Tap 7 Half-Foam at 1,500 Feet
You manage a 16-tap taproom at 1,500 ft elevation. Your house pale ale is carbonated to 2.6 volumes and served at 38°F through 10 ft of 3/16-in. ID vinyl line at 12 psi. Tap 7 pours roughly half foam into a pint, while taps 8–12 pour normally from the same batch. Only tap 7 is affected. The line was cleaned yesterday, the keg was tapped this morning, and the keg was delivered warm and chilled overnight in the walk-in. The foam is large-bubbled and collapses to about 1 in. of head. You have a Zahm CO2 tester, a calibrated thermometer, a pressure gauge, and a spare coupler and faucet. The taproom opens in two hours, and the manager wants a fix before service.
1. What is your differential diagnosis, and in what order do you check it? For each check, state what result would confirm or rule out a cause.
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Differential diagnosis, in order:
- Pour, glass, and faucet hygiene – swap the glass and then the faucet on tap 7. A dirty glass or partially open faucet can nucleate foam; if the swap fixes it, the keg is not at fault.
- Beer temperature at the tap and keg – measure with the calibrated thermometer. If tap 7 beer is above 40°F while others are 38°F, CO2 breakout from warm beer is likely. A warm keg or a line run near a door or heat source fits.
- Restriction and balance – inspect for a kinked line, partially closed flow-control lever, or mismatched restrictor. Large-bubble half-foam with normal pressure often points to restriction at the faucet or coupler.
- Carbonation – use the Zahm on tap 7 and a known-good tap. If tap 7 is above 2.7 vol, the keg may be over-carbonated or filled from a different tank. Same-batch taps pouring normally make a system-wide carbonation error unlikely.
- Coupler and spear – swap the coupler and, if possible, the keg. A defective or loose spear gasket causes turbulence and foam.
- Pressure gauge – confirm 12 psi at the regulator and at the keg. A stuck gauge or closed manifold valve can mislead.
Most likely, tap 7 has a warm or mechanically restricted keg or coupler, not a recipe or carbonation-setpoint problem, because taps 8–12 from the same batch are fine.
2. If the keg at tap 7 is measured at 44°F while the other kegs are 38°F, what is the likely effect on foam, and what immediate and root-cause corrections would you make? Use the 12 psi setpoint in your reasoning.
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At 44°F and 12 psi, the beer's equilibrium CO2 is roughly 2.2–2.3 vol, not 2.6 vol. The keg is warm relative to the others, so dissolved CO2 comes out of solution in the line and faucet; large bubbles and half-foam are classic breakout. Cooling the keg back to 38°F restores solubility. Immediate action: move the keg to a cold, circulating spot in the walk-in, or immersion-cool it; do not simply lower serving pressure, which would worsen breakout. Hold at 12–13 psi. At 1,500 ft, the gauge pressure needed for 2.6 vol at 38°F is slightly higher than sea-level charts, about 12.7–13 psi, but the 44°F fault dominates. Check line temperature too. Root cause: the keg was placed near the walk-in door or tapped before reaching 38°F. Correction: enforce a 24-hour chill before tapping, log keg temperatures, rotate stock, and insulate or reroute the tap-7 line if it runs near a heat source.
3. What prevention steps would you institute for the next keg and for line cleaning?
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Prevention steps: (1) write a keg-receipt SOP requiring 24 hours at 36–38°F and a temperature log before tapping; (2) verify line length, inside diameter, and flow rate for 3/16-in. line at 12 psi, and use a flow-control faucet only as a tuning aid; (3) after line cleaning, rinse with potable water and sensory-check the first runnings; (4) keep walk-in doors closed, add a strip curtain, and map warm spots; (5) train staff to check the first pour and use a clean glass; (6) keep a spare coupler and faucet for swap tests. During service, if the keg cannot be cooled in two hours, move tap 7 to a known-good keg and chill the suspect keg. Never serve half-foam pints as normal; that masks the fault and wastes beer.