Dwyer PMT - Manual (Page 23)

Particulate Monitoring Systems
Installation & Operating Manual
Document No. 210-1015-F Page 17 ©2007
8.
Interpreting Readings and Adjusting 4-20mA Range
8.1 Fabric Filter (Baghouse) Applications
Particulate flow is very dynamic in nature, thus the output signal is also usually very dynamic. This is
more often the case with fabric filter and dust collection exhaust monitoring applications where filter
emissions and filter cleaning systems can cause wide ranging variations in the particulate levels.
When monitoring downstream of fabric filter, it is often possible for the difference between baseline
readings and peak readings following cleaning cycles, to vary by a factor of 10 or even 100. This is
the reason for the logarithmic output (linear output is also easily selected).
The logarithmic scale provides the ability to simultaneously monitor and resolve the baseline and
peak readings. It is not uncommon to have baseline readings of less than 10pA while at the same time
peak readings may be over a hundred or more.
Particulate levels listed below are typical for new or well maintained bag or cartridge filter dust
collection system. Many factors, other than generic bag wear may contribute to high particulate levels
including but not limited to: Improper filter installation, bad tube sheet seals, improper filter media
for process conditions, high differential pressure or a lack of a filter cake buildup.
PARTICULATE READING GUIDE FOR FABRIC FILTERS
• The guide below is only an approximate guide for modern, highly-efficient baghouses
• With larger or older baghouses, readings can be significantly higher than the ranges shown below
• Shaker and reverse air baghouses will have higher peak readings as compared to pulse jet
• Readings tend to be higher when new filters are installed and a filter cake has yet to form
• With small cartridge filters, the readings tend to be at the lower end of the ranges
• Readings tend to also be lower with highly-efficient filter media such as Gore-Tex® fabric (Gore-
Tex is a registered trademark of W.L. Gore & Associates.)
Typical Readings and Guide for New Efficient Fabric Filters
AVERAGE
BASELINE READINGS
PEAK READINGS
(after cleaning cycle)
FILTER CONDITION
1 – 10 pA Less than 50pA No significant emissions
10 – 100 pA Less than 500pA Onset of emissions
100 – 1000pA Greater than 500pA Significant emissions present
IMPORTANT