Dwyer DPM - Manual (Page 25)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1003-M Page 18 ©2007
6.6 4-20mA Analog Output Scaling
Particulate levels may be transmitted to external devices with the 4-20mA analog output. Typical
applications include remote monitoring of particulate levels with a PLC, chart recorder or panel
meter. The 4-20mA output is transmitted as linear or multi-decade logarithmic output.
Two parameters determine the type of output signal. The “4-20mA Minimum Scale” parameter
determines the pA equivalent of the 4mA output. Setting this parameter to “0.0” enables the Linear
scale. A value other than “0.0” initiates Logarithmic scale. The “4-20mA Output Span” parameter
determines the pA equivalent of the 20 mA output.
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.
4-20mA Linear Output
Linear output is selected when the “4-20mA Output Minimum Scale” is set to “0.0” pA. To convert
the 4-20mA output signal back to pA, use the following formula:
pA = ((4-20mA Output Span) * (mA - 4)) / 16
Example:
Where: 4-20mA Minimum Scale = 0.0 and
4-20mA Output Span = 1000pA (from Menu #2)
4-20mA Output pA
4 0.0
5 62.5
6 125.0
7 187.5
8 250.0
9 312.5
10 375.0
11 437.5
12 500.0
13 562.5
14 625.0
15 687.5
16 750.0
17 812.5
18 875.0
19 937.5
20 1000.0