Person
‘Person’ enables the operator to monitor in personal safety mode. This mode automatically stores
an average N
2
O reading every 15 minutes. This is used to calculate and display an eight hour TWA
(time weighted average) reading which is the operator’s personal exposure level to N
2
O over their
working day (often called OEL - occupational exposure limit).
The TWA is measured over a 24 hour period.
The TWA calculation used by the instrument is that described by the Health and Safety Executive
EH40/2002 Occupational Exposure Limits 2002. The TWA is represented mathematically by the
following equation:
(C1T1 + C2T2...CnTn) / 8hrs.
Where C1 is the average reading for the time period T1.
In this mode the instrument will alarm when the operator has exceeded the recommended TWA limit
(the OEL for N
2
O = 100ppm). This operating mode incorporates a confidence chirp which sounds
every 30 seconds to give the operator confidence that the monitor is still operating correctly. There is
also an option which allows the operator to set an additional alarm for the actual N
2
O level, i.e.
greater than 300ppm.
It is possible using Analyser Data Manager to change the OEL level for the TWA limit (default
100ppm). We would also recommend that the operator changes the ‘Site’ and ‘ID’ fields to ‘Site’ and
‘Name’ using the ‘Configuration’ tab.
Once IDs have been selected and accepted, readings commence immediately and are monitored
continuously. To stop monitoring the operator selects the soft-key ‘Person’, enters the stop code
5709 and presses the ‘Enter’ key. Readings are automatically stored.
The following points should be noted.
The 15 minutes averages used for the TWA calculation are the result of readings taken each
second. Thus short peaks of concentration will be correctly recorded and added into the TWA
If the instrument is turned off when in ‘Person’ mode, the readings will be assumed to be zero
for the TWA calculation during the time when the instrument is off.
The TWA value displayed on the instrument is not downloaded to Analyser Data Manager
(ADM). ADM recalculates the TWA based on the downloaded raw data and user set parameters.
Area:
‘Area’ enables the operator to monitor in area or room mode. This mode automatically stores a rolling
average N
2
O reading every ‘n’ minute(s). It is indicated by ‘AV’ on the screen.
The calculation is represented mathematically by the following equation: (C1 + C2...Cn) / elapsed time
in hrs.
This mode would typically be used to monitor a ventilation system in an operating theatre or similar
where N
2
O is used throughout the day with different people coming and going. Primarily for room or
area monitoring, this mode is intended to give the operator a little more flexibility and can be user
modified to suit the application. The monitoring interval and duration can be adjusted by the
operator. There are alarm options which allow the operator to set an alarm for the average and
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