Appendix J: 4-20 mA Input and Output
Page J-8
Lasair II Particle Counter Operations Manual
•
If a channel is set to 1000 for the scale, then the readings will be within 10 counts
(i.e. 1%).
•
The minimum detectable change is about 2 counts (i.e., 1000/(16 mA/0.025 mA)
or 1.56).
In summary, setting the scale factor is a balance between setting the scale large
enough to cover the worst case expected count output, while at the same time setting
it small enough to accurately show normal events.
Example:
The user would like to differentiate between 99, 100, and 101 counts.
Then:
1
Range = 16 mA = 20-4
2
With min. detectable change of 0.025 mA, this means the range can be divided
into a max of 640 channels = 16 mA/0.025 mA.
3
Then by setting 4 mA = 0 counts and 20 mA = 640 counts
4
We derive:
99 Counts = 6.475 mA
100 Counts = 6.500 mA
101 Counts = 6.525 mA
