| Q1 |
When using Zirconia Oxygen Analyzer, what kind of matters that require attention (effects of flammable gas) are there? |
| A1 |
The sensor (zirconia element) is heated to 750℃ during measurements. If the process gas contains combustible gases such as carbon monoxide, hydrogen, and methane, these gases burn in the detector and consume oxygen, causing the oxygen concentration measured by the oxygen analyzer to be smaller than the actual value. Therefore, zirconia oxygen analyzers should be used only when the effect of coexisting combustible gases can be ignored or when their effect on oxygen concentration can be corrected.
Generally, exhaust gases after combustion that are emitted from combustion equipment such as boilers and industrial furnaces have been completely burned; the volume of combustible gases such as carbon monoxide is very small in comparison with oxygen, and so their influence can be ignored. However, if the excess air ratio is extremely small or if combustion is non-uniform, causing carbon monoxide to be produced, care is required.
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| Q2 |
How does the humidity in reference gas (air) influence it? |
| A2 |
Zirconia oxygen analyzers use a gas whose oxygen concentration is known and always consistent, as the reference gas.
In general, air is used as the reference gas. The oxygen concentration of dry air is constant at 20.95%; however, air generally contains water vapor, in which case the oxygen concentration varies with temperature and humidity.
In zirconia oxygen analyzers, a measurement error is caused if the temperature or humidity of the reference gas (air) varies significantly between calibration and measurement. When instrument air is used as the reference gas, this error can be ignored, but if it cannot be used, care is required.
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| Q3 |
Explain the precautions when shutting down a furnace or boiler. |
| A3 |
If operation of the zirconia oxygen analyzer is stopped during plant shutdown, moisture may condense on the detector's probe in contact with gas, causing dust to adhere to it. If operation is restarted in this condition, dust on the sensor will become firmly adhered and severely affect its performance. In addition, if condensed water accumulates, the sensor may be broken by heat shock, making the analyzer unusable.
When stopping the zirconia oxygen analyzer, it is important to do the following:
- Keep on supplying power to the zirconia oxygen analyzer. The reference gas should also be
left flowing. If it is difficult to do this, remove the detector from the system.
- If it is impossible to keep on supplying power to the analyzer or remove the detector from the system, keep on flowing air through the calibration gas piping at a flow rate of approx. 600 ml/min.
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| Q4 |
Notes for when running and stopping the analyzer repeatedly on a regular basis |
| A4 |
When you use the analyzer for an application in which you have to run and stop the operation repeatedly, continue to supply power to the zirconia oxygen analyzer to prevent unnecessary changes in the temperature around the sensor.
It is also recommended to continue to introduce the calibration gas (instrumentation air) flow.
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| Q5 |
What types of Zirconia Oxygen Analyzers exist, and how are they selected? |
| A5 |
Yokogawa offers the following zirconia oxygen analyzers.
Select an appropriate one according to your application and usage.
| • ZR22G/ZR402G |
Direct In-Situ Zirconia Oxygen Analyzer(Separate type) |
| • ZR202G |
Direct In-Situ Zirconia Oxygen Analyzer (Integrated type) |
| • ZR22S/ZR202S |
Explosionproof Zirconia Oxygen Analyzer |
| • AV550G |
Zirconia Oxygen Averaging Converter |
| • OX400 |
Low Concentration Zirconia Oxygen Analyzer |
(1) ZR22G/ZR402G Direct In-Situ Zirconia Oxygen Analyzer(Separate type)
Separate type zirconia oxygen analyzers need not use a sampling device.
They can also be used in a variety of manufacturing applications.
They alｌow direct installation of the probe in the wall of a flue or furnace to measure the concentration of oxygen in the stack gas to 700℃ . In addition, with the high temperature detector, they can allow to 1400℃ .
The converter uses a digital display and incorporates a LCD display with touchscreen for ease of operation.
(2) ZR202G Direct In-Situ Zirconia Oxygen Analyzer (Integrated type)
The integrated type zirconia oxygen analyzer combines probe and converter.
The wiring cost and the installation fee can be decreased compared with the separate type by the integrated one.
The converter can be operated in the field using an optical switch without opening the cover. On the other hand, there are some restrictions such as the sample gas temperature (700℃ or less) and functions.
(3) ZR22S/ZR202S Explosionproof Zirconia Oxygen Analyzer
Two types are available explosionproof direct in situ zirconia oxygen analyzer.
The ZR22S/ZR402G is a separate type which consists of a ZR22S explosionproof probe and a ZR402G non-explosionproof converter.
The ZR202S is an integrated type which combines a probe and a converter.
Separate and integrated type Zirconia oxygen analyzers do not need a sampling device, and allow direct installation of the probe in the wall of a flue or furnace to measure the concentration of oxygen in the stack gas.
The converter displays the cell temperature and cell emf in addition to the oxygen concentration.
This analyzer is most suitable for monitoring combustion and controlling the low-oxygen combustion of various industrial furnaces in explosive atmosphere at petroleum refinery, petrochemical plant, and natural gas plant.
(4) AV550G Zirconia Oxygen Averaging Converter
The AV550G Averaging Converter can accept inputs from up to 8 zirconia oxygen detectors ZR22G.
With large boilers used in the utility industry, the oxygen concentration varies in different zones across the flue. In order to obtain the most reliable oxygen data, the most common method used is the arithmetical averaging of several measuring points using an external averaging unit. The model AV550G Averaging Converter not only averages the signals but fully controls all of the individual detectors thereby eliminating the need for costly, redundant hardware or DCS programming.
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| Q6 |
Overview of the calibration of zirconia oxygen analyzer |
| A6 |
The electromotive force E (mV) is given by Nernst's equation.
The following figure shows the relation of oxygen concentration and sensor (cell) electromotive forces, when the zirconia element is heated up to 750℃ .
The measurement principles of a zirconia oxygen analyzer have been described below.
However, the relationship between oxygen concentration and the electromotive force of a cell is only theoretical. Usually, in practice, a sensor shows a slight deviation from the theoretical value. This is the reason why calibration is necessary. To meet this requirement,
an analyzer calibration is conducted so that a calibration curve is obtained, which corrects the deviation from the theoretical cell electromotive force.
Oxygen concentration in a Measurement Gas vs Cell Voltage  (21% O 2 Equivalent) → Return to questions
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| Q7 |
What gas is used for calibration? |
| A7 |
A gas with a known oxygen concentration is used for calibration.
Normal calibration is performed using two different gases: a zero gas of low oxygen concentration and a span gas of high oxygen concentration (two-point calibration).
In some cases, only one of the gases needs to be used for calibration.
However, even if only one of the gases is normally used, calibration using both gases should be done at least once.
The zero gas normally used has an oxygen concentration of 0.95 to 1.0 vol%O2 with a balance of nitrogen gas (N2).
The span gas widely used is clean air (at a dew-point temperature below -20℃ and free of oily mist or dust, as in instrument air).
Note:
An N2gas of oxygen concentration of 0% can not be used for zero gas.
A zirconia oxygen analyzer detects a change in the oxygen partial pressure (PX) on the measurement side against the oxygen partial pressure (PA) on the reference side as an electromotive force (E).
Since this electromotive force (E) is logarithmically proportional to PX divided by PA, calibration cannot be performed when PX is 0% because the electromotive force becomes infinite.
Zero calibration is usually performed using 1 % O2 gas.
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| Q8 |
What are two-point calibration and one-point calibration? |
| A8 |
There are two calibrations for zirconia oxygen analyzers ;
two-point calibration using zero and span gases,
and one-point calibration using only a span gas.
(1) Two-point Calibration :
The following figure shows a two-point calibration using two gases: zero and span.
Cell electromotive forces for a span gas with an oxygen concentration p1 and a zero gas with an oxygen concentration p2 are measured while determining the calibration curve passing between these two points. The oxygen concentration of the measurement gas is determined from this calibration curve.
In addition, the calibration curve corrected by calibration is compared with the theoretical calibration curve for determining the zero-point correction ratio represented by B/A × 100 (%) on the basis of A, B and C shown in the bellow figure and a span correction ratio of C/A × 100 (%).
If the zero-point correction ratio exceeds the range of 100±30 % or the span correction ratio becomes larger than 0±18 %, calibration of the sensor becomes impossible.
Calculation of a Two-point Calibration Curve and Correction Factors using Zero and Span Gases
(2) One-point Calibration
The next figure shows a one-point calibration using only a span gas.
In this case, only the cell electromotive force for a span gas with oxygen concentration p1 is measured.
The cell electromotive force for the zero gas is carried over from a previous measurement to obtain the calibration curve.
The principle of calibration using only a span gas also applies to the one-point calibration method using a zero gas only.
The way of "Zero-point correction Factor" and "Span correction Factor" is the same as the case of Two-point Calibration.
Calculation of a One-point Calibration Curve and Correction Factors using a Span Gas
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| Q9 |
Calibration operation modes and calibration time setting |
| A9 |
There are three calibration modes available :
| Manual calibration which allows zero and span calibrations or either one manually in turn; |
| Semi-automatic calibration which lets calibration start with the touchpanel or a contact input, and undergoes a series of calibration operations following preset calibration periods and stabilization time; |
| Automatic calibration which is carried out automatically following preset calibration periods. |
Calibrations are limited by the following mode selection:
(1) When the calibration mode is in Manual :
First, set the output stabilization time. This indicates the time required from the end of calibration to entering a measurement again.
This time, after calibration, the measurement gas enters the sensor to set the time until the output returns to normal.
The output remains held after completing the calibration operation until the output stabilization time elapses. The calibration time set ranges from 00 minutes, 00 seconds to 60 minutes, 59 seconds.
(2) When the calibration mode is in Semi-automatic :
In addition to the above output stabilization time and calibration time, set the interval, set the output stabilization time and calibration time.
The calibration time is the time required from starting the flow of the calibration gas to reading out the measured value.
The set calibration time is effective in conducting both zero and span calibrations.
The following figure shows the relationship between the calibration time and output stabilization time.
Calibration and Output-stabilization Time Settings
(3) When the calibration mode is in Automatic :
In addition to the above output stabilization time and calibration time, set the interval, start date, and start time.
Interval means the calibration intervals ranging from 000 days, 00 hours to 255 days, 23 hours.
Set the first calibration day and the start-calibration time to the start date and start time respectively.
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| Q10 |
Pressure compensation function |
| A10 |
If the in-furnace pressure is high or there are variations in the in-furnace pressure, the pressure balance between the inside of the furnace and the reference air is lost, making it difficult to measure the oxygen concentration accurately.
In this case, the detector's pressure compensation function can be used to return the reference air to the furnace to maintain the pressure balance between the inside of the furnace and the reference air side. This enables reliable measurements to be made.
This function allows in-furnace pressures of up to 250 kPa to be measured.
Pressure compensation function
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| Q11 |
What is preventive maintenance functions of the zirconia oxygen analyzer? |
| A11 |
The following information necessary for routine maintenance can be displayed.
This information can be used to determine the calibration period and prepare for the zirconia cell in a timely manner.
(1) Span-gas and Zero-gas Correction Ratios
These are used to check for degradation of the sensor (cell).
If the correction ratio is beyond the Correctable range, the sensor should no longer be used.
These ratios can be found by calculating the data as shown below.
Span-gas and Zero-gas Correction Ratios
(2) Response Time
The cell's response time is obtained in the procedure shown in below figure.
If only either a zero-point or span calibration has been carried out, the response time will not be measured just as it will not be measured in manual calibration.
The response time is obtained after the corrected calibration curve has been found.
The response time is calculated, starting at the point corresponding to 10% of the analog output up to the point at 90% of the analog output span. That is, this response time is a 10 to 90% response.
Response Time
(3) Robustness of a Cell
The robustness of a cell is an index for predicting the remaining life of a sensor and is expressed as one of four time periods during which the cell may still be used: more than a year, more than six months, more than three months, less than one month.
The above four time periods are tentative and only used for preventive maintenance, not for warranty of the performance.
This cell's robustness can be found by a total evaluation of data involving the response time, the cell's internal resistance, and calibration factor. However, if a zero or span calibration was not made, the response time cannot be measured. In such a case, the cell's robustness is found except for the response time.
(4) Cell voltage
The cell (sensor) voltage will be an index to determine the amount of degradation of the sensor. The cell voltage corresponds to the oxygen concentration currently being measured. If the indicated voltage approximates the ideal value (corresponding to the
measured oxygen concentration), the sensor will be assumed to be normal.
The ideal value of the cell voltage (E), when the oxygen concentration measurement temperature is controlled at 750℃ ., may be expressed mathematically by:
E = -50.74 log (Px/PA) [mV]
where, Px: Oxygen concentration in the measured gas PA: Oxygen concentration in the reference gas, (21% O2)
Oxygen Concentration Vs. Cell Voltage (cell temperature: 750℃ )
(5) Cell's Internal Resistance
A new cell (sensor) indicates its internal resistance of 200 Ω maximum. As the cell degrades, so will the cell's internal resistance increase.
The degradation of the cell cannot be found only by changes in cell's internal resistance, however. Those changes in the cell's internal resistance will be a hint to knowing the sensor is degrading. The updated values obtained during the calibration are displayed.
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| Q12 |
After replacement of sensor, what are the tasks that is necessary? |
| A12 |
After replacement of sensor, carry out surely Two-point Calibration using zero and span gases.
Note that calibration should usually be performed in the measurement state (in the operation state of the furnace when the analyzer is installed near the furnace).
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| Q13 |
Measures against applications with a lot of dust |
| A13 |
When the sample gas includes a lot of dust like with a recovery boiler or cement kiln, the probe becomes clogged within a short time. To prevent this from happening, the use of air pressure for blowing back is effective in removing dust.
There are two blow back modes available:
•Semi-automatic blow back :
In this mode, touchpanel operations or contact input signals will start and perform blow back operations according to a preset time and output stabilization time.
•Automatic blow back :
Performs blow back operations automatically according to a preset interval.
The following figure shows a timing chart for the operation of blow back.
To execute blow back with a contact input, use a contact input with an ON-time period of one to 11 seconds.
Once blow back starts, a contact output opens and closes at ten-second intervals during the preset blow back time.After the blow back time elapses, the analog output remains held at the preset status until the hold time elapses .
As the hold (output stabilization) time, set the time until the measured gas is returned to the sensor and output returns to the normal operating conditions, after completing blow back operations.
The operation of blow back
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