A Peculiar Luxmeter!

When a user buys a meter, be it a voltmeter, a water-flow meter, a UV-A irradiance meter, a luxmeter, he expects the manufacturer has designed an accurate equipment, that the supplier is able to calibrate the meter and that confidence in the equipment is a “no-question asked” given.

One of us, who worked in a company performing many calibrations of UV-A radiometers and luxmeters, had his attention drawn by a colleague on a curious luxmeter designed by a French manufacturer for NDT applications.

As the peculiarity of the meter seemed so…peculiar, the colleague wanted to let others have a look at this extraordinary performance.

The luxmeter under test, sent by a user for recalibration, displayed only even numbers! A 2 lx-step, never seen before in any unit, especially in a unit designed to measure 20 lx maximum in UV-A inspection booths!

You may imagine the tremendous error when reading small figures, even 20 lx. This unit displayed 0 lx for any real value from zero to 1.9 lx; then 2 lx for values between 2 and 3.9 lx; and so on, up to 20 lx for values between 20 and 21.9 lx. The relative error is tremendous! Add to this that every manufacturer states that the figure on any digital display is always +/-1 digit.

Unfortunately, along the years we met meters unsuitable for our specific NDT needs: for instance, luxmeters used to check the visible light illuminance in UV-A booths, whose diffuser fluoresces under UV-A light, producing visible photons that the sensor adds to the photons detected in the booth. In fact, we saw a luxmeter which gave a reading of 3,500 lx in a UV-A booth where there were an irradiance of 3,000 µW/cm², and an illuminance of only 12 lx when measured with the right luxmeter.

Once again, our paper is there to make readers and users, as well as auditors, or people in command of choosing an equipment, or people responsible of selecting a calibration subcontractor, or people in charge of Quality Assurance, in short … almost everyone involved in NDT (and not only PT or MT)! ... think twice, or even thrice, before writing specs, purchase orders, calibration procedures/requirements, etc.

By the way, we think it a very good idea to pay a visit to your calibration labs. Dare we give you another short story? ... Yes we dare!

Some ten years ago NDT people of the French Air Force were wondering how the manufacturer of their radiometers carried out calibration. So after a phone call they went to the small company’s plant, and asked the manager to show them the process. The manager refused, stating that it was very technical, secret; he did not want anyone to have a look on his equipment, fearing that others could copy it.

The Air Force specialists insisted; they were shown that the secret equipment was behind a curtain. They went to say that, if not shown the process, they would cancel the contract for the yearly calibration of 32 meters, and prevent any new purchase from this supplier.

Reluctantly, the company’s manager drew the curtain, and very embarrassed, explained that the UV-A source for the calibration was a … mere, standard 100 W UV-A source, not even voltage-stabilized! The “calibration” was done by first putting the “reference sensor” under the source, then replacing it with the sensor under test.

You may guess the reaction of these technical people … whose attention, by the way, had previously been drawn on this specific point by one of us, who had explained and showed them some things about what is a reliable calibration equipment. The French Air Force people were able to compare this equipment with the “very secret, technically ahead of any other installation” calibration installation of this manufacturer.

In the '60s a UK Company had a motto: "For every surface treatment problem, there is a product xxxxxx" (trademark we do not display).

Engineers and Commercial people in this Company had made a "translation": "With every surface treatment product xxxxxx (trademark we do not display), there is a problem".

This anecdote is there only to remind everyone that problems met in workshops may be due to the suppliers/manufacturers as well as to the users.

Our idea in these documents is NOT to target anyone, but on the contrary to bring to your knowledge some interesting cases which may prevent you to duplicate the same mistakes while performing Penetrant Testing (PT) or Magnetic Testing (MT).

All the ministories you will read are TRUE. We think they will be helpful:
• First as examples of specific technical --or non-technical-- requirements or peculiar problems.
• Second to let you see that the problems do not always come where you think they should come from.
• Third so that users feel free to ask for help from people (the experts) who may know more than they do.