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Servicemans Log

Not all faults in equipment are hardware or software based but are instead due to carbon-based failures or CBFs, as in carbon-based life forms. Unfortunately, CBFs can really waste your time.

by the Serviceman

Items Covered This Month

• Carbon-based failures
• The mosquito that died happy
• Is it electrical or fuel?
• Faulty air-conditioning controller

My first story this month comes from T. M. of St Agnes, SA and concerns faults in biomedical equipment. Here’s the story in his own words . . .

For the last 19 years, I have worked as a Biomedical Engineering Technician for one of the largest pathology providers in Australia. For most of that time, I have worked in a team that looks after all our regional labs. These can range from quite large labs with several hundred staff to small labs at country hospitals with only four or so staff members.

As with most modern equipment, the majority of problems are not strictly electronics related. Most of the gear in this industry is built in the USA, Europe or Japan by large corporations with huge design budgets. Failures in the medical industry can sometimes literally be a life or death situation and so the design and construction of such equipment is to a very high standard.

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Certainly, the companies involved are not interested is sacrificing component quality just to save a few dollars.

As a result, most of our equipment is quite reliable. Even so, I have a team of seven guys who work hard at fixing the analysers that the supplier’s sales reps tell us (at the time of purchase) should never break down.

One morning, I received a call from a Lab Manager complaining that his main chemistry analyser was reporting an error along the lines of “Can’t find reagent B”. Chemistry analysers usually have one or more thin probes attached to robotic arms that pick up samples or reagents and place them into a small cuvette (or tube), where a chemical reaction takes place. Light is then shone through the cuvette and the absorbance of the light at different wavelengths is measured.

By measuring the absolute figure, rate of change or a peak value, the analyser can determine the amount of a certain chemical that was originally present in the sample. The results mean something to our medical fraternity but usually don’t mean much to the engineering staff, especially as the chemical name is usually shortened to a code such as “T4” or “Dig” etc.

The volumes of both the sample and the reagent are kept very small for each test. Apart from anything else, keeping reagent volumes to a minimum is a cost-saving exercise as the number of tests each machine does in a week can run into the thousands.

Normally, a liquid-level sense circuit is incorporated on the probe, allowing the probe to draw a reagent or sample from the top of the fluid. This is done to minimise “wetting” of the outside of the probe, thus reducing carry over and contamination. The liquid level sensing is usually achieved by monitoring the capacitance of the probe and looking for a step change as it touches the surface.

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