IR Overall Status Display – Menu Page 2

 

The IR Overall Status Display provides the user with the calculated carbon percentage (%C) from two different sources (probe and infrared).  It provides information to allow the atmosphere controller to be “tuned” to match the information from the 3-gas analyzer.

 

To obtain the most information from this screen, some data must be manually entered.  This is done at the right hand side of the screen under the heading Operator.  Using the keypad to enter numbers, and the arrow keys to move the highlighted area up and down, the following data should be entered:

 

FC TC= This is the furnace thermocouple value, or the furnace temperature.

PB MV= This is the millivoltage from the carbon probe.

PB TC= This is the probe thermocouple value, or the probe temperature.

COF= This is the CO Factor value read from the SSi, Honeywell, Barber Colman, Yokogawa, or other atmosphere controller.

PF= This is the Process Factor value read from the Marathon Sensors atmosphere controller.

Temperature Units = This determines the measurement units for temperature.  Enter 0 for Fahrenheit or 1 for Celsius.

 

Measurement of Infrared % Carbon (IR %C)

To accurately measure the % carbon in a furnace atmosphere, the instrument will need to know the values of each of the three gases and the temperature of the gas being measured.  At the left side of the screen, under the heading Measured, are the real-time values of CO, CO2, and CH4.  The values of these three gases, plus the furnace temperature (FC TC) value, will result in the calculation of the IR % Carbon (IR %C).  This is displayed in the center of the screen, under the heading Calculated.  Please note that if the furnace temperature 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, large changes in CO Factor or Process Factor should be verified by shim stock analysis or other means to confirm the significance of the change.