1 About Axetris LGD in general
1.1 What is the Axetris LGD?
The Axetris LGD is an universal Laser Gas Detection OEM module (LGD) sub-system for extractive measurement of gases such as ammonia (NH3), methane (CH4), carbon dioxide (CO2) and others.
Based on near-infrared Tunable Diode Laser Spectrometry (TDLS), the Axetris LGD offers gas detection from sub ppm to percentage concentrations. All LGD OEM modules use telecommunication type laser diodes and feature an innovative, patented measurement principle without the need for a physical reference channel.
The LGD sensor modules are targeted at system integrators that would like to benefit from the intrinsic advantages of tunable diode laser spectrometry, without having to develop the technology in-house. A modular, self-contained and stand-alone product design as well as standard industrial interfacing assure rapid integration by OEMs.
1.2 What are the main advantages and customer benefits of an Axetris LGD compared to other gas sensors?
- Highly selective to target gas, virtually no cross-sensitivity
- No direct contact with the gas, no degradation or poisoning, fast recovery
- High dynamic range: sub-ppm to percentage-range
- No need for a physical reference channel due to innovative patented measurement principle
- Stable & accurate, practically no 0-drift
- Long re-calibration cycles / calibration-free
- Easy integration / Self-contained
- Easy to operate
- Low cost-of-ownership
- No consumables
- Low maintenance
- Long life-time (around 7 years)
- Continuous sensor status monitoring
- Low cost potential for high-volume applications
1.3 Why are there two different versions of the LGD available?
Axetris offers the LGD Compact-A that is designed to measure gases at ambient temperatures in a range of -10°C up to 65°C. Here, the gas and the measuring device are typically thermalized and at the same temperature. This series is indicated by an “A” that stands for ambient.
Certain process or emission control applications require hot-gas measurements. Therefore, the LGD F200 H, indicated with an “H” for heated is offered. Here, the measurement cell is heated to a certain predefined and fixed temperature which can be as high as 220°C. The ambient temperature needs to be in a range of 15°C up to 50°C. This version is larger compared to the ambient modules and typically performance for hot gas measurements (in particular precision) is slightly lower due to spectroscopic reasons. As the temperature of the measurement cell is taken into account for calibration, it is factory-set to the application specification of the customer.
1.4 How many different gases can be measured with one single LGD module?
Each LGD module is designed to measure one particular gas since the tuning range of a laser diode is limited to a few nanometers. However, it is sometimes possible to measure two gases with the same laser if the absorption lines are closely interlaced (e.g. water and ammonia or methane and carbon dioxide)
1.5 Why can the LGD measure without a reference channel and why is this advantageous?
In a traditional non-dispersive infrared gas detector (NDIR), the light is emitted by a light source, passes through the gas volume to be measured and then is captured by an infrared detector. As the light incident on this detector depends on both gas concentration and emitted light intensity, NDIR gas detectors need a second infrared detector (and a second electronics channel) to yield information about the performance of the light source, which degrades over time and misaligns.
In contrast to this, the innovative, patented measurement principle incorporated into Axetris’ LGD modules allows extraction of both concentration and light intensity information from one single photodiode due to laser modulation and lock-in technique. This measurement principle eliminates the need for an additional detector/electronics channel and offers the associated cost savings. It also enables stable and robust gas detectors because the possibility of differently degrading measurement and reference channel (leading to false readings) is excluded.
1.6 What are the advantages of the LGD set-up as an instrument for extractive gas measurements?
Different set-ups for a laser gas detection device are possible. While the Axetris LGD module is a stand-alone system that can be integrated for extractive gas measurements, cross-duct type systems are also possible. The advantages for our extractive set-up are as follows:
- Integration into standard housings (19-inch racks).
- Sub-system: offering the LGD as part of a tailor-made end-user solution.
- Combination with other analytical modules.
- Easy & safe maintenance for mounts in measurement houses/containers.
- Small and light: Can be mounted in “Semi in-situ” housings on a stack with gas-extractive probes in the same place.
1.7 Can the internal parts of the LGD be cleaned, like the measurement cell, the cell windows…?
It is not recommended to open the device to try to clean the cell and cell windows because this will on one hand void the warranty (warranty stickers) and on the other hand degrade the performanc of the LGD modules.
Only operators trained by Axetris can proceed to internal cleaning and repair as this needs special knowledge and special equipment to verify performance after the repair is performed.
1.8 What is the MTTF (mean time to failure) for the device or the laser?
In general, the LGD has been designed in such a way that an average lifetime of about 7 years can be expected. Whether regular maintenance or repair is necessary depends very much on the application and proper integration. For example, in many applications it is very important to ensure proper filtration of moisture and particles. If this is not given, contamination of the gas cell by particles can occur, which in turn disturbs the measurement. In general, the heated versions of the LGD are more affected by maintenance and service due to the harsh conditions in the applications concerned.
The lifetime of the laser is included in the lifetime of the LGD. Laser diodes are affected by drift due to their characteristics, but this is intercepted by the software algorithm of the LGD and also automatically corrected if necessary. Therefore, premature failure of the laser is not to be expected.
2 Facts on the LGD measurement performance
2.1 What is the difference between measurement precision and measurement accuracy?
Measurement precision refers to the closeness of agreement between measured values obtained by repeated measurements on the same objects under similar/same reference conditions. Measurement precision is usually expressed numerically as the standard deviation or variance at reference conditions as well as to define measurement reproducibility and intermediate measurement precision.
Instrument performance is governed by the “Precision” when the temperature of the instrument is stable (i.e. operated in stable environmental conditions or when measures for temperature stabilization are taken).
Target analogy for accuracy and precision
Precision is the “short-term noise”, which mostly stems from electronic Johnson noise and/or analog/digital conversion noise. Precision can be improved by measuring over longer periods, e.g. by using moving averages. As a general rule the precision improves with the square root of the number of measured seconds (as the systems gives out one value per second). E.g. a 10s average should result in 3.3 times better precision.
Accuracy is an important parameter to characterize the performance of a measurement device. The measurement accuracy of a system is the degree of closeness of a measurement to the true value. It includes all systematic and calibration errors, in particular zero, and span-drift.
Significantly improved performance values can be achieved when measurement conditions can be held constant and span & off-set values are adjusted in the actual application environment. System performance is then governed by the Precision values.
The target analogy shown in the sketch above illustrates the difference between measurement accuracy and precision.
2.2 What is the lower limit of detectoin (LoD)?
In analytical chemistry, the detection limit, lower limit of detection, or LOD (limit of detection), is the lowest quantity of a substance that can be distinguished from the absence of that substance (a blank value) within a stated confidence limit. The detection limit is estimated from the mean of the blank, the standard deviation of the blank and some confidence factor. Another consideration that affects the detection limit is the accuracy of the model used to predict concentration from the raw analytical signal.
The limit of detection is related to the reproducibility of the spectral background and the instrument sensitivity. It is found by two or three times the standard deviation repeated measurements of the blank times the sensitivity, as determined by measurement(s) of a standard.
Most companies define it at 3 times the standard deviation (or Root Mean Square “RMS” noise s). Achievable detection limits, accuracy and precision (2s) depend on the infrared absorption characteristics of each gas as well as on optical system parameters. All three can be lowered by increasing the physical size of the absorption path (twice the absorption length gives an improvement of a factor of two). The detection limit can also be reduced by increasing the measurement time as already described in the section on precision (see above).
2.3 How good is signal linearity?
The TDLS measurement principle yields a perfectly linear relation between signal and concentration over a very wide dynamic range (see above: on measurement range). The exact linearity range depends on the gas, measurement conditions, gas matrix, path length, etc.
2.4 What is the difference between Instrument Detection Limits (IDL) versus Method Detection Limits (MDL)?
The method detection limit (MDL) considers real-life matrices.
A rule of thumb typically used by optical emission spectroscopists is that the MDL can be anywhere from about two to five times worse than the IDL.
However, this rule of thumb may not necessarily apply across all spectroscopy techniques and applications.
2.5 What is offset and span and why do they need adjustment?
Our systems are shipped with a customer specific calibration that is as close as possible to the application conditions. However, often it is not possible to represent all of the application parameter space in our calibration. Also, our calibration system and calibration gas has a certain accuracy and systematic error.
In order to give the customer the possibility to adapt the factory calibration to their measurement set-up and application, an offset and span adjust can be performed by our software. For the procedure, please refer to the Operation Instructions. A schematic representation is shown below:
The regular period for a new offset and span setting depends very much on the application itself. Some applications have very good conditions for the LGD, which hardly affect the performance at all. Other applications, on the other hand, have much harsher influences. In general, an annual check is highly recommended, regardless of the application. In many applications this is already required by legal regulations. In general, however, an incorrect span and offset setting can also be detected very quickly with a defined concentration of test gas or with nitrogen. If the measured concentration does not match the expected concentration a new span and offset calibration should be performed.
New values for the span and offset can be conveniently entered and saved in the LGD Customer Frontend (LGD Customer Frontend -> Settings -> Sensor Parameter). Alternatively, the command can be sent directly to the sensor using HEX code as described in the manual (Chapter Commands).
2.6 What is the T90 time?
The T90 is defined as the rising time needed to reach 90% of the final step value. It is commonly accepted in industry that by purging 3 times a volume vessel, the outlet will read 95% of the change. (for 99% it is five times the vessel volume).
With this statement it is easy to calculate the T90 of an LGD module at a flow of 3 l/min for the LGD F200 and for the LGD Compact with 2 l/min respectively (as recommended in the product datasheet):
- The volume of the measurement cell of an LGD F200 is 15 mL.
- The time needed to purge 3 times the 15 mL (45 mL) @ 3 l/min (50 ml/s) is then ~1s.
- The T90 of a LGD F200 is then 1s.
- The volume of the measurement cell of a LGD Compact is 19 mL.
- The time needed to purge 3 times the 19 mL (57 mL) @ 2 l/min (33.3 ml/s) is then ~2s.
- The T90 of a LGD Compact is then 2s.
2.7 What is the measurement range of LGD modules?
The detection system has a high standard dynamic range of about 2’000. This is an advantage over many other technologies where only low or high concentrations can be measured with a dynamic range of 100 or 200. In most applications rather good linearity can be obtained from the low-ppm-range up to several thousand ppm.
In some applications, amplification switching allows measurements from the low-ppm-range up to two-digit %-values of the gas, or even 100 % (e.g. CH4). At higher concentrations, the dominant accuracy limitation will be ±2 % of the measured concentration value, but usually accuracy and precision requirements are less stringent at higher concentrations.
2.8 What is the integration time and how will this influence the measurement?
The Integration time is a function of reducing the noise level by means of averaging the measurement data. Increasing the integration time reduces the noise level but as consequence increases the reaction time.
The measured values continue to be updated at the same frequency. However, if a change in concentration occurs for a gas, it takes longer for this to be displayed by the LGD, depending on the integration time set. Due to the continuous averaging over the last x measured values (x = selected integration time), it takes longer until the LGD reacts to a change in concentration.