s Scope The goal of this project was to develop a miniature, standalone PID sensor in the form factor of an electrochemical sensor (City Technologies 4P Series). The PID sensor must be compatible geometrically and electrically - with portable multisensor instruments, existing on the market. Another objective was for the sensor to be competitive performance wise with other PIDs on the market. 3. Design The main three elements, defining the design and performance of a PID, are: - The UV Lamp - Detector Cell - Detector Electronics (amplifier and lamp driver circuits) Special VUV lamp has been developed for the sensor. The lamp s working media is Krypton. It is the smallest lamp commercially available at the moment: only 14 mm long and 6 mm in diameter (Fig. 1). Small lamp size typically leads to a short life, due to the increased relative outgassing inside the bulb. Thanks to the special sealing and conditioning techniques, our lamps have the lifetime of more than 10,000 hrs, which is more than 1 year of continuous operation. The lamp is excited by the high frequency electric field, through the capacitive coupling. The novel approach is that the excitation conductors are placed directly on the surface of the bulb, with the help of vacuum sputtering. They are made in the shape of two rectangular pads, located on the opposite sides of the bulb (see Fig. 1). This design is especially important for achieving low power consumption and miniaturizing device maximize the lamp s efficiency and output, since it significantly improves electrical-to-light efficiency (by 30 to 50%). This also allows using smaller components in the lamp driver circuit Another important advantage of the integrated excitation approach is that it helps with the ignition of the lamp. As compared to the external electrodes design, we see the reduction of ignition voltage by 10-15%. Besides relaxing the 3

4 requirements to the lamp driver circuit, lower ignition voltage also helps to make the whole device safer from the standpoint of the explosion hazard. Fig.1 VUV Lamp The construction of the PID detector cell should meet a number of requirements that are often contradictory. It should provide: - Lowest detection limit possible - Widest dynamic range - No sensitivity to moisture (due to the surface conductivity) - Minimal Moisture Quenching effect - Mechanical stability - Ease of maintenance in the real world conditions he optimal design of detector was realized in the form of a single, self-contained part, the chip (Fig. 2). It represents a rectangular piece of Teflon sheet, 0.5 mm thick, with a plurality of 0.5 mm through holes and conductive cladding on both sides. Each cladding layer (electrode) is connected to one of the two connector pins. 4

5 Fig. 2. Detector Cell The negative polarization voltage (~70V) is applied to one electrode, while the other electrode (collector) is connected to the input of the amplifier circuit (Fig.3). The chip is placed adjacent to the UV lamp window. When lamp is on, the UV light passes through the holes, which compose an ionization/ ion collection volume. The total volume of the detector cell is about 2 microliters, which makes it ideal for the fast resoponse applications. The geometry of the individual holes (both the depth and aspect ratio) as well as the strength of electric field are both providing for an efficient collecting of ions. This results in extremely wide dynamic range (>10 5 ) and low minimum detectable (<10 ppb). For achieving the best minimum detectable it is imperative to minimize the background signal of detector. With the proper construction materials selected, the main source of the background current is electrons, ejected from metal parts by photoemission. In our design, photoemission is effectively suppressed by two means. First, the electrode which is exposed to the UV (collector) has the positive potential relatively to the collector. Second, the negatively polarized collector electrode (which could be source of the photoelectrons) is completely out of 5

6 view of the lamp s window. The combination of this two measures leads to the detector s background current of less than 1 pa. -70 V Ionization Cells Electrodes Teflon Substrate Amplifier Lens UV Lamp US Pat 6,646,444 Fig. 3. Detector All the sensor s elements described above, along with the associated electronic circuits are packed in the plastic housing 17 mm tall and 20.5 mm in diameter. The sensor is complete with the three connector pins: Common, Power Voltage and the Output Signal (Fig.4). The top cover has a 5 mm opening, next to the detector cell, which allows the sample gas to enter the sensor. The detector is protected by two layers of filter material, which prevent particles and liquids from entering the cell. The sensor requires 3 to 5 Volts DC to run. The output signal range is 0 to 2.9 Volt. There are two versions of sensor available low sensitivity Black Label and high sensitivity Silver Label. 6

7 Fig. 4. PID Sensor 4. Performance The Fig. 5 illustrates the Minimum Detectable Concentration (MDC) characteristic of the sensor (Silver Label). Peak-to-peak short-term noise here does not exceed 2.5 ppb Isobutylene equivalent. Therefore, MDC is less than 8 ppb. The Black Label has MDC of 100 ppb, but its the upper concentration limit goes up to 2000 ppm versus 20 ppm for the Silver Label. Hence, both versions cover together the range of 10 ppb to 2000 ppm. Linearity of the sensor s signal (Black Label) is demonstrated by the graph in the Fig. 6. Based on the 20% linearity criteria, the linear range is, at least, from 0 to 2000 ppm. Even though the characteristic is not linear above 2000 ppm, the signal keeps growing up to 5000 ppm. In other words, the sensor s Useful Range is ppm. For the better accuracy beyond the linear range, one should calibrate the sensor with the concentration that is as close as possible to the expected application value. 7

8 Signal, ppb Peak-to-Peak Noise=2.5 ppb MDC=7.5 ppb Time, min Fig. 5. Minimum Detectable Concentration The sensor has been designed to work in the wide range of environmental conditions, to facilitate a variety of indoor/ outdoor, real-world applications. Some special measures were taken to minimize the signal s dependence on the temperature. The lamp and detector properties are rather constant over a wide temperature range. The main source of such dependence is the sensor s electronics (namely, the lamp driver circuitry). The electronics characteristics were optimized in such a way that: a) temperature dependence is minimal b) it is uniform (and even better, linear) so it can be compensated, e.g. digitally by a microprocessor Result of this optimization is reflected in temperature dependence characteristic (Fig. 7). The signal (response to gases) changes no more than 30% over the 40 to +60 C range. The zero air signal over the same range changes insignificantly (~ 0.2 ppm equivalent). 8

9 Reported Concentration, ppm Actual Concentration, ppm Fig. 6. Signal Linearity Real world applications also demand minimal interference from other environmental factors, such as moisture content in the sample gas. In the typical PID, 90% relative humidity in sample causes the instrument s signal to drop by 40 to 60%, relative to the dry sample conditions. This phenomenon ( moisture quenching ) is caused by the water molecules absorbing UV and thus reducing the radiation flux in the detector chamber. UV light absorption is proportional to the molar concentration of water vapor. Thanks to the optimal detector cell geometry, PID sensor has a very minimal moisture quenching (Fig. 8). At 90%RH, signal reduction due to this effect does not exceed 10%. Due to the extremely small volume of the ionization cell, sensor exhibits very fast response time. When the sample gas is flowing across the top cap of the sensor at the flowrate of 150 cc/min, it takes less than 3 seconds for the signal to reach 90% of the final readings. Response time, of course, will vary depending on the conditions of sample delivery to the sensor. 9

10 125 Response to Isobutylene, % of C Ambient Temperature, C Fig. 7. Temperature Dependence Reported Concentration, ppm Relative Humidity, % Fig. 8. Moisture Quenching 10

11 5. Conclusion The first commercial stand-alone, ultra miniature PID sensor has been developed (Trade Name pid-tech ). While keeping competitive characteristics, the sensor offers the portability and compatibilty with existing portable instrumentation. It complements the electrochemical sensors, adds the ppb-level VOC measurement capability to the portable multi-gas monitors. Sensor features Minimum Detection Limit down to 10 ppb, Linear Range up to 2000 ppm. Operating temperature range is from 40 to +40 degrees C. Sensor can be employed for portable environmental VOC monitoring, indoor air quality, personal safety applications and others. The sensor s design is covered by U.S. Patent 6,646,