Dwyer PMT - Manual (Page 27)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1015-F Page 21 ©2007
Linear Scale: Linear scaling provides direct, proportional, linear change in the 4-20mA output signal
over the selected pA range. Allowable ranges are 0-100pA, 0-500pA, 0-1000pA, and 0-5000pA. To
convert the 4-20mA linear signal back into pA in the remote plc/dcs, the following formula is used:
) ( *
) ( 0 . 16
) ( 0 . 4 ) (
pA Range
mA
mA mA Measured
pA
−
=
If unsure where to set the linear output range, a good generic selection for fabric filters is a range that
is equal to or greater than 25 times the measured baseline signal. For example, if the baseline signal is
15pA, the linear range would be selected as 15pA * 25 = 375pA … choose 500pA linear range.
Resolution: The 4-20mA output is transmitted at a resolution of 12 bits or 4.88uA.
8.4 Time-constant Adjustment
The particulate transmitter contains an adjustable digital filter. Particulate flow is very erratic in
nature. Filter cleaning cycles can also cause dramatic changes. Setting the filter time constant higher
(in seconds) will “smooth” out the signal. Setting the filter time constant lower (in seconds) will
increase the responsiveness of the signal and make it more “bouncy”. Keep in mind that setting the
filter time constant too high may reduce the ability to use peak readings to determine the onset of
filter wear.
The recommended filter time constant for fabric filters is 3.0 seconds. If the output signal is too
bouncy with this setting, the time constant should be increased until a smoother reading is obtained. If
the output signal is steady but peak readings are not seen during the activation of the filter pulse-jet
cleaning cycle, the time constant should be decreased until the peak readings can be measured.