| Q1 |
Measurement Principle of Zirconia Humidity Analyzer |
| A1 |
A zirconia high temperature humidity analyzer is designed to measure oxygen concentrations in the air and calculate the moisture content. Accordingly, moisture content in other than in the air, for instance, moisture content in the combustion exhaust gas, cannot be measured.
The following describes the measurement principle of the zirconia high temperature humidity analyzer.
A solid electrolyte like zirconia exhibits conductivity of oxygen ions at high temperature.
As shown in the figure below, when porous platinum electrodes are attached to both sides of the zirconia element to be heated up and gases of different partial oxygen concentrations are brought into contact with the respective surfaces of the zirconia, the device acts as an oxygen concentration cell.
This phenomenon causes an electromotive force to be generated between both electrodes according to Nernst's equation.
| where, |
R : |
Gas constant |
| |
n : |
4 |
| |
F : |
Faraday's constant |
| |
T : |
Absolute temperature (K) |
| |
PX : |
Oxygen concentration in a gas in contact with the positive zirconia electrode (%) |
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PA : |
Oxygen concentration in a gas in contact with the negative zirconia electrode (%) |
Measurement principle chart
When the gas sample consists of two components, namely, air and vapor as shown in bellow figure, the relationship between the oxygen partial pressure, vapor partial pressure, and zirconia element (cell) electromotive force can be shown as follows.
For the vol%H2O measurement
ｙ＝(100－x)× 0.21 ………(2)
x : Assuming that H2O vol% in a mixed gas is measured:
From the above equations (1) and (2), we obtain :
E=-K log(1－0.01x) ………(3)
Using the above equation (3), we can calculate the water vapor in vol% from the electromotive force.
For the "mixing ratio" measurement
Assuming that the mixing ratio is rkg/kg, then "r" can be calculated from the value of H2O vol% as follows:
With Equation (5), we can obtain the mixing ratio rkg/kg from the electromotive force.
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| Q2 |
Definitions of moisture content, mixing ratio (absolute temperature) |
| A2 |
(1) Definition of moisture content :
Same as the volume fraction of vapor in the air
(2) Definition of mixing ratio (absolute humidity)
The ratio of the mass of vapor contained in wet air to the mass of dry air obtained by removing vapor from the wet air.
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| Q3 |
Can the dew point be obtained using the zirconia high temperature humidity analyzer? |
| A3 |
The dew point is the temperature at which a water vapor pressure in the moist air is
equal to the saturated water vapor pressure.
The vapor pressure p (kPa) in the wet air can be obtained using the moisture content W (vol%H2O) and gas absolute pressure P (kPa).
p＝PW
The relationship between the saturated vapor pressure of single-component fluid and the temperature can be expressed using the following empirical equation (Antoine equation)
p : Vapor pressure p (kPa)
T : Temperature (°C)
A, B, C: Antoine constants (empirically defined constants)
Since the constants of water are as follows: A = 7.07406, B = 1657.46, and C = 227.02, the dew point TH (°C) can be obtained using the following equation.
Accordingly, the dew point can be calculated using the moisture content W (vol%H2O) measured with the zirconia high temperature humidity analyzer and the gas absolute pressure P (kPa) measured with the pressure transmitter.
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| Q4 |
The condition of sample gas? |
| A4 |
The following shows the sample gas conditions for measurement with the zirconia high temperature humidity analyzer.
• Sample gas temperature : 0 to 700°C
• Sample gas pressure : -5 to 2 kPa
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| Q5 |
Measurement Range and Output Signal |
| A5 |
Moisture quantity
Measurement range : 0 to 100 vol%H2O
Minimum output range : 0 to 25 vol%H2O
Maximum output range : 0 to 100 vol%H2O
(in 1 vol% H2O), or partial range
0 to 25 through 0 to 100 vol%H2O (in 1 vol%H2O), or partial range.
Mixture ratio
Measurement range : 0 to 1.000 kg/kg
Minimum output range : 0 to 0.2 kg/kg
Maximum output range : 0 to 1.000 kg/kg
0 to 0.2 through 0 to 1.000 kg/kg (in 0.001 kg/kg), or partial range.
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| Q6 |
What is the output signal? |
| A6 |
Two current outputs (4 to 20 mA DC/maximum load resistance: 550 Ω) are available for the ZR402G Zirconia High Temperature Humidity Analyzer.
The output items can be selected from the following. By factory default, the two outputs are set to Moisture quantity.
• Oxygen density
• Moisture quantity
• Mixture ratio
Furthermore, the following output function can also be set.
• Smoothing factor setting
When the measurement value changes rapidly and these values are used for control, some problems will occur; for instance, the on and off operation must be performed frequently. In such a case, the changes in the signal can be smoothed by providing a proper time constant.
Output damping : 0 to 255 seconds.
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| Q7 |
Can the output be held during maintenance? |
| A7 |
The output can be held at the last measured value or at a preset value during the warm-up period, during calibration, and when an error occurs. However, output 1 and output 2 cannot be set separately.
The following table shows the relationship between the device status and the analog output hold values that can be set.
Output Hold Setting
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| Q8 |
What are the contact inputs? |
| A8 |
Up to two contact inputs can be connected to the ZR402G Zirconia High Temperature Humidity Analyzer. The contact input performs a preset function by receiving a dry (non-voltage) contact signal from the outside.
The following table shows a list of the setting items and functions.
| Item |
Function |
| Calibration-gas press. low |
While the contact signal is on, neither semi-automatic nor automatic calibration is possible. |
| Measuring range change |
While contact input is on, analog output 1 range is switched to 0 to 25%O2.
While range is switched by the contact input, [Range] is displayed on the screen. |
| Calibration start |
If the contact signal is applied, semi-automatic calibration starts (only if the semi-automatic or automatic mode has been setup).
Calibration is started with an applied one- to 11-second time interval single-output contact signal. Even if a continuous contact signal is applied, a calibration is not repeated.
If you want to perform a second calibration, turn the contact signal off and then back on. |
| Process upset |
If the contact signal is on, heater power will be switched off.
Upset (A one- to 11-second time interval single-output signal is available as a contact signal.)
If this operation starts, the sensor temperature decreases and an error occurs.
To restore it to normal, turn the power off and then back on, or reset the analyzer. |
| Blow back start |
The contact signal is on, blow-back starts. (A one- to 11-second time interval single-output signal is available as a contact signal.)
Even if a continuous contact signal is applied, calibration is not repeated. If you want to make a second calibration, turn the contact signal off and then back on. |
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| Q9 |
What are the contact outputs? |
| A9 |
Four contact outputs are available for the ZR402G Zirconia High Temperature Humidity Analyzer.
Three of the output points can be selected to either normally energized or normally deenergized status.
Contact output 4 is set to normally operated, fixed error status.
The following functions are programmable for contact outputs.
(1) Alarm
• high-high alarm
• high-alarm
• low-alarm
• low-low alarm
• Calibration coefficient alarm
• Startup power stabilization timeout alarm
• Error
(2) Others setting
• warming-up
• calibrating
• maintenance
• Range switching answer-back
• Blowback start
• calibration-gas pressure decrease (answerback of contact input)
• Flameout gas detection (answerback of contact input)
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| Q10 |
How is error diagnosis performed? |
| A10 |
An error is detected if any abnormality is generated in the detector or the converter, e.g., in the cell (sensor) or heater in the detector, or the internal circuits in the converter.
(1) Error
If an error occurs, the converter performs the following:
| • |
Stops the supply of power to the heater in the detector to insure system safety. |
| • |
Causes an error indication  in the display to start blinking to notify of an error generation. |
| • |
Sends an output contact if the error is set up for "Output contact setup" for that contact. |
| • |
Changes the analog output status to the one set in "Output hold setting".
When the display shown in Figure appears, pressing the error indication brings up a description of the error. |
The content of errors that are displayed includes those shown in next table.
Types of Errors and Reasons for Occurrence
Error
No. |
Type of error |
Reason for Occurrence |
| Error-1 |
Cell voltage failure |
The cell(sensor) voltage signal input to the converter falls
below -50 mV. |
| Error-2 |
Heater temperature failure |
The heater temperature does not rise during warm-up, or it falls
below 730°C after warm-up is completed. |
| Error-3 |
A/D converter failure |
The A/D converter fails in the internal electrical circuit in the converter. |
| Error-4 |
Memory failure |
Data properly are not written into memory in the internal electrial circuit in the converter. |
(2) Alarm
When an alarm is generated, the alarm indication  blinks in the display to notify of the alarm.）
If an alarm is generated, such measures as turning off the heater power are not carried out.
The alarm is released when the cause for the alarm is eliminated.
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| Q11 |
Replacing existing models |
| A11 |
When replacing the existing models (ZO21D sensor and HA400 converter), special attention should be paid to the following points.
| • |
When leaving the existing HA400 converter as is and only replacing the existing sensor with a ZR22G :
The ROM must be replaced and a cold conjunction temperature compensation board must be installed.
It is recommended that this work be performed by a Yokogawa's service technician.
Part number of ROM :
K9293HQ (Humidity unit: vol%H2O / when you use English)
K9293HU (Humidity unit: kg/kg / when you use English)
Part number of cold conjunction temperature compensation board: K9471JA
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| • |
When leaving the existing Z021D sensor as is and only replacing the existing converter with a ZR402G:
The converter can be connected directly because it is compatible with the existing model. |
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