|
In this chapter we enter the realm of measuring the values of various aspects of power, and the instruments used to do this. Imagine if you arrive on site, take a reading, hand it to the customer saying nothing is wrong, but is later proved to be false! Ok, a lot depends on what you're taking the reading of, but when it comes to power quality surveys and investigations then.... If the measurements you are going to take have any legal recourse, then the equipment you use to do this has to be regularly verified as operating within specification. A company or individual has the right to chose whoever they wish to carry out the calibration of equipment, but there are a few issues listed below that should be noted before handing over precious gear to just any old so-called 'calibration house'. Although this appears as common sense, scary stuff is happening in the field! The PQ world is not large and it came to light that one calibration facility apparently use nothing more than a variac on the mains, coupled to a multimeter, to provide what should be a stable voltage source for calibrating mains input instruments. Apart from the mains not being at all stable, questions arise regarding distortions etc. that affect some forms of RMS convertors. Just one more thing before we get to the list; Some companies have equipment and laboratories that would make even 'standards houses' blush and green with envy. However, if a company calibrates its own instruments they leave themselves open to legal issues if the question ever arises about the validity of calibration dates and accuracies. C'mon, it doesn't take a lot of brains to 'doctor' a calibration certificate - especially if the company want to protect their butts! Here's my list of questions one should ask when vetting a calibration concern and/or the personel carrying out the calibration procedures: The time between each calibration is defined by company policy or, if appropriate, governing body. The common frequency is calibration every 12 months, although it has been heard that some have decided that every 6 months is more appropriate to their needs. The choice, as they say, is yours. Verification and/or calibration is only half the story. This step purely ensures the readings from the instrument are correct. There are also further tests that may need to be carried out with regards safety. One such test is the PAT (Portable Appliance Test) with appropriate certificate. If the same calibration house can offer all required tests, well, what a bargain! On the point of the 'one-stop-shop' approach; Some companies prefer to deal with just a single concern who then takes care of all their calibration requirements. However, herein lies rather significant dangers - that's right, plural, i.e. more than one danger, and what experience has proved more than once! The reason is the single concern will have to outsource work they are not capable of doing, especially if the equipment has complex calibration procedures. There is an increased danger that your equipment is damaged in transit as it is now subjected to twice as many trips (attracting twice the carriage charge!). There is not only the delay in having the equipment travel twice as far, but also a distinct possibility of further delays if the concern don't keep their accounts in order with the outsourced calibration house. And on a final note, the 'middle-man' is also going to want his cut so all-in-all you land up a lot paying more for the priviledge for doing things this way! Mmmm......
© 10.10.02 |
MEASUREMENT TECHNIQUES: