Super Systems Gold Probe - Manual (Page 5)

4
SSi Gold Probe Manual Rev C
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 Equation (1).
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 Equation (2), 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).
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.
ZIRCONIA O
2
RESPONSE
E
c
= 0.0276T
R
log (P
f
/P
a
) millivolts (1)
Zr PROBE ALGORITHM
%C= ( E
C
, %CO, T
R
) mVDC (2)
Where: 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.