Emerson Rosemount Oxymitter 4000, Справочное руководство, Страница: 18 / 198

oxidized). The probe is permanently positioned within an exhaust duct or stack and
performs its task without a sampling system.
The equipment measures oxygen percentage by reading the voltage developed across a
heated electrochemical cell, which consists of a small yttria stabilized zirconia (YSZ) disc.
Both sides of the disc are coated with porous metal electrodes. When the equipment is
operated at the proper temperature, the following Nernst equation gives the millivolt
output of the cell:
EMF = KT log10(P1/P2) + C
Where:
• P2 is the partial pressure of the oxygen in the measured gas on one side of the cell.
• P1 is the partial pressure of the oxygen in the reference air on the opposite side of the
cell.
• T is the absolute temperature.
• C is the cell constant.
• K is an arithmetic constant.
Note
For best results, use clean, dry, instrument air (20.95 percent oxygen) as the reference air.
When the cell is at an operating temperature and there are unequal oxygen
concentrations across the cell, oxygen ions will travel from the high oxygen partial
pressure side to the low oxygen partial pressure side of the cell. The resulting logarithmic
output voltage is approximately 50 mV per decade. The output is proportional to the
inverse logarithm of the oxygen concentration. Therefore, the output signal increases as
the oxygen concentration of the sample gas decreases. This characteristic enables the
transmitter to provide exceptional sensitivity at low oxygen concentrations.
The transmitter measures net oxygen concentration in the presence of all the products of
combustion, including water vapor. Therefore, it may be considered an analysis on a "wet"
basis. In comparison with older methods, such as the portable apparatus, which analyze
on a "dry" basis, the "wet" analysis will, in general, indicate a lower percentage of oxygen.
The difference between the "dry" and "wet" analyses will be proportional to the water
content of the gas stream.
1.4 Configuration
Rosemount
™
Oxymitter 4000 Transmitters are available in three length options, giving the
operator flexibility to use an in situ penetration appropriate to the size of the stack or duct.
The options are:
• Length: 18 in. (457 mm), weight: 29 lb. (13.15 kg)
• Length: 3 ft. (0.91 m), weight: 34 lb. (15.42 kg)
• Length: 6 ft. (1.83 m), weight: 44 lb. (19.96 kg)
The electronics control probe temperature and provide an isolated output, 4-20 mA, that
is proportional to the measured oxygen concentration. The power supply can accept
voltages of 90 - 250 Vac and 48/62 Hz; no setup procedures are required. Modulating the
duty cycle of the probe heater portion of the electronics maintains the oxygen sensing cell
Description and specifications Reference Manual
October 2019 00809-0100-4340
18 Emerson.com/Rosemount