XZR250 User’s Manual
Michell Instruments
41
APPENDIX B
P1
(O
2
PARTIAL PRESSURE
TO BE MEASURED)
COMMON
P2
(CHAMBER PRESSURE)
SENSE
V
SENSE
PUMP
SENSING PLA
TE
PUMPING PLA
TE
i
Sensing Plate:
A difference in oxygen pressure across the second ZrO
2
square generates a Nernst
voltage which is logarithmically proportional to the ratio of the oxygen ion concentrations
(see Nernst Voltage). As the pressure inside the chamber (P2) is always kept less than
the pressure outside of the chamber (P1), the voltage at SENSE with respect to the
common is always positive.
This voltage is sensed and compared with two reference voltages and every time either
of these two references is reached, the direction of the constant current source is
reversed. When the PPO
2
is high, it takes longer to reach the pump reversal voltages
than it does in a low PPO
2
atmosphere. This is because a greater number of oxygen ions
are required to be pumped in order to create the same ratio-metric pressure difference
across the sensing disc.
Nernst Voltage:
Two different ion concentrations on either side of an electrolyte generate an electrical
potential known as the Nernst Voltage. This voltage is proportional to the natural
logarithm of the ratio of the two different ion concentrations.
Δ
V =
k T
ln
B
c
1
c
2
( (
e
O
kB
Boltzmann constant (kB = 1.3x10-23J/K)
T
Temperature in K
e
0
Elementary charge (e
0
= 1.602x10-19C)
ci
Ion concentration in mol/kg
