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HF Monitor HA1

Hydrogen Fluoride Monitor HA1, Open Path

The HF monitor HA1 was the first gas monitor developed by Norsk Elektro Optikk AS (NEO). It was developed in co-operation with Hydro Aluminium and the main development was done during year 1990. More than 35 units have been produced including a similar short path version HF Monitor SR1.

The NEO HF Monitor HA1 and SR1 are discontinued products and have been replaced by the LaserGas counterparts.

Principles of operation

The HF monitor HA1 (and SR1) was based on single line spectroscopy and in-situ measurements giving correct measurements under a variety of process conditions. It was for many years the only reliable monitor for correct measurement of hydrogen fluoride.

A halogen light source emitted light in the infra red and in the visible range. This light is sent through the gas sample like emission in a potroom in a primary aluminium smelter and the light is collected by the analyser or "monitor". The optical part of the monitor filters out all light except around 2.4 micro metres where there is a HF absorption line.

Optical system of third prototype

This picture shows the optical components of the third prototype. To the left we have the fore optics (cylindrical tube), next 2 solid etalons and an interference filter (black retainer ring). Then we have the square centre plate and a scanning Fabry-Perot interferometer (black cube). Behind the connectors we have the detector optics and the detector (PbS) (cylindrical tube) and to the right the detector amplifier. The interference filter is an optical band pass filter while the solid etalons (air-spaced) acts like a sharp bandpass filter only letting the area around the HF absorption line through. The scanning Fabry-Perot interferometer scans across this area giving light signals on the detector as shown in the figure below.

HF Absorption (left), No HF (right)

To the left we have the detector signal with HF present in the optical path and to the right without HF. The dip in the curve will depend on the HF concentration in the path.

In the Potroom

HF Monitor light source (left) and the Monitor (right)

To the left we can see the light source mounted in the roof structure in the potroom. To the right we can se the monitor itself. The optical path lengths covered by this HF monitor was up to more than 300 metres.

Typical example of HF potroom emission

This figure shows typical curves from potroom emission. Peak correspond to work procedures in the potroom where covers are opened anodes changed etc. As can be seen from the curve, typical levels are around 0.1-0.2 mgHF/m3 when no operations are ongoing. (Primary aluminium smelter, prebake technology.)

The NEO HF Monitor HA1 and SR1 are discontinued products and has been replaced by the LaserGas counterparts.

The HF monitor HA1 technology has been patented world wide. The following patent publications describes this technology: AU5578494 A, BR9307468 A, CA2149663 A1, EP0670035 A1, NO924443 A, NO924443D D0, SK63895 A3, US5606419 A, WO9411713 A1.

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