Differentiating amplifier helps detect small leaks

DN Staff

September 22, 1997

4 Min Read
Differentiating amplifier helps detect small leaks

Columbus, OH--Users of high-efficiency particulate air (HEPA) filters and ultra low penetration air (ULPA) filters (used in the nuclear industry and in cleanrooms) need to detect leaks as quickly and reliably as possible. A widely employed method of detecting leaks relies upon a photometer that measures light scattered from an aerosol. Typically, engineers atomize what's called a challenge agent, for example, dioctyl phthalate (DOP), upstream of the filter to generate the aerosol. A probe moving across the surface of the filter captures aerosol at the leak and delivers it to the photometer.

To be sure of detecting leaks, the probe should reside over the center of the hole for several detector time constants, i.e. long enough to build up aerosol concentration in the photometer's optical chamber. When the operator encounters small leaks, or leaks not directly under the probe, such concentrations often don't develop satisfactorily. Until recently, when using any photometer, operators faced a choice of either missing some leaks or working very long hours scanning filters at very slow speeds.

Photometers gather aerosols and use light focused on a photomultiplier to measure aerosol concentrations. The light stop creates a shadow that keeps direct light out of the photomultiplier.

Designed by Bela Kovach, Eric Banks, and Walter Wikoff of NUCON International Inc., this scanning hand-held probe makes use of a differentiating amplifier to observe the first fraction of the rising signal produced by an aerosol. The amplifier also removes slow signal drift and zero offset, preventing them from interfering with a reading. This approach reduces the time required to detect leaks by 50% to 90%, according to the NUCON team.

A photometer basically consists of a light source and light stop, a sample chamber shaped like two cones joined at their apexes (the sensing zone), a lens that collects light scattered by particles in the sensing zone, and a photodetector. Drawn by a pump, the sample moves into the first cone-shaped region, passes through the sensing zone between the cones, and exits the chamber. Photodetector output is an analog signal, a varying voltage proportional to the density of the aerosol. A conventional system amplifies and displays that signal. It appears as a value added to photodetector drift and the constantly changing background signal.

In the system designed by NUCON's team, the probe's two op amps receive the signal. Any basic differential op amp produces an output signal equal to the difference in voltage appearing at its input terminals. In the NUCON design, an op amp with a gain setting of one drives the main display of an LCD meter to present the aerosol concentration seen by the photodetector to the operator.

Engineers preset the second display of the analog bar graph LCD meter to a value equal to 50% of meter range. Doing so enables the meter to display both positive and negative movement. It's fed by an ac-coupled op amp with a gain of 10. This ac-coupled differentiating op amp causes the display to show a positive movement as the probe passes over a leak, and a negative movement as the probe moves away from the leak. At the same time, an LED flashes to warn the operator of a sudden change in aerosol concentration. These signals enable the operator to realize that he or she just passed a leak. Next, the operator moves the probe back over the leak, and reads the leakage concentration value from the main LCD meter display.

According to the NUCON engineers, tests performed using a NUCON detector reduce the time needed to scan for leaks by 50% to 90%. Such time reduction allows operators to scan more filters per unit of time, and to reduce both their exposure to radiation fields and time spent in confined spaces.

Other Applications

  • Test equipment

  • Pollution monitoring

Additional details...Contact Curt Graves, NUCON International Inc., 7000 Huntley Rd., Columbus, OH 43229, (614) 431-0858.

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