Eurotherm 394, User Manual

Page: 81 / 142

HA250385
Issue 7 Nov 06
Page 77
MULTIPOINT CIRCULAR-CHART RECORDER: USER GUIDE
Probe temp Ch 1
Probe EMF Ch 1
Reference 1.00
P
P
E
T
2
1
10
0 0496
=
×
.
7.2.2 LEVEL 2 EQUATIONS (Cont.)
ZIRCONIA PROBES
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 temperature and to the log of
partial pressure of oxygen difference between its two ends.
OXYGEN CONCENTRATION MEASUREMENT
In order to measure oxygen concentrations, one end of the probe is inserted into the atmosphere to be measured,
while the other is subjected to a reference atmosphere. For most applications, air provides a suitable reference (refer-
ence input = 20.95% for air).
The temperature of the probe is usually measured using a type K or a type R thermocouple. The temperature effect on
the thermocouple is such that for successful operation with the recorder, the probe temperature must be greater than
973K (700˚C).
The probe output obeys a law, described by the Nernst oxygen equation:
where, P
2
= Partial pressure of oxygen in the sampled gas (%)
P
1
= Partial pressure of oxygen in the reference atmosphere (%) (20.95% for air)
E = Electromotive force across the probe in mV
T = Probe temperature in Kelvins
In order to obtain a useful result, it is necessary to scale the inputs and outputs correctly. The channel measuring the
probe voltage will normally need a scale of 0 to 100 mV. The temperature measuring channel will probably be scaled
at 273 to 1800K, while the output scaling would typically be 0 to 5 % for boiler flues, and 0 to 20% in kilns.
CONFIGURATION PAGES
Enter channel numbers for Probe temperature, Probe emf and reference % measurements.