aspida
– User Manual
Document Ref: PGA-801-10
-
December 2014
Page 15
Time-weighted average (TWA) monitoring of carbon dioxide
Carbon dioxide is toxic to the human body as concentrations increase. Short term exposure to the gas
can be potentially lethal. Longer term exposure to more moderate levels of carbon dioxide can also be
detrimental to health, so health and safety bodies (i.e. UK HSE) define occupational exposure levels
(OEL) for the gas. These OELs provide a recommended safe exposure to carbon dioxide whilst under
working conditions. The occupational exposure levels are based on a calculated time-weighted
average (TWA) level of carbon dioxide in any one 24 hour period. The TWA calculation is based on
average carbon dioxide levels weighted to an assumed standard 8-hour daily working shift.
The Analox
aspida
has a facility which automatically calculates a TWA value on a per-user basis.
When a user powers on the device and selects a user ID, the Analox
aspida
begins calculating and
continuously updating that user’s own TWA. The device also takes into account any historic data from
the last 24 hours which is relevant to the selected user. TWA data is logged alongside the actual gas
readings in the internal data log.
NOTE:
If no users IDs are registered to the device, the TWA values will all be attributed to a default
user.
An alarm feature is provided which will warn of a high TWA exposure to carbon dioxide. In addition to
the normal 3 available gas alarms per sensor, carbon dioxide sensors have a 4
th
alarm (priority 4)
which is triggered when the calculated TWA value exceeds the TWA set-point. This alarm is set by
default to the UK HSE recommended OEL for carbon dioxide of 5000ppm (0.5%). When this alarm is
triggered, the red LEDs, vibration and horn warnings will activate as with other gas alarms. In the case
of a TWA alarm, ‘TWA’ will be shown below the carbon dioxide reading on the display, as shown
below. The TWA alarm is the lowest priority alarm and as such will be over-ridden by any gas alarms
triggered by a short term increase in carbon dioxide levels.
Figure 7 - The main gas display showing that to user has exceeded their TWA
exposure to carbon dioxide
As with the standard gas alarms, the TWA alarm can also be fully configured using the
aspida
configuration software.
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