Eurotherm 6100, User Manual

Page: 134 / 397

100/180MM DISTRIBUTED GRAPHIC RECORDER: USER GUIDE
Page 118 HA029324
Issue 11 Oct 14
4.3.11 FUNCTION (Cont.)
ZIRCONIA PROBE
A zirconia (oxygen) probe consists of two platinum electrodes bonded to a pellet or cylinder of zirconia.
At elevated temperatures, such a probe develops an emf across it which is proportional to probe tem-
perature and to the log of the difference in oxygen partial pressure between its two ends.
OXYGEN CONCENTRATION
In order to measure oxygen concentrations, one end of the probe is inserted into the atmosphere to be
measured, whilst the other is subjected to a reference atmosphere. For most applications, air provides a
suitable reference (reference input = 20.95 for air).
The temperature of the probe is normally measured using a type K or type R thermocouple. The tem-
perature effect on the thermocouple is such, that for successful operation the probe temperature must
be greater than 973K (700˚C).
The equation solved by the maths function is:
Function
Probe temperature
Probe emf
Reference Partial Pressure
Maths Number
Value
Units
Units
Apply Discard
Zirconia Probe
Channel 1
1) Math 1
0.0000
Channel 2
Units
20.95
Figure 4.3.11r Zirconia probe function confi guration menu
To obtain a useful result, it is necessary to scale correctly.
The channel which is measuring the probe output would normally need to be set to: Input Type = mV;
Input low = 0; input high = 100..
A typical temperature-measuring channel might be set up as:
Input Type = Thermocouple; Lin type = Type K; Range low = 273; Range high = 1800, Range units = K.
The maths channel scaling would typically be confi gured as:
Units = %; Scale Low = 0; Scale High = 5 (for boiler fl ues) or 10 (for kilns).
where:
P2 = Partial pressure of oxygen in the sampled gas (%)
P1 = partial pressure of oxygen in the reference atmosphere (%) (20.95% for air)
E = Electromotive force (emf) across the probe in mV
T = Probe temperature in Kelvins
Figure 4.3.11r shows the confi guration menu. Figure 4.3.11s shows oxygen concentration versus probe
emf for various temperatures.
P2

P1
10
E
0.0496

T