Tunable Diode Laser Gas Analyzer - Answer -
Optical Path Length
| Q1 |
How is the optical path length defined? |
| A1 |
The optical path length is the distance travelled by laser light in sample gas.
 Return
to questions
|
| Q2 |
What is the maximum optical path length? |
| A2 |
The maximum optical path length of the standard type is 6 meters, while the one of the divergence type is 30 meters. Where the optical path length exceeds the limit, the laser beam diameter expands with distance, and thus the photodetector cannot receive a sufficient amount of transmitted light.
The TDLS 200 can be used in applications where the optical path length is 1 meter or less. However, the sensitivity will decrease.
Return
to questions
|
| Q3 |
How long is the laser
beam diameter? |
| A3 |
The standard type (the optical path length up to 6 meters) has
a collimated beam with a diameter of approximately 20 mm. The divergence type
(the optical path length exceeding 6 meters) has a laser beam diameter of max.100
mm.
Return
to questions
|
| Q4 |
What is the allowable decrease in the light intensity? |
| A4 |
A 10% decrease from the factory default is typically allowable.
Return
to questions
|
| Q5 |
How far is the laser scan range? |
| A5 |
The laser scan range depends on application.
In the case of oxygen measurement, the TDLS200 has a laser linewidth of 0.0002 nm and a scan width of 0.1 to 0.2 nm. It measures oxygen concentration by scanning the absorption line in a 0.1 to 0.2 nm width.
Return
to questions
|
Optical Axis
| Q1 |
How is the optical axis aligned (when the optical path length is up to 6 meters)?
How often should the optical axis be aligned? |
| A1 |
The optical axis
can be aligned by monitoring the transmission values on the display on the Launch
Unit. The alignment frequency depends on the installation conditions.
Thermal
expansion due to temperature change at the furnace or other factors may cause
axis misalignment. The TDLS200 is designed to minimize the effects of axis misalignment.
If axis misalignment occurs, realignment is required.
Return
to questions
|
| Q2 |
How is the optical axis aligned when the optical path length exceeds 6 meters? |
| A2 |
Same as when the optical path length is up to 6 meters. Align
the axis using the divergence type alignment flange so that the transmission value approximates
100%. The divergence flange will be included with shipment when the Suffix Code
"-A" is specified for the optical path length on TDLS200 MS code.
Return
to questions
|
| Q3 |
For the alignment of laser beam axis at the Launch and Detect units, vertical and horizontal alignments are performed two times each. Are there any ways (data) to check the accuracy? |
| A3 |
The analyzer display can show the amount of light received, by which the accuracy can be checked.
Return
to questions
|
Top of this page
Purging
| Q1 |
Why does the instrument need to be purged by gas? |
| A1 |
The process windows need to be purged to prevent the dust accumulation and to cool the windows. The instrument is purged to prevent the influence on the measurement. In the case of oxygen measurement, the instrument and the process windows need to be purged with nitrogen gas to prevent ambient air ingress to the optical path.
In the case of moisture measurement, the instrument, including the enclosure, needs to be purged with nitrogen gas to prevent the effect of trace moisture content.
In case of Explosion Proof Type, the purge gas is also needed to maintain inner pressure purge.
Return
to questions
|
| Q2 |
What determines the flow rate and type of purge gas? |
| A2 |
The type of purge gas is determined by the target gas to be measured.
In the cases of trace moisture and oxygen measurements, nitrogen gas is used
as a purge gas. For other target gases, instrument air is used for purging. The
amount of purge gas required depends on the application, but typically 5-50 l/min.
Return
to questions
|
| Q3 |
Does dirt adhere to the pane that comes in contact with the process gas? |
| A3 |
When the purge gas is supplied at the proper pressure, process gas hardly come in contact with the windows. If any, it will not affect the measurement.
But when the transmission goes down to alarm level by the dirt on the windows, the window can be cleaned on site.
Return
to questions
|
Top of this page
Typical System Configurations
Top of this page
Measurement Method
| Q1 |
What is the peak area integration method? |
| A1 |
In the peak area integration method, the concentration of the target gas is calculated by measuring the peak area of one absorption line of interest. As compared with the second harmonic (2f) method, the peak area integration method provides stable measurements even in the presence of background gases change. In the 2f method the width of the absorption line of the target gas is affected by background gases. Further more advantage is that the measurement is not affected by changes in ambient temperature and pressure at the same time.
Yokogawa’s TDLS200 is preset to suit your application so that the absorption peak of interest can be scanned. Also, whether the peak is surely scanned or not can be checked by the software.
Examples of spectra of 10% oxygen in different background gases by the peak area integration method and the second harmonic (2f) method, are shown above. In the peak area integration method, the peak area of the spectrum of 10% oxygen is not affected by any background gases (90%) and so remains constant.
Return
to questions
|
Top of this page
Dust
| Q1 |
Does the dust affect the broadening of absorption spectrum? |
| A1 |
No.
Return
to questions
|
| Q2 |
Is the TDLS200 affected by dust? |
| A2 |
The effect of dust content is dependent on the optical path length. When the dust concentration is 10 g/Nm3 over a 1 meter path length or less, it is believed that the measurement is less affected.
Typically, the stable measurement can be kept on down to 10% transmittance.
Return
to questions
|
Top of this page
Interference, Temperature and Pressure Effects and Compensation
| Q1 |
If spectral interference exists, are there any ways to separate it? |
| A1 |
We have measurement programe to separate the target component signal from over lapped interference spectrum.
But for a new application it is needed to check the spectral wavelength to be used before order acceptance.
The setting cannot be changed in the field. The wavelength cannot be changed after shipment.
Before order, please send application condition information.
Return
to questions
|
| Q2 |
Is the TDLS200 affected
by the temperature change (sample gas, ambient air)? |
| A2 |
No.
The affection by the temperature change of sample gases can be compensated automatically.
Return
to questions
|
| Q3 |
The gas temperature effect on peak area is non-linear.
How is it compensated? |
| A3 |
It is compensated based on the database of the temperature correction curve.
Return
to questions
|
| Q4 |
Is the target component
gas affected by oxygen (O2), moisture (H2O), or other different components
that coexist in the process gas? |
| A4 |
No. However, preliminary check is required for unknown applications.
Return
to questions
|
Top of this page
Other Specifications
| Q1 |
As for the message of the indicator, is Japanese also possible? |
| A1 |
It is only a current state, and English.
Return
to questions
|
| Q2 |
What is the limit of operating temperature of the Launch and Detect units? |
| A2 |
As mentioned in the General Specifications, the Launch and Detect units should be used in the temperature range of 0 to 50 deg C. The ambient temperature should also be in the range of 0 to 50 deg C.
Return
to questions
|
| Q3 |
What is the maximum distance between the Process flange and the Launch/Detect unit? |
| A3 |
For details, please consult Yokogawa.
Return
to questions
|
| Q4 |
Does duct vibration or other vibration affect the integral-type converter?
Does vibration cause misalignment of the optical axis? |
| A4 |
As mentioned in the General Specifications, the instrument should be installed in a place with minimal vibration.
Return to questions
|
| Q5 |
Do water droplets on the window affect the measurement? |
| A5 |
Yes. The drop of water becomes a turbulence influence.
Purge the window properly.
Return
to questions
|
| Q6 |
How much is the warming-up time? |
| A6 |
It requires approximately 60 minutes for warm-up in order to ensure stable measurements.
Return
to questions
|
| Q7 |
The General Specifications say that the upper limit of ambient temperature of the TDLS200 is 50 ℃.
The ambient temperature of the analyzer may easily get higher because of radiant heat from a furnace.
|
| A7 |
For some applications such as boilers, an optional shield box is available and a Vortex Cooler can be used for cooling.
They have been used successfully in some applications in the U.S. and so can be recommended.
Are there any other means?
Return
to questions
|
