Super Systems PGA3500 - Manual (Page 11)

SSi Manual 4559 Page 11 of 26 3-Gas Analyzer PGA3500
Revision Level - D
information has not been entered correctly the resulting carbon calculation will NOT be
accurate.
Measurement of Probe % Carbon (PB %C)
The measurement of the probe % carbon requires three pieces of information to be entered.
These are the probe millivolts, probe temperature, and either the COF or the PF (depending
on the type of atmosphere controller you are using). The probe millivolts and probe
temperatures are entered on the right side of the page. Before entering either a COF or a PF,
you will need to determine the manufacturer of the atmosphere control instrument you are
using. If SSi, Honeywell, Barber Colman, Yokogawa, or anyone other than Marathon
manufactures the instrument, then it will contain a CO Factor Adjustment variable. If you are
using a Marathon Sensors instrument, then this variable is called a Process Factor
Adjustment. Only one of these (COF or PF) will be used at any one time, and the other will
not be relevant. If your instrument has a CO Factor Adjustment variable, then use the arrow
keys to go to the appropriate location and type in the value that is stored in your atmosphere
control instrument. After the value is entered, you will see an * appear next to the number.
This lets you know that the CO Factor is being used for the calculations, not the Process
Factor. If your instrument has a Process Factor Adjustment variable, then this value should
be entered in the PF= slot, which will result in the * appearing by the number that you
entered.
The calculation of probe % carbon is no different from the % carbon as displayed on the
atmosphere controller. The algorithm used by both instruments in their calculations is
identical. The reason the information is entered into the PGA is not to calculate the probe %
carbon, but to enable the PGA to compute the suggested COF / PF. Without knowing the
current instrument values, it cannot compute the suggested values.
What is a CO Factor or a Process Factor?
The carbon probe is measuring the amount of oxygen in the atmosphere. Knowing the
amount of oxygen, the atmosphere controller can determine the percentage of carbon. The
calculation that the instrument uses to translate oxygen concentration into % carbon is based
on a theoretically pure atmosphere being present in the furnace. The composition of this
theoretically pure atmosphere is 40% hydrogen (H2), 40% nitrogen (N2), and 20% carbon
monoxide (CO). In many situations, the measured amount of CO is less than the theoretically
exact 20%. This can be due to a variety of factors including seasonal changes in natural gas
composition and incomplete gas cracking in an endothermic generator. The CO Factor and
Process Factor adjustments are intended to make adjustments to the calculation to
accommodate differences between theoretical and actual gas compositions.
Suggested COF / PF
Between the two methods of determining % carbon (probe and infrared), the infrared is
considered to be more accurate. This is because the infrared analyzer evaluates the levels of
three gases (CO, CO2, and CH4) to make its calculation, instead of just using % oxygen like
the probe does. At the bottom of the middle display column, you will see suggested COF and
PF values. These values are determined by making a comparison between where the probe
is actually reading and where it should be reading. The suggested COF and PF values can be
entered into your atmosphere controller to make it display the same % carbon reading as the
PGA3500. By performing periodic evaluations with the PGA3500 and making the suggested
modifications to the adjustment factor in the atmosphere controller, you can be assured that
your continuous source of process data (the probe) is as accurate as possible. Of course,