Sweep and degas: what the two buttons on a digital generator do

Two functions separate a digital generator from an analog one in daily use. Both are simple, both matter, and both are misunderstood often enough to be worth a page.

Sweep: no standing waves

Sound in a tank reflects off the walls and the surface. At a fixed frequency the outgoing and reflected waves add up into a standing wave: a fixed pattern of points where the pressure swings hard (antinodes) and points where it barely moves (nodes). At 40 kHz in water the nodes sit about 18 mm apart, at 28 kHz about 27 mm. A part resting across a node cleans poorly along that line, and the pattern shows on the foil test as regular stripes.

Sweep modulates the generator frequency continuously across a band of about ±1 to 2 kHz around nominal, a few times a second. The node pattern moves with it, so over a cycle every point in the tank sees the full pressure swing. The result is an even sound field and no striping on the part.

Sweep also helps the generator: as the load and temperature change the transducers' resonance drifts, and a sweeping generator crosses the true resonance every cycle instead of hunting for it.

When to leave it on: almost always. The only reason to switch sweep off is a specific process that has been validated at a fixed frequency, or a diagnostic where you want to see the node pattern on foil.

Degas: getting the air out

Tap water holds roughly 20 mg of dissolved air per litre at room temperature. A freshly filled tank is saturated. In that state the low-pressure half of every sound cycle pulls dissolved gas out of solution into the growing bubbles; the bubbles fill with gas instead of vapour, and gas cushions the collapse. The bath hisses, the surface fizzes, and the part comes out barely cleaner than it went in.

Degas mode pulses the ultrasound on and off, typically a few hundred milliseconds each, at full power. During the on period bubbles form and coalesce; during the off period they are large enough to rise and leave the liquid instead of being broken up again. After five to ten minutes the fizzing stops, the sound of the tank changes to a sharper crackle, and cleaning power is back.

Heating the bath drives gas out as well, which is why a tank warmed to working temperature before use needs less degassing than a cold one. Detergent lowers the surface tension and speeds the process up.

When to use it: after every fill or top-up, and after the bath has stood for more than a day. On the UMX Pro and the digital ST generators degas is a programme step, so it can be part of the daily start-up.

What you see on the foil

The foil test shows both effects clearly. A degassed bath with sweep on perforates the foil evenly across the sheet within a minute. A gassy bath leaves the foil almost intact. Sweep off on a degassed bath leaves horizontal bands of holes with clean strips between them, spaced at half a wavelength.

Which machines have them

Sweep and degas are functions of the generator, not the tank. On our range they are standard on the UMX Pro series and come with the digital generator option of the ST series; an analog generator has neither. For the benchtop STS models the product page lists what each generator offers. A machine without degas still cleans, but it needs a warm, rested bath to do it well, which is one reason to specify the digital generator when cleaning is a daily production step.