by Dr. Jack Driscoll

The industrial hygienist is called upon whenever there is a problem that affects human health. In some cases, there is little time to react, just grab the gear and go. PID's 102+ with Snap on Heads are perfect  for this application since all sensors can be calibrated with a simple 2 point calibration, the logging function is the same and a wide variety of heads and sensors are available for a multitude of applications.. The approach for the multiple heads comes from our Model 101 () where we used multiple  probes to optimize the light sources for each lamp. This new approach employs a compact multisensory head to replace the probes.
 
The process of photoionization occurs when a photon of sufficient energy is absorbed by a molecule and results in the formation of an electron plus an ion :

R+ + hv  =  R+ + e-

where:

R = an ionizable species

hv = a photon with sufficient energy to ionize species R

The ion current is proportional to the concentration.  A PID consists of an ion chamber, a UV lamp (9.5, 10.6 or 11.7), high voltage sources for the accelerating electrode and lamp, an amplifier and a digital display.  
The simplest and most common  head is, of course,  the 10.6  PID Head. There are three types of PID heads (9.5, 10.6 and 11.7 eV). Each of these are interchangeable and can be easily snapped into place. The readout unit when snapped in will display the appropriate PID head and library of response factors for that head. The Analyzer can be calibrated on any gas in the library and the Respond As feature will correct for the direct response for any other gas in the library. The calibration data is stored in memory.

The most common head is the 10.6 eV. It has the longest lifetime (5,000-6,000 hours). The 11.7 was described in a previous paper (email jen@pid.bz for a copy; put "11.7 paper" in subject line). The 10.6 has the highest sensitivity and is 20  times more sensitive than the 9.5 or 11.7 In addition, with the 102+, 1- 3 additional sensors can be accommodated in the same head. Another option is to have a separate confined space head (CG or IR, H2S, O2, CO) head which can be snapped into place. Other options are also available. Other heads can have an infrared (IR), combustible gas (CG) , thermal conductivity (TCD) sensor, a mixture of the previous sensors with one to three electrochemical (EC) sensors, or 1-3 EC sensors. Each head can be setup differently and snapped into a single readout module. 
Some applications for the different PID heads can be seen in Table I where the 9.5 head is useful for easily ionized species such as amines, aromatics, the 11.7 is useful for low molecular (tightly bonded) species formaldehyde,acrylonitrile, methanol, and chloroalkanes that cannot be detected with the 10.6 lamp. The 10.6 is the most sensitive general purpose lamp.

Table I
Response for Various Lamps
Lamp eV Response
9.5 Styrene, mercaptans, amines
10.6 Alkanes > C4, alkenes, aromatics,hydrides ( H2S, NH3, PH3, AsH3)
11.7 Alkanes (except CH4), aromatics, alkenes,, chloroalkanes, CAN, acrylonitrile, formaldehyde 
   
Other Sensors 
Dual beam miniature IR detectors for CH4 (% LEL to 100%) and CO2 (ppm to 100%)   
  
CG  library
 
TCD library H2
 
EC sensors
             
Signals from electrochemical sensors result from oxidation or reduction reactions some of which are shown below:
 
The signal can be positive or negative with respect to ground and the output can be  voltage or current. We have designed a device that can amplify  low level voltage or current signals which are then processed by a bipolar 16 bit ADC and fast mictroprocessor (50 MIPS).

With these flexible electronics and software, it is easy for customers to interchange sensors without worrying about a limited (range = 64,000:1) range or polarity of the various sensors. In conclusion, the Model 102+ is the most versatile multichannel analyzer available.