Questions and Answers about the LGD F200
FAQ about LGD F200
1. LGD F200 - Best practice
1. LGD F200 - Best practice
1 LGD F200 - Best practice
- NH3-slip measurements in Selective Catalytic Reduction (SCR) de-NOx installations for:
- Ammonia monitoring in process control, e.g. carbo-nitration in steel treatments
- Incineration monitoring (CEM): NH3, HCl
- Semiconductor (Scrubber Wet/Dry)
The LGD F200 is designed as a self-contained and stand-alone device. Customers need only to supply power for the electronics and power for the cell heating elements. Furthermore, conditioned gas needs to be supplied. Filtering for dust and avoiding condensation in the measurement cell is required. The temperature of the incoming gas has to be adapted to the set cell temperature. In most cases the LGD F200 is either integrated into industry standard (e.g. 19-inch rack mount) or into OEM specific housings. The housings might contain other analytical elements as well as gas pump, filtration elements and power supply.
The LGD is a self-contained instrument with an integrated flow through cell for extractive gas measurements. Because Axetris’ customers have many different applications with diverse requirements, we cannot recommend standard equipment. However, it is typically necessary to use a pump to pull the target gas through the measurement cell. Furthermore, filtration for dust is necessary to avoid any contamination of the inside of the measurement cell. For hot-gas applications and industry standard heated gas sampling lines, hot-gas extractive probes with adequate filtration are required.
The LGD F200 is heated to a predefined value, typically 190 or 220°C. The temperature of the incoming gas has to be adapted to the set measurement cell temperature. If the temperature difference from the gas compared to the heated cell is too big (> +/-10°C) this can create extra noise and decrease the Precision of the device (Also, see the target analogy graph in Axetris_LGD_FAQ).
The LGD F200 is a rather compact self-contained and stand-alone device that can be integrated into housings that can be mounted in different settings. In SCR process control or CEM-type applications, on-stack semi in-situ/gas extractive mountings are possible and avoid long sampling lines. Another popular set-up is to use already installed gas conditioning equipment like extractive probes, filters and heated lines and to introduce the LGD F200 in the measurement house just before the gas-cooler that is used to condition the gas for other analyzers like NOx or CO2. Furthermore, the extractive set-up allows for simple 0-gas and span-gas measurements, as required daily in certain emission monitoring applications. In some applications it can be necessary to carry out multi-point measurements in order to achieve optimum process control (e.g. SCR) – in this case the extractive set-up of the LGD allows for stream-switching set-ups, contributing to a significant reduction in investment cost.
Cross-duct system cannot offer simple 0-gas and span-gas measurements as they are installed in-situ and measure across the stack. Furthermore, the installation of emitter and receiver increases installation cost for one measurement point compared to an extractive system. Also, cross duct systems do not allow for stream switching set ups when multiple point measurements are required, which increases capital expenditure. High dust-loads and misalignment from stack distortion can lead to significant signal reduction or loss.
Purge gas can be used to eliminate the absorption influence of the dead space between laser diode respectively detector and measuring cell and mirror if target gas is present in the surrounding of the sensor module.
It can also be used if the gas matrix contains potentially dangerous mixtures. Purging can add additional safety in case of cell leakage.
As already stated above, the best performance can be achieved when the module temperature is stable, i.e. not exposed to ambient temperature changes.
When ambient temperature changes occur (e.g. during the warm-up time or when turning on the room’s air conditioning), the performance of the instrument is governed by the “Accuracy” value as given in the product data sheet. (Also, see the target analogy graph in Axetris_LGD_FAQ).
When the temperature of the instrument is stable and as well as the temperature around the instrument, the performance is governed by the “Precision” value in the specifications. This “short-term noise” can be improved by averaging measurements over a certain period of time (e.g. 5,10 or 30 s). Generally, Precision improves with the square root of the number of measured. E.g. a 10s average should result in 3.3 times better precision.
The 190°C/220°C measurement cell heating will heat up the housing. The fan should then stabilize the temperature in the box to 45 or 50°C, with maximum precision (e.g. 1°C). However, the ambient temperature around the instrument (or inside of the box) should not exceed 50°C. This is because the global heat budget from the ambient temperature added to the intrinsic temperature of the instrument (190°C cell heating) would be too high for electronics and electro-optics. Therefore the Product Data Sheet indicates the ambient temperature range as 15 – 50°C.
When a sample gas touches the walls of the tubing a few of its molecules stick to that surface, some more than others. This is called adsorption and when someone is working on ppm level analysis this loss of molecules can be significant.
The strength of adsorption effect is depending on molecules nature. The NH3 for example, as polar molecule, is strongly affected by adsorption effects and such can lead to longer T90 and delay in reaction. Imagine that the NH3 will need to cover all tubing surfaces and filter surfaces before reaching the LGD F200.
Avoiding this physical effect is difficult and special care needs to be taken when selecting material choices for tubing, filters, connectors … contact firstname.lastname@example.org for more information and assistance.
It’s very important that a separated heater power supply is used for LGD F200 H, and this should deliver only 24Vcc (with max 4A).
The connection should be done as shown in following schematics:
The electronic box of an LGD F200 must not be detached from the body. As the electrical signals going from electronic box to optical parts are very small, they are easily affected by EMI surrounding fields and can decrease dramatically the performance of an LGD F200.
All EMI tests were performed with an electronic box attached to the LGD body and Axetris can’t ensure same results with an electronic box detached.
As the LGD F200 has an opto mechanical adjustment it’s important to avoid big mechanical vibrations or mechanical shock that could lead to system misalignment and performance degradation.
As mentioned in the operating & integration instructions, for environments where important vibrations can’t be avoided, dampers can be mounted on the LGD F200 to attenuate the mechanical vibrations.
Due to adsorption effects of polar molecules like for NH3 the gas molecules are “stored” in the measurement cell walls. By heating the measurement cell these molecules are released in the flow and create a signal.
If the flow is not big enough, typically <1 l/min, some oscillations on the concentration reading will be seen corresponding to the heating/cooling cycle of the measurement cell.
These are physical effects that can only be avoided by ensuring enough flow through the LGD F200 H.
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