BASIC OPERATING THEORY

 

Carbon potential of a conventional furnace atmosphere is defined as the %C achieved in a coupon of carbon steel shim stock equilibrated in the furnace atmosphere. Unfortunately, equilibration time is long, so it is impossible to continuously control the atmosphere on the basis of shim stock measurements. A zirconia sensor, however, can be used to measure and control the carbon potential precisely, and on a continuous basis.

Strictly speaking, the zirconia probe is not sensing carbon at all. It is an oxygen sensor with a mVDC output described by the following equation.

ZIRCONIA  O₂  RESPONSE

       E_c  = 0.0276T_R log (P_f /P_a) millivolts

Fortunately, an empirical (experimental) relationship exists between oxygen concentration and carbon potential, and this relationship has been used in carbon control instruments since the early ‘70’s. The equation used by most control manufacturers today is illustrated by the Equation below, which states that there are only three variables affecting the measured millivoltage. Because the actual equation used is somewhat complex, it is not reproduced here. A full description of probe theory will be found in SSi technical bulletin T4401 (Zirconia Sensor Theory).

 

Zr PROBE ALGORITHM

 

%C= Φ( E_C , %CO, T_R) mVDC

Where:

• Φ means ‘is a function of’
• %C is the carbon potential
• %CO is carbon monoxide percentage
• T_R is the absolute temperature in degrees
• Rankine (deg. F + 460).
• and E_C is the probe output in millivolts.

 

All competitive probes will invariably agree within one or two millivolts when exposed to the same atmosphere under equilibrium conditions. Differences in values listed by probe vendors relate to differences in manufacturers’ source data, but the true value of the zirconia probe is its repeatability.