Environnement MP101M, Technical Manual

Page: 45 / 196

Duplication prohibited
MP101M
Environnement
S.A
2–3
APRIL 2014
2.1. BETA GAUGE PRINCIPLE (FIGURE 2–1 AND FIGURE 2–2)
A BETA gauge consists of a radioactive source of Carbon 14 (
14
C) radioelement, emitting soft Beta
radiation and a radioactive radiation detector: Geiger-Müller (G.M.) tube. The G.M. is mounted at a
given distance downstream of the filter ribbon which gathers the particles suspended in the air.
Drum-type
source
holder
Dust
deposit
Pay-out
reel
Radioactive
source
C.14
Filter
ribbon
Take-up
reel
Geiger-Müller counter
Figure 2–2 – Beta gauge
When determining a mass deposit, at the end of each period or cycle, the source is aligned with the
dust deposit and the counter (G.M.).
The low-energy Beta rays are absorbed by the matter by collision with the electrons whose number
will be proportional to the density.
The matter consists of the glass fibre filter, the dust deposited and the air between the source and the
G.M.
The absorption is governed by an exponential law and is independent of the physico-chemical nature
of the matter.
The measurement consists in calculating the difference in absorption observed on the blank filter at
the start of the cycle and on the charged filter at the end of the period or cycle.
This differential measurement is used to compensate for the non-uniformity of the filter when
calculating the mass of the deposited dust. The monitor also compensates for the variation in
temperature of the weight of the air knife.