Table of Contents
Advertisement
2
D-Guard
23.
APPENDIX F:
23.1. Environmental Effects on Detector Performance
Environmental changes in temperature, pressure and humidity can alter the performance of the sensing elements
2
within D-Guard
.
23.2. Temperature
The use of inherently stable electronics and built-in software routines provide compensation for static or slow moving
changes in ambient temperature. However, rapid changes in temperature, if severe enough, can impart transient
shifts in the detector's output signal. The effect is generally low level, but in systems where alarm points are set close
to the normal zero operating point, false alarms could occur. For this reason, wherever possible, locate the detector
out of direct sunlight or other sources of heat that may be cycled.
23.3. Pressure
2
Static or slow rates of change of atmospheric pressure will have little effect on the performance of D-Guard
Toxic gas
detectors. Transient pressure changes can however cause spurious responses generally lasting for tens of seconds.
Oxygen sensors are more severely affected than other 'electro-chemical' type detector types.
Infra red sensors will show a response to pressure changes that is described by the ideal gas laws. The molar
concentration of a gas (n) is proportional to pressure according to n = pv / rt, where p = pressure, v = volume, r = ideal
gas constant and t = absolute temperature.
Detectors will respond to both positive and negative pressure transients but the response in a toxic gas sensor to
positive transients is about 3 times greater than for negative transients. Infra-Red sensors will respond equally to
positive and negative pressure shifts. Detectors should not be located in sealed rooms with doors that may be
opened and closed rapidly, or in locations where ventilation systems can create pressure pulses.
23.4. Humidity
2
The electrochemical cells used in D-Guard
detectors use an aqueous sulphuric acid electrolyte. This electrolyte
remains in an equilibrium state of sulphuric acid to water content at a relative humidity of 60%. If the detectors are
subject to a relative humidity greater than 60%, then the electrolyte will absorb water. Conversely if the exposure is
to a relative humidity less than 60%, water will be lost from the electrolyte.
For exposure to dry atmospheres where water is lost from the electrolyte, the cell will eventually 'settle' and adopt
the new conditions as its operating point. Prolonged exposure to dry atmospheres will eventually prevent the cell
2
from operating and will present a fault condition from the D-Guard
detector. Normal sensor operation can be
restored by exposure to atmospheres within the 15% to 90% RH range. Re-calibration of the detector will however be
required.
For exposure to a very high relative humidity, the cell may not be able to accommodate the increased water volume
through absorption, and may leak. Once leakage has occurred, the cell is deemed to have failed and will require
replacement.
2
It is important that the D-Guard
detectors are installed and operated in conditions that meet the product
specification of 15% to 90% RH per the product specification.
62
Advertisement
Table of Contents
