How to choose an effective ultrasonic detector

Many electrical maintenance technicians rely on ultrasonic detection for the preventive maintenance of high-voltage insulation components on transmission lines and at distribution centres. These components need to be inspected regularly for mechanical problems, corona leaks, cracked insulators, ground faults, leaks in air breakers, cracked skirts, tracking, loose connections and worn bearings.

The adaptation of ultrasonic detection for these inspections provides operators with an all-in-one tool that is robust, versatile and portable.

Ultrasonic detection is convenient. Visual inspections can only be done when safety lockout procedures are enforced. Since there are few opportunities to shut down a substation, these inspections are often rushed or neglected. Moreover, most of the problems can only be found when the systems are energized. Ultrasonic detection provides the opportunity for random inspections that can be conveniently conducted at any time.

What to look for

Consumers have many choices when specifying tooling. As in most growth markets, technology is usually developed by a good R&D firm and then duplicated by the masses for fast profit. As with many other products, ‘buyer beware’ is the critical theme in selecting an ultrasonic detector. So what should you look for?

By definition, ultrasound is any frequency occurring above the human range of hearing. Since that definition is not very specific, and relies on everyone hearing the same, 20,000 Hz and beyond is commonly referred to as ultrasound. So to qualify as an ‘ultrasonic detector’, all one needs to do is build a sensing device capable of detecting a frequency in the ultrasonic range.

Combine this loose definition with crafty marketing techniques and you are sure to see all sorts of devices appear on your purchasing agent’s desk — many of which are unsuitable for the job.

Signal/data processing

The ability to detect ultrasound is quite simple. What is done with the ultrasound after it is detected makes all the difference between equipment that is useful and equipment that gathers dust. To provide truly meaningful data to the operator, an ultrasonic detector must be more than an amplifier of noise.

First of all, a good device must use current techniques to transform a specific (not random) ultrasonic frequency from its naturally occurring frequency into a signal that is both audible and measurable.

To be of any use to the operator the audible signal must be recognizable. Whether one is listening to a bearing, an air leak or corona discharge, the sound in the headset must be representative of the specific phenomena. This is achieved using a technique called heterodyning.

Heterodyning simply means to receive an input signal, mix it with a fixed frequency signal and amplify it. This allows the conversion of ultrasound to audible sound while protecting its quality and integrity. The converted ultrasound maintains its properties, allowing humans to listen at an elevated frequency. Detectors that use older, or no heterodyning technology, will have a lot of static noises in the headset that are confusing to the operator and make it difficult to discern between a leak, corona or even someone shuffling his feet nearby.

Signal/data measuring

Listening to sounds allows the operator to make a qualitative interpretation of the condition of the equipment. But for comparisons between multiple operators, it is important that the ultrasonic detector have an accurate and repeatable means for measuring and displaying input signals.

The highest-quality ultrasonic detectors use True RMS measuring techniques to average and linearize the analogue input signal. Since few manufacturers use this technique, and even fewer salespeople can explain it, be certain you ask for it in order to qualify your selection. True RMS is a proven way to guarantee repeatability when measuring an analogue input signal.

Presenting the measured data in a meaningful way is equally critical. A simple bar graph or needle is useless for benchmarking as no true measurement scale is specified. The measurements should be displayed on a reference scale that is logical and meaningful.

Presenting ultrasonic values on a digital LED display was first implemented in the mid-1980s and is now being adopted by other manufacturers 15 years later. Beware the crafty marketing use of the word ‘digital’. To use a digital display is one thing, and to process data from its analogue origins to a digital format (analogue to digital conversion) is completely something else. A manufacturer’s documentation can be worded in such a way to make it sound as though it is a ‘digital’ detector, when in reality it is only displaying an analogue input signal on a numerical scale.

The commonly accepted method for presenting ultrasonic data is to use the logarithmic scale known as the decibel. The decibel is not a measuring unit like Volt, Ampere or any other unit, but it is a ratio between a reference unit and a measured value.

The decibel originally comes from quantifying signal strengths in terms of relative loudness as registered by the ear. Because of this relationship to the human ear, the decibel is the logical scale to use.

For example, if a person estimates that a signal is twice as loud when the transmitter power is increased from 10 watts to 100 watts, he or she will also estimate that a 1,000-watt signal is twice as loud as a 100-watt signal. The human ear has a logarithmic response. This fact is the basis for the use of a relative power unit called the decibel (written as dB). A decibel is one tenth of a bel, the unit of sound named after Alexander Graham Bell.

When making reference to acoustics or audible sounds, we specify the sound in dBA. Voltages are sometimes given as decibel values with respect to 1 Volt (dBV) or to 1 mV (dBmV). An ultrasonic detector is detecting a microvolt analogue signal and so to display the measured value properly, the exact technical term should be dBmV or dBV (decibel/microvolt or 0dB = 1 mV).

Some manufacturers introduce confusion to the equation by misusing or misunderstanding the term decibel. To simply display a measured result as 35 dB is meaningless. It is as dubious as stating 35 kilos. Does it mean 35 kilograms, kilolitres, kilometres or kilovolts?

Here are some questions to ask when selecting an ultrasonic detector:

* Is there any memory capacity to store values

* Can it download values to a PC?

* Does the manufacturer supply database software to accept and organize the reading?

* Can the device be upgraded?

* Can it be repaired locally?

* What is the warranty?

* What accessory sensors exist to expand functionality?

Ergonomics

Functionality is important and equally important is comfort. Pay attention to ergonomics because a device that is not comfortable to use becomes a dust collector. Is it heavy? Can it be held comfortably in the hand?

When using a parabolic dish or other attachment, can the buttons be accessed without having to set down the accessory? Are the accessories designed with thought to the applications?

Accessories

The most common accessory for corona detection is the parabolic dish. This lightweight sensor extends the detectable distance to make even very tall transmission lines accessible from the ground.

It should be constructed of aluminum with provisions made for aiming both with laser and left or right eye. Plastic see-through dishes are hard to use on sunny days since the operator can’t always choose to have the sun at his back. Consider also that if the dish is stored in a truck on a very hot day, the high temperatures can easily distort the shape of the plastic parabola and reduce accuracy.

The parabolic dish is a delicate senso
r and should be protected in a sturdy case. Depending on the manufacturer, the sensor can be shipped in anything from a cardboard box, to a vinyl case, to an aluminum case. If you’re going to be working in rugged environments, choose a rugged case.

Finally, remember that the parabolic dish is an accessory to a more versatile device. It should not require any additional electronics to make it heavy. Its job is to detect a signal, not process it.

An ultrasonic detector should be capable of detecting both airborne and structure-borne ultrasound. Airborne ultrasound (corona, air leaks) is detected with the parabolic dish, a flexible wand style sensor, or an integral airborne sensor. Structure-borne ultrasound is detectable using either a contact-style needle probe, a magnetic sensor or a permanent threaded sensor. These accessories are used to monitor the condition of bearings, gearboxes, motors, valves, steam traps, hydraulics and pumps.

A complete ultrasonic package will offer these as either standard or extra. The important thing is that they are offered.

Allan Rienstra is with SDT Diagnostic Technologies, Cobourg, Ont. For more information, visit www.sdtnorthamerica.com.