Using Ultrasonic Leak Detection Equipment to Test Exhibit Case Seal

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Ultrasonic leak detection equipment can be used to pinpoint leaks in a well-sealed case design.

What is an ultrasonic leak detector?

An ultrasonic leak detector uses short-wave, high frequency signals (airborne ultrasound) to identify and locate leaks. Commercial units are manufactured for steam pipe maintenance and for use in the automobile industry to pinpoint windshield and window leaks. Leak detection devices can also be used to evaluate the seal of exhibit cabinetry and are particularly helpful for inspection of cases under construction.

The procedure relies on decoding non-audible ultrasonic signals emitted from an ultrasonic tone generator placed inside the case. Ultrasonic leak detection equipment requires three components: A tone generator (which generates a continuous warbling sound), a scanning receiver (used to capture, amplify, and convert the frequency), and a speaker or set of headphones (which make the ultrasonic message audible. Portable ultrasonic detection units are battery powered and can weigh as little as two pounds.)

• Ultrasound generator: A small ultrasonic transmitter is placed inside the case to be tested. It floods an area up to 3,000-4,000 cubic feet with ultrasound. The emissions travel readily through air and voids but are stopped by solid materials, including transparent glazing.
• Receiver/scanner: The ultrasound emission is made audible to the human ear by a handheld receiver/scanner. Some units incorporate a meter to visually display intensity changes in incoming frequencies. Receivers have a sensitivity dial (similar to a volume control) for calibrations of the signal and to discriminate between different wall thicknesses and leak sizes. To help direct and focus the scanner, some units use a rubber focusing probe.
• Speaker/headphones: A monaural speaker or set of headphones allows incoming emissions to be heard and interfering noise to be blocked out. Headsets can, to a certain extent, be used as low volume speakers, allowing multi-listeners to monitor the leak detection equipment.

How is ultrasonic leak detection equipment used in exhibit applications?

It is important to understand that because of the nature of the technology, ultrasound detection should be used to conduct qualitative analysis of exhibit cabinetry rather than quantitative. The technique pinpoints unsealed or poorly sealed areas but does not determine the number of air exchanges that will occur because of the leaks. It is only possible to judge the relative size of a leak by changes in sound volume and meter intensity.

A computer employing sound-sensitive software can be used to objectively measure and record a leak which is located with this detection equipment. This method requires the receiver/scanner to be plugged directly into the computer; the decibel output from a sealed area is recorded on a graph, then a comparison graph is made of the decibel level of the suspected leak. A modification of the technique uses a tape recorder to capture decibel levels (such as in a production shop or exhibit hall); upon returning to the office the recorder is plugged into a computer with sound-sensitive software for a visual recording of the sound level.

The threshold for acceptable leakage should be established by the exhibit team and depends on the specific function and expected performance of the case. The size of a case leak can be established with some difficulty. The technique used by leak detection firms is to verify the equipment's sensitivity using established references. The references used are standardized metal orifices. An orifice or set of orifices can be purchased; each is inserted into a hole that is drilled into the case. Comparing the decibel level emitted through a known sized orifice to the sound level emanating from a leak establishes the size of the leak. The technique requires that a threaded orifice be tapped and screwed into a metal case wall or a non-threaded in-line connector orifice be inserted into a standard, plastic bulkhead fitting (1/8" ID) that is mounted into a non-metal case wall.

• The tightest measurable hole that air passes through is .00004 of an inch (or 10 microns). Ultrasonic leak detectors can detect this size leak.
• A hole size of .0008 allows standing water to pass through it. Leaks in cases can be restricted to less than this size hole.
• A set of orifices (e.g., .002 to .016 of an inch) can be purchased at a relatively low cost.

When is detection equipment used in exhibit applications?

Ultrasound detection is a useful aid throughout the exhibition process: during fabrication, as an inspection tool for acceptance or rejection of a case; during final installation of the case; and for follow-up maintenance checks. The equipment can be used to test cases designed as well-sealed, hermetically-sealed, or ventilated. Problems and the likely causes of the leakage are:

1. Leaks along joints, seams and holes caused by:

• Deficient construction
• Need for caulk or sealant

2. Leaks along doors and movable panels caused by:

• Deficient construction or closure design
• Insufficient closure mechanisms

3. Leaks around gasketry caused by:

• Unseated gaskets
• Insufficient gasket design or size
• Poorly joined gasket corners and splices

Are there any special techniques for using the equipment to evaluate an exhibit case?

As with any testing equipment, the accuracy of results depends on proper usage. Follow the four steps outlined below, making sure that the unit's battery is recharged frequently following manufacturer's instructions.

STEP 1: Placement of the tone generator

• Position the generator in a central location in the empty case
• Emitted sound waves will travel around corners, however, relocation of the generator is necessary to test separate chambers, e.g., the lighting chamber

STEP 2: Scanning for leaks

• The hand held receiver/scanner is used to scan the exterior, working from gross to fine for specific areas, crisscrossing to pinpoint problem areas
• The focusing probe should be directed over all seams, doors, and other areas of possible leakage, including screws and bolts

STEP 3: Discriminating sound and identifying leaks

• Adjust the sensitivity dial to the lowest level where the leak is just audible but registered on the meter; note the warble and meter level
• Continue scanning, recording location and thresholds for all leaks (tape markers are useful)
• compare to established sealed standards— known good (sealed) and known bad (unsealed) areas

STEP 4: Establishing a protocol for repeatable measurements

• Use identical tone generator placement and scanner movement between different cases
• Standardize sensitivity of dial setting
• Standardize sensitivity adjustments for differing materials and wall thickness (e.g., small leak in a thick wall produces less sound than in a thin wall)
• Comparison to standard reference orifices, if required (optional)

Products, Manufacturers, and Suppliers

Mention of a product, manufacturer, or supplier by name here is for information only and does not constitute an endorsement of that product or supplier. Listed materials have been used successfully in past applications. It is suggested that readers also seek alternate product and vendor information to assess the full range of available supplies and equipment.

Leak Detector

Untraprobe 500 STG, UE Systems, Elmsford, NY, 10523-2336

Ultrasonic Leak Detector 5550, Triple 5 Industries, Trenton, NJ, 08620

Decibel Sensitive Software

Spectrapro Software, UE Systems, Elmsford, NY 10523-2336

Leak Standard Orifices for Hole Size

In-line Connector Orifices Test Kit, Bird Precision, Waltham, MA, 02454-0569

Standard Orifices, O'Keefe Controls, Trumbull, CT, 06611

Air-tight Plastic Fittings

Bulkhead-Mounted Luer Fitting, Value Plastics INc, Ft. Collins, CO, 80525