MSA Orion G Operating Manual page 31

Chapter 3, Using the Orion G Gas Utility Instrument
Time Weighted Average (TWA) (FIGURE 3-13)
• The TWA flag will appear
in the upper portion of the
display to show the
average exposure since
the TWA reading was
reset.
• When the amount of gas
detected by the Orion G
Multigas and Leak
Detector is greater than
the eight-hour TWA limit:
• Alarm Sounds
• Alarm Lights Flash
• The TWA
flag flashes.
To Reset the TWA:
1. Access the TWA page.
2. Press and hold the CHANGE/RESET button until the TWA
flag flashes.
3. Push the ON-OFF/PAGE button to reset the TWA.
The TWA alarm is calculated over an eight-hour exposure.
Calculation examples are as follows:
• 1-hour exposure of 50 PPM:
(1 hour x 50 PPM) + (7 hours x 0 PPM )
8 hours
• 4-hour exposure of 50 PPM
4-hour exposure of 100 PPM:
(4 hours x 50 PPM) + (4 hours x 100 PPM )
• 12-hour exposure of 100 PPM:
(12 hours x 100 PPM)
8 hours
3-16
Figure 3-13. Exposure Page with
8 hours
= 150 PPM
Operating Manual
TWA Alarm
= 6.25 PPM
= 75 PPM