Introduction; General; Principle Of Operation - Honeywell 705 Operating Instructions Manual

MAN0364.P65 Issue 03, Mar 01
00705M5003

1.1 GENERAL

The Type 705 Carbon Monoxide Sensor is a toxic gas detector
certified for installation in a hazardous area. This version
uses an Electrochemical (ECC) Sensor, in conjunction with
an electronic circuit to form a three wire 4 - 20mA transmitter.
The sensor comprises an aluminium housing containing an
Electrochemical Cell with a sintered stainless steel disc
forming the face of the sensor. The housing has a 47mm AF
body with a 3/4 NPT mounting thread at one end and an M36
accessory thread on the other end. A plastic Filter Housing
screws on to the accessory thread and holds a Hydrophobic
Barrier in position when accessories are not fitted. There are
a number of accessories available for the 705 Sensor which
utilise the M36 accessory thread. Refer to Section 8 for
alternative systems to which the 705 Sensor can be
connected.
Note:

1.2 PRINCIPLE OF OPERATION

The sensor contains an Electrochemical Cell. Electrochemical
Cells are dynamic devices which generate a current in
proportion to gas concentration within the cell. As the current
output from the cell is in microamps, it is amplified and
processed by the electronics in the terminal enclosure. The
resulting signal in the range 4 - 20mA (representing
0 - 500ppm CO) can then be connected to equipment in the
safe area to provide alarm and concentration indications.

1. INTRODUCTION

New sensors are despatched with a protection disc
in place of the Hydrophobic Barrier. This disc should
not be removed until the sensor is installed or
commissioned.
7
705 Sensor CO