This article first appeared in the 1983 inaugural issue of Corrosion Control magazine, Kay Publishing. The science has grown since then, so don't take issue with the technique as outlined, this is just a writing sample, not a current technical article.

 

Caveat emptor

Eddy-Current Testing

It's good testing system - if operator skilled

by
RG Liberty



One of the most effective and cost efficient methods of testing heat exchanger tubing systems is the Eddy Current method, but users must be sure the people doing the testing know what they're doing.

Industry sources say that increasingly, companies hoping to cash in on the use of the method are fielding teams of ill-trained testing people who can't really interpret the results they receive from the instrumentation.

That means a lot of money is being wasted on such testing and in many cases damaging corrosion is going undetected.

The Eddy-Current method has been widely recognized throughout North America as one of the best non-destructive systems developed to date for applications such as commercial heating, ventilation and air conditioning (HVAC) systems.

Testing is accomplished by passing a four inch probe, attached to a flexible tube housing wires, through the entire length of a tubing system, or into bundles of tubing, to locate faults such as pitting, cracking, wear and erosion.

The probe holds four coils. Two emit Eddy-current fields which measure smaller defects within the tubing and the other two measure large defects, and both sets display them on a cathode ray screen.

It is then up to the technician to interpret the information provided by the scope. If he can't, the testing is worse than useless, because the end user is out the cost of having his unit tested, and may still end up with system damage. And in a $150,000 system, it's not uncommon to spend $50,000 for repairs when leakage occurs.

The cost of testing varies from add up, in some cases, to $1,600 for an entire system. If, for example, the client does have a faulty tube in his system, the Eddy-Current method will detect and locate the problem.

The cost of replacing a tube might run to $100, but if the fault is not repaired and water eventually leaks into the system, the damage can, conceivably, amount to the cost of a whole new system.

It's not too difficult to see the advantages of regular testing with this method. It's also easy to understand why the person doing the testing must be a fully-trained, highly qualified technician able to read the results accurately.

Hunter Blakely & Associates of Charlotte, North Carolina, one of the pioneers in developing the Eddy-Current method specifically for air conditioning units, have a training course for their technicians which lasts a year or more. The course adheres to the guidelines set down by the American Society for Non-Destructive Testing. Only on completion of the course is a technician sanctioned for testing.

As Hunter Blakely says, "An Eddy-Current instrument has considerable potential if used in proper hands. There are a very large number of people out there who have purchased instruments and set themselves up in the testing business, but there is also very little attention paid to the training aspects."

Some manufacturers offer short training programs to purchasers in the use of the instrument, after which the buyer is given a certificate stating he is a testing technician. Blakely is opposed to this practice, as are Canadian service companies like D'Arcy Sweeney Ltd.

Mike Lambert of DSL told us. "A little training is better than none, but people who buy these instruments and set themselves up in business without proper training are unwise.

It would be like me buying an electrocardiogram machine and testing patients for heart disease. You just don't do that. This is an art as well as a science and it takes time and thorough training to become competent."

In Canada, so far, there is no regulatory body or sanctioning group involved with systems testing using the Eddy-Current method. Although there haven't been the numbers of unqualified personnel in the field that plague the American market, the few who are out there doing improper testing are casting a pall over the credibility of reputable service companies. The end user receiving erroneous results from the method doesn't always differentiate between the method and the testing technician.

The problem is compounded in the United States, and to some extent in Canada, because many insurance companies are beginning to insist that system owners and maintenance managers have their units tested regularly. A commercial air conditioning unit should be tested every three years until a fault is located, then each year thereafter until the fault is corrected. Then the testing should resume on a three year basis, says Lambert.

One method suggested is to set out testing guidelines relevant to the training and qualifications of the field personnel, as the American Society for Non-Destructive Testing does.

The best advice Lambert and Blakely offer to owners and managers of HVAC systems is to "screen companies thoroughly before any testing begins."