Prepared by Mustafa AliSenior Metering Tech. OMV Sawan13-11-06

Types of Oxygen analyzers (By detection method)

1 Direct methodZirconia Oxygen analyzer

2 Indirect methodParamagnetic Oxygen analyzer

Heating the element allows different partial oxygen concentrations of the gasses to come into contact with the opposite side of the zirconia’s creating an oxygen concentration cell.

These ions travel through the zirconia’s element to the other electrode. At that point, the Nernst expression applied to calculate the force by measuring the electromotive force generated between the two electrodes.

Zirconia is one of the few compounds that actually become conductive at high temperatures & more conductive as its temperature increases

Zirconia starts out with a very high resistance at room temperature greater then one trillion ohms / cm , Having less then 1000 ohms / cm resistance at 1000 degrees Celsius & become a very good conductor

Zirconia principle

A process gas (A) with an unknown oxygen concentration flows around the outside of heated Zirconium –oxide measuring cell (B) A reference gas (C) with a known oxygen concentration flows inside the measuring cell At an optimum temperature a voltage (U) in mV is created between the two surfaces of the cell which are dependent only on the ratio of oxygen concentration (partial pressures) in (A) & (C) Using air as a reference gas the measured voltage is a direct measure of the oxygen concentration in the process gas (A)

Zirconia principle

The paramagnetic oxygen analyzer is based on the scientific principle that oxygen is a paramagnetic material

When a test sample containing oxygen is introduced into the test cell, the oxygen in the sample is attracted to the point of maximum field strength. 

The magnitude of dumbbell displacement is proportional to the amount of oxygen in the sample.  The movement of the dumbbell is detected by a light beam from a light source exterior to the test cell. 

The light beam is reflected from a mirror on the dumbbell body to an exterior photocell.  The output of the photocell is amplified and transmitted to an indicating unit that is calibrated to read out the oxygen content in the test sample in percent. 

Paramagnetic principle

When a test sample containing oxygen is introduced into the test cell, the oxygen in the sample is attracted to the point of maximum field strength.  The magnitude of dumbbell displacement is proportional to the amount of oxygen in the sample.  The movement of the dumbbell is detected by a light beam from a light source exterior to the test cell.  The light beam is reflected from a mirror on the dumbbell body to an exterior photocell.  The output of the photocell is amplified and transmitted to an indicating unit that is calibrated to read out the oxygen content in the test sample in percent. 

Paramagnetic principle

Novatech Zirconia Oxygen analyzers are being use for combustion control / optimization in Incinerators Hot oil heaters & Regenerators , vital to fulfill the EPA & ISO 14001 requirements

Novatech Zirconia Oxygen Sensor Novatech Zirconia Oxygen Controller

Train # 1 Incinerator control through Oxygen analyzer

Controller display Oxygen % 1.19%

Impedance 2.1 K

EMFParameters

60 mv

Number of sensors 1

Sensor offset -0.4

Output channel 1 cal man cal

Reference voltage=1 27.5

Reference voltage=2 193.3

Reference voltage=3 1198

Reference voltage=4 2479

Sensor 1 Type 34

Thermocouple Type k

Transmitter output chan 1 scale

liner 02

Transmitter span chan 1 30%

Top line display unit %

Deg C or Deg F C

Flue pressure Fix / Var fix

Flue pressure mm/inch/kp mmwg

Ref air pump Int,ext,inst air inst air

Reference RH 5%

Damping 5

Common reference parameters of Oxygen analyzers

Testing & calibration of Zirconia Oxygen analyzer

SENSOR OFFSET

SENSOR IMPEDANCE

SENSOR HEATER

SENSOR THERMOCOUPLE

A/D (Four channels Milli Volts) CALIBRATION

D/A (4-20mA output) CALIBRATION

Sensor offset

Calibration of the probe generally only requires the Sensor Offset to be set.

If the offset for a sensor is not set the error will generally be less than 5% of the actual oxygen reading. By setting the offset the error will be less than 1% of the oxygen actual reading.

With the probe or sensor heated to over 700°C, Connect reference air to the probe calibration check port. 50 to 500 cc/ minute,

To check a probe offset on site, the probe must be sensing air, with reference air, and allowed to settle at the probe operating temperature for 30 minutes. Read the offset ‘Sensor EMF’ in ‘RUN’ mode in milli volts on the lower line. Switch back to ‘set-up’ mode and enter ‘Sensor Offset’ of equal value and the same polarity.

Sensor Impendence

The zirconia sensor impedance is a basic measurement of the reliability of the oxygen reading. A probe or sensor with a high impedance reading will eventually produce erroneous signals. The analyzer checks the zirconia sensor impedance every 24 hours and if the impedance is above the maximum level for a specific temperature then the impedance alarm (Sensor Fail) will be activated. Typical sensor impedance is 1 K to 8 K at 720°C. With the probe or sensor heated to over 700°C, Connect reference air to the probe calibration check port. 50 to 500 cc/ minute

To test the source impedance set the meter to read ohms and take a measurement, within a couple of seconds, reverse the meter and repeat the reading. Take the average of the two readings, if the impedance is above 10k, then the probe or sensor needs to be replaced. The reason that impedance measurements need to be performed quickly, is that the zirconia sensor polarizes with the DC voltage from the milli volts across it.

Sensor Heater

For heated probe or sensors, a heater failure will cause a ‘SENSOR UNDER TEMP’ or ‘HEATER FAIL’ alarm. Heaters can be tested with a continuity test. The heater element impedance should be approximately 100 ohms. The heater insulation should be approximately 10 mega ohms, should the heater be open or short-circuited, replace the probe or sensor.

Sensor Thermocouple

The analyzer has an alarm function that will advise the operator of an open circuit thermocouple; however bench testing can be performed by simply measuring the thermocouple continuity.

A/D (Four channels Milli Volts) Calibration

The analyzer maintains its accuracy over a very long period by continuously checking itself against internal references. The only calibration required is to set the actual values of these references into battery backed memory. The analyzer will read these references every minute and update its zero and span correction factors. These references should be checked every 12 months.

Set-up items 7 to 10 are used to calibrate the A/D of the instrument. This should be done 30 minutes or more after the instrument has been on, approximately once every year. The calibration constants are retained in battery backed memory unless a ‘COLD START’ is performed. Connect a 3 1/2 digit multi meter negative lead to the test point marked ‘C’ to the right of the PCB on the inside of the door (labeled ‘REF VOLTS’). Measure the four voltages on the test point marked 1 to 4 with the positive lead. Enter the measured values in set-up items 7 to 10. Whenever new values are entered the D/A Section should be re-calibrated

D/A (4-20mA output) Calibration

The ‘Manual Cal’ mode is selected in set-up 13 Set the 4mA calibration first and then the 20mA calibration.

1. Select ‘Set 4mA Trim’ in set-up 13

2. Return to RUN mode.

3. Measure the output on the channel to be calibrated with a digital multi meters. If the current is not exactly 4.00mA, return to set-up mode and change the 4mA calibration factor in set-up 14

4. Re-measure the current while back in RUN mode until the current is within 3.9 to 4.1mA.

5. Return to set-up mode and select ‘Manual Cal’ in set-up 13 Set the 20mA calibration factor.

D/A (4-20mA output) Calibration cont..

6. Select ‘Set 20mA Trim’ in set-up 13

7. Return to RUN mode.

8. Measure the output on the channel to be calibrated with a digital multi meters. If the current is not exactly 20.00mA, return to set-up mode and change the 20mA calibration factor in set- up 15

9. Re-measure the current while back in RUN mode until the current is within 19.9 to 20.1mA.

10. Return to set-up mode and select ‘Manual Cal’ in set-up 13

This calibration is now saved in battery backed memory until the factors are changed in the manual calibrationThe analyzer is forced into a COLD-START

Zirconia sensor probe life expectancy

Zirconia electrode material loss can be measured in an attempt to determine the remaining life of the probe. The impedance changes must be taken as a series of readings over a period of several weeks. The best indication of an impending probe failure is in a series of impedance readings showing a definite and rapid increase.

Common problems Condensation

Vibration