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How to Align an Opacity Monitor

Correct optical alignment is essential for any opacity monitor. If the instrument is misaligned, its readings will be unstable. There may also be an offset, and this can be either positive or negative.

Aligning Opacity Monitor Figure 1AMETEK Land opacity monitors project a light beam that is larger than the retroreflector (Figure 1). They are carefully designed so that the intensity of the projected beam is very even. This design minimizes the instrument’s sensitivity to the inevitable changes in the stack or duct geometry which are caused by solar heating, process conditions, etc. (Figure 2)

All Land opacity monitors feature a transceiver alignment target which consists of two concentric rings. The image of the retroreflector is focused on this target and can be viewed, either through a window or by removing a cover. The instrument will operate within its design specification provided the centre of the retro image is within the inner ring (Figure 3a). Note that multi-element retro elements will show multiple spots (Figure 3b). Alignment of the retro itself is also required, though this is less critical than the transceiver. You need to remove the retroreflector element to before the alignment target can be used (Figure 4).

Before beginning the alignment process, you should ensure there is a moderate amount of compression on all three of the spring washer stacks. This will allow the alignment nuts to be turned in either direction and still maintain good tension.

The Land mounting flange has three studs, arranged in an L-shape (Figure 5). This allows for independent horizontal and vertical adjustments. The nut on the bottom right will adjust the instrument vertically. The nut on the top left will adjust it from side to side. NOTE: an internal folding mirror causes the movements on the alignment target to be rotated, so that the alignment spot moves from side-to-side when the transceiver is adjusted vertically.

It can be tempting to align an opacity by maximizing the signal level. This is usually a mistake because the central part of the light source is designed to be very homogeneous, and the brightest areas usually occur around the edge of the light spot. Therefore, aligning for maximum signal will almost certainly result in poor stability and drifting opacity readings.

Bright sunshine can make it difficult to see the green spot on the transceiver alignment target, especially on long-pathlength applications. In this case, it can be helpful to undo the clips on the retroreflector and swing it open. You can then see the bright circle of daylight through the retro purge housing much more easily than the green retro image. The retro is accurately centered on the purge mount, so you can align the transceiver by centering the circle on the target. 

After aligning the instrument, you should check that all three spring washer stacks are compressed. Using one hand, firmly push the instrument up, down, left and right. If you feel it loose, you need to tension all three nuts and repeat the alignment.


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