Thermo Scientific MIRAN 205B Series, Instruction Manual, Page: 77 / 214

Chapter 7 Configuration and Setup
7-23
Unit
To select a unit for the application, press
1
from the Set Parameters Menu. The Gas Unit
Selection Menu is displayed. The last unit configured is shown on the display. If this is
the unit of the present application, press
ESC
. The display returns to the Application
Setup Menu. To change the unit, press
1
. The Gas Unit Selection Menu is displayed.
Select the unit for the application. The display responds "-ACCEPTED-" and then
returns to the Analysis Setup Menu Page 4.
┌────────────────────────────────────────┐
│╒════ METHANE SUMMARY ════╕ Select │
││ HRL: 100.0ppm │ gas unit: │
││ High: 100.0ppm │ │
││ Low: 50.00ppm │ ppm │
││ STEL: 5.00ppm @ 15min │ │
││ TwaWt: Off │ 1=ppb │
││ MolWt: 16.0 D: Off │ 2=ppm │
│└───────── Gas1 ──────────┘ 3=Percent │
└────────────────────────────────────────┘
Gas Unit Menu
The following definitions apply to the Gas Unit menu, Alarm Level menu, Alarm Level
Enter menu, and the STEL Time Select menu:
 HRL is the high range limit.

High is the high level alarm setpoint.

Low is the low level alarm setpoint.

STEL is the Short Term Exposure Limit setpoint.

TwaWt is the group time weighted average factor. Many times when several
compounds are present in an area, the combined effect of the gases needs to be
considered for industrial hygiene purposes. For example, two gases (Gas A with
a TWA of 5 ppm and Gas B with a TWA of 10 ppm), are each below the TWA
value for the 8-hour period. However, when both gases are present, it is
important to report the combined effect. This is sometimes referred to as a
synergistic TWA. The factor that is typically applied to each gas is 1/TWA. So
for this example, the factor for Gas A is 1/5 and for Gas B is 1/10. In the
calculation of the Group TWA, the instantaneous concentration is multiplied by
the factor. Therefore, if Gas A has a concentration of 3 and Gas B, a
concentration of 7, the Group TWA is:
1.3
10
1
(7)
5
1
) 3 (















MOLWT is the molecular weight of the gas, and is used for conversion to mg/m
3
.

D is the density.