LB 470RID Level
3 System Description
56925-1BA2 Rev.02, 05/2021
21
3.3.3
Secondary Channel
As is shown in illustration 3, a second channel (SC) with a different threshold set-
ting is provided (threshold 2). Only pulses which cross the threshold 2 voltage
(height) level are processed in channel SC, the secondary channel. The number of
pulses coming from the interference radiation source which fulfil this criterion is
small and can be neglected in most cases. Hence, in contrast to measuring channel,
the pulse rate in the secondary channel remains uninfluenced by interference ra-
diation.
Conversely, a large number of the Co-60 impulses also fall below threshold 2 and
do not contribute to the pulse rate in the secondary channel, which is therefore
smaller than the pulse rate in the measuring channel. This difference in sensitivity
compared with the measuring channel results is bigger statistical variations of the
pulse rate registered by the secondary channel.
To sum up, it can be said that the LB 470 measuring system-RID contains two chan-
nels which measure at the same time and independently of each other a change in
pulse rate dependent on the continuous level:
•
The highly sensitive Measuring Channel (MC) which shows only low pulse
rate variations but is however, susceptible for interference radiation.
•
The less sensitive Secondary Channel (SC) with larger pulse rate variations
which is, however, at the same time insensitive to interference radiation.
3.3.4
Interference Radiation Suppression
If interference radiation hits the detector, the count rate in the measuring channel
MC rises quickly and the resulting level signal drops. At the same time, the change
in counts and therefore also the resulting level signal in the secondary channel SC
remains uninfluenced by interference radiation. The resultant difference between
the levels represented by the secondary channel SC and the measuring channel MC,
the channel difference CD, is a measure of the strength of the interference radia-
tion. If the CD value exceeds a value preset by the user, the interference radiation
tolerance
T
(factory set: 10%), the active channel is switched from the measuring
channel MC to the secondary channel SC, i.e. under interference radiation condi-
tions, the indicated level is determined by the secondary channel SC.
Equation Channel Selection Criteria between the Measuring
Channel MC and the Secondary Channel SC
𝐶𝐶𝐶𝐶 ≥ 𝑇𝑇
𝐶𝐶𝐶𝐶
=
𝐿𝐿
𝑆𝑆𝑆𝑆
− 𝐿𝐿
𝑀𝑀𝑆𝑆
In Equation 1, L
SC
and L
MC
represent the level measurements from the Secondary
Channel or Measuring Channel; CD is the channel difference (live value) and
T
is
the interference radiation tolerance (Fixed value, factory set = 10%)
A channel difference smaller than the interference radiation tolerance
T
is due to
statistical variations or differences in the calibrations and does not result in switch-
ing channels.
Due to the low pulse rate in the Secondary Channel the level measurement varia-
tions are somewhat greater during periods of interference. However, the level is
measured continuously and reliably.
When the interference ends, the count rate in the measuring channel drops to nor-
mal and the channel difference decreases. When the channel difference falls to less
than the value of
T
the instruments switches back to its normal configuration.
Summary of Contents for DUO XPERT LB 470RID
Page 2: ......
Page 12: ...1 About this Operating Manual LB 470RID Level 12 56925 1BA2 Rev 02 05 2021...
Page 16: ...2 Safety LB 470RID Level 16 56925 1BA2 Rev 02 05 2021...
Page 34: ...3 System Description LB 470RID Level 34 56925 1BA2 Rev 02 05 2021...
Page 42: ...4 Installation LB 470RID Level 42 56925 1BA2 Rev 02 05 2021...
Page 207: ...Level F llstand Technical Information Technische Information 56925TI1L Rev No 04 05 2021...
