IAN HANBY - CHARTERED ENGINEER
MICROCONTROLLER ABSORPTIOMETER
CONTENTS OF KIT
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Absorptiometer
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Sample Beaker
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Plugtop Power Supply (230v 50-60Hz)
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Instructions
BENEFITS
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Used to evaluate ten-day obscuration percentage from dust deposit gauge samples
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Quick and easy to calibrate and use
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Microcontroller system gives linear performance in the range 0-70%
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Spares and repair service always available
- Low power consumption - 10 watts
OPERATING PRINCIPLE OF THE ABSORPTIOMETER
This instrument is a ‘third generation' version of the Absorptiometer described in BS 1747:Pt.5:1972. and basically consists of a 150ml Pyrex Low-form Beaker held between a Selenium Cell and a light-source consisting of a ring of 9 White LEDs. This light source is controlled using an adjustable constant current generator. The beaker has an internal mask which restricts the visible diameter of the beaker's contents to 50mm.
The output from the selenium cell varies with the amount of obscuration between it and the light source. The cell has a sensitive area larger than the bottom of the beaker and effectively gives an output based on the average of the light energy over the entire area of the sample.
Although the output from the selenium cell versus transmitted light energy is not particularly linear, a linear relationship between transmitted light energy and final meter reading has been achieved by mathematical manipulation of the cell's output within the microcontroller, using a two-stage quadratic least-squares fit for values of obscuration between zero and 70%. The user has therefore only to make one adjustment to set up the instrument (to compensate for differing opacity of sample beakers, aging of the LED light source, etc). This adjustment is effected by adjusting the current in the constant current generator referred to above and therefore mimics the procedure used in the original 1972 BS1747 Pt5 design. A small screwdriver or trimtool should be used to carry out this adjustment at the point marked ‘calibrate'. Note that the ‘calibrate' potentiometer is an 18-turn device with slipping clutches at each end of its travel.
Calculations, various checks and display generation are performed by a PIC microcontroller running at 8MHz. The results are presented on a 2x16 backlit LCD display, and are updated at intervals of approximately 650ms.
METHOD OF USE
The instrument should be plugged in using its dedicated Mascot 9583 plug-top 12-volt regulated power supply and allowed to stabilize for a few minutes during which time the internal temperature of the electronic components will settle. (When not in use, unplug power supply to save energy)
Calibration is simply achieved as follows:
Fill the beaker with clean water up to the 100ml mark and adjust the calibration control to give an LCD display reading of 0%. Note that filling the beaker to this mark is not the same as introducing 100ml water !
Using a new beaker (which must have a 50mm diameter mask) will necessitate a new calibration.
As calibration is a very quick and easy procedure it is recommended that it be carried out at least once per day when the absorptiometer is in use.
Measuring the obscuration of samples is simply a matter of pouring the sample into the beaker up to the 100ml mark and reading the display
The Reset button is basically a ‘panic' button; it returns the system to the starting point (Master Clear Reset). Turning the power off and on has the same effect.