Dwyer PMT User Manual
Page 26
Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1015-F
Page
20
©2007
8.3 4-20mA Output Range Adjustment
For proper selection of 4-20mA output range, both baseline and peak levels must be taken into
account. The 4-20mA output range should be selected to allow accurate measurement of low baseline
signals while still allowing resolution of higher peak signals that occur during pulse-jet cleaning
cycles.
The 4-20mA analog output represents the linear or logarithmic equivalent of the “pA” particulate
levels. Once the analog output has been transmitted to the PLC or chart recorder, it is recommended to
convert the 4-20mA signal back into pA to assist in data interpretation, alarm level determination and
historical data comparison. This can be of particular importance for EPA regulatory applications. The
following two examples show the formulas used to convert the 4-20mA signal into pA.
Logarithmic Scale (Recommended): Logarithmic scaling provides a method of transmitting signals
such that low-level signals are expanded for increased resolution and accuracy while high-level
signals are compacted for increased signal range. Allowable ranges are 0.5-500pA and 5-5000pA.
Both ranges are three decades wide. To convert the 4-20mA logarithmic signal back into pA in the
remote plc/dcs, the following formula is used:
1. Scale mA input to proper log(10) argument:
333
.
5
)
(
00
.
4
)
(
mA
mA
Measured
Y
−
=
2. Convert log(10) argument to pA:
Y
pA
ge
MinimumRan
pA
10
)
(
∗
=
Example: 0.5 – 500pA log scale with measured loop current of 9.33mA:
1. Scale mA input to proper log(10) argument:
0
.
1
333
.
5
)
(
0
.
4
)
(
33
.
9
=
−
=
mA
mA
Y
2. Convert log(10) argument to pA:
)
(
0
.
5
10
)
(
5
.
0
10
)
(
5
.
0
0
.
1
pA
pA
pA
pA
=
∗
=
∗
=
Three decade logarithmic scale gives the following equivalent mA outputs:
4-20mA Scaling 0.5 – 500pA LOG:
4mA = 0.5 pA
9.33mA
=
5.0pA
14.66mA = 50pA
20mA = 500 pA
4-20mA Scaling 5 – 5000pA LOG:
4mA = 5.0 pA
9.33mA
=
50pA
14.66mA = 500pA
20mA = 5000 pA