The pressure gauge on top of a bright tank tells you the pressure in the headspace. Your filler doesn’t draw from the headspace. It draws from the bottom of the tank, where the pressure is higher. That gap is easy to forget, and it matters on packaging day.
Liquid has weight
Pressure in a liquid rises with depth. The extra pressure from a column of liquid is its density × gravity × height (ρgh), the same reason your ears pop at the bottom of a pool.
For beer, with a density close to water, that works out to roughly 1.4 PSI for every metre of liquid (illustrative arithmetic using ρ ≈ 1,010 kg/m³). A tank with two metres of product in it has nearly 3 PSI more at the outlet than at the head gauge. As the tank empties, that extra pressure falls away.
Why it matters for packaging
When you pick a packaging pressure from the head gauge, you’re choosing based on a number the filler never sees. The filler sees head pressure plus the weight of the liquid above the outlet, and that changes through the run as the level drops.
FizzWizz’s Head Pressure Mode holds the headspace steady during packaging: on the run shown in our head transfer video, head pressure varied by about 0.02 PSI through the whole fill. The hydrostatic part, though, depends on liquid level.
What the Bottom Pressure Sensor does
The optional Bottom Pressure Sensor (also called the Foot Pressure Sensor) lets FizzWizz read pressure at the foot of the tank during packaging, instead of in the headspace. Because it reads at the outflow, hydrostatic pressure drops out of the pressure control equation, and the filler gets a more stable, consistent pressure as the product level falls. BrewWizz says that cuts setup time and reduces fill inconsistencies (brewwizz.com).
What you need to know:
- Where it goes: at the tank’s foot or on the CIP arm, in the outflow line.
- Which units: the FizzWizz Bubbles Anywhere Edition running code version 2.0.5 or later. All Bubbles Anywhere units can be updated to the latest code.
- One port, two sensors: the sensor shares a port with the temperature probe. The Temperature / Pressure Switch keeps both connected: select the temperature probe for carbonation mode and the bottom pressure sensor for packaging mode, with no plugging and unplugging.
- Build: 316 stainless steel with a tri-clamp connection, rated up to 100 PSI and splash proof (System & Accessories).
Gauges drift too
There’s a second, separate reason a head gauge can mislead: the gauge itself. One FizzWizz customer found the unit showed their head pressure gauges were slightly off, and checked FizzWizz’s carbonation against an Anton Paar CO2 meter:
“It taught me that my head pressure gauges are slightly off and it seems accurate with the carbonation.” Evan Cronshaw, Whitetooth Brewing Co.
If your packaging numbers don’t match what the gauge says, check both: where you’re measuring, and what you’re measuring with.
Want to know if it fits your line? Contact us, or order at brewwizz.com.
Sources
- Hydrostatic pressure (ρgh) is standard fluid statics. The 1.4 PSI per metre figure is our own arithmetic, using a beer density of about 1,010 kg/m³.
- brewwizz.com, Bottom Pressure Sensor and Temperature / Pressure Switch product pages.
- FizzWizz System & Accessories (construction details), Testimonials (Whitetooth) and Head Transfer Mode (0.02 PSI).