Detcon PI-700 - Manual (Page 5)

Model PI-700
PI-700 Instruction Manual Rev. 2.5 Page 1 of 54
1. Introduction
1.1 Description
Detcon Model PI-700 VOC gas sensors are non-intrusive “Smart” sensors designed
to detect and monitor a wide range of VOC and Toxic gasses in air. Ranges of
detection for target gasses are from 0-1ppm up to 0-5,000ppm. The sensor features
an LED display of current reading, fault and calibration status. The Sensor is
equipped with standard analog 4-20mA and Modbus™ RS-485 outputs. A primary
feature of the sensor is its method of automatic calibration, which guides the user
through each step via fully scripted instructions displayed on the LED display.
The microprocessor-supervised electronics are packaged in an encapsulated
module and housed in an explosion proof casting, called the ITM (Intelligent
Transmitter Module). The ITM includes a four character alpha/numeric LED used
to display sensor readings, and the sensor’s menu driven features when the hand-
held programming magnet is used.
Sensor Technology
The sensors are based on plug-in replaceable miniature PID (Photo-Ionization Detector) sensor technology.
The sensor is sensitive to ambient gases that have ionization potentials of < 10.6eV, making it highly sensitive
but extremely non-specific. The sensor responds to most toxic VOC compounds and many other toxic gases
as well. The sensor is comprised of a UV lamp covered by a specific optical filter which projects only
radiation in the 10.6eV range. Target gases that diffuse into the sensor chamber with ionization potentials of <
10.6eV, are ionized by the radiation and give up free electrons. The free electrons are captured by the high
voltage collection grid and provide a current that is directly proportional to the concentration of the target gas.
Figure 1 Typical Sensor Cell
1.2 Sensor Electronics Design
Intelligent Transmitter Module
The PI-700 Intelligent Transmitter Module (ITM) is a fully encapsulated microprocessor-based package that is
universal in design and will accept any Detcon intelligent plug-in PID gas sensor. The ITM design uses an
internal intrinsically safe barrier circuit that lifts the requirement for use of flame arrestors to achieve Class 1,
Division 1 (Zone1) area classification.
This facilitates fast response times and improved calibration
repeatability on strongly absorbing gas types. The ITM circuit functions include extensive I/O circuit
protection, on-board power supplies, internal intrinsically safe barrier circuit, microprocessor, LED display,
magnetic programming switches, a linear 4-20mA DC output, and a Modbus™ RS-485 output. Magnetic
program switches located on either side of the LED Display are activated via a hand-held magnetic