Foam on beer detectors and how they cut waste at the bar
A foam on beer detector, usually called a FOB, is a small chamber in the beer line with a float inside. When a keg empties, it shuts the line before foam can fill it. On a long draw system, that saves beer at every keg change. Here is how it works, how to use it and where it earns its cost.
A FOB is a float that shuts the line
The detector sits in the beer line, usually on a wall in the cold room close to the kegs. Inside a clear chamber is a float. While beer flows, the chamber stays full and the float rides high. When the keg runs dry, gas and foam enter the chamber, the liquid level drops, the float falls and seals the outlet.
The result is simple. The beer line from the FOB to the tap stays full of beer. Instead of foam spluttering out of the tap and filling the line, the tap just stops pouring. The bartender knows the keg is empty and nothing has been lost downstream.
Why the line volume matters
Without a FOB, the end of a keg sends foam all the way up the line. After the new keg is tapped, that foam has to be pushed out and poured away before clear beer reaches the glass.
The volume adds up. A 3/8 inch bore trunk line holds roughly 70 ml per metre. A 20 metre long draw line holds about 1.4 litres. If most of that has to be poured away as foam and part filled beer at every change, a busy bar changing 15 to 20 kegs a week loses many litres a month. On a short counter mount system with 2 metres of line, the loss is small and a FOB matters far less.
Resetting a FOB after a keg change
- Change the keg as usual and seat the coupler fully.
- Open the vent or bleed valve at the top of the FOB chamber. Gas and foam escape.
- Watch the chamber fill with clear beer from the bottom.
- Close the vent once the chamber is full and foam free.
- Lift or reset the float using the lever or button on the unit.
- Pour a short test into a jug, check it is clear and taste it.
Train every person who changes kegs on this routine. A FOB that is reset with foam still in the chamber lets that foam straight into the line.
Cleaning FOBs with the lines
The FOB is part of the beer line, so it needs cleaning every time the lines are cleaned, typically every two weeks. Most units have a clean or bypass setting that holds the float open so cleaning solution and rinse water can flow through. If that setting is missed, the float can trap solution or leave part of the chamber uncleaned.
Strip the chamber, float and seals by hand from time to time. Look for yeast or protein deposits and replace any perished seals. A dirty FOB taints every pint that passes through it. Our guide to draught line cleaning covers the full routine.
When a FOB pays for itself
FOBs make the most sense when:
- Lines run more than a few metres from cold room to tap.
- The bar changes many kegs a week.
- The beer is expensive craft beer where every litre lost costs real money.
- Staff turnover is high, so keg changes are not always done by experts.
Work out your own number. Multiply the volume of each line by the number of keg changes on that line each month. That is the beer at risk. If even half of it is poured away today, a FOB on your long lines will usually pay back quickly at Indian craft beer prices.
FOBs help with quality too
Waste is only half the story. A line full of foam and air between kegs exposes the inside of the tube to oxygen and lets beer sit in an odd mix of gas and liquid. A FOB keeps the line full and sealed, which is better for the next beer as well.
There is a staff benefit too. A keg change on a line with a FOB is calmer and quicker, because nobody has to stand at the tap pouring jug after jug of froth while customers wait. In a crowded bar on a Saturday night, those minutes matter.
It also takes the guesswork out of empty kegs. Without one, a spluttering tap during a rush can be read as a gas problem, a line problem or a keg problem. With one, the tap simply stops and the empty is obvious. For more on reading a pour, see our guide to judging a beer's head.
Count what you pour away
The case for a FOB is easier to make with numbers from your own bar. Put a measuring jug next to the taps for one week. Every time staff pour off foam after a keg change, they tip it into the jug and note the amount on a sheet. Multiply the weekly total by your cost per litre.
Your first step: measure the length of your longest beer line from keg to tap and multiply by 70 ml per metre for a 3/8 inch line. That is the most you can lose at a single keg change on that tap, before counting the foam from the new keg.
Common questions
What does a FOB do in a draught system?
It shuts the beer line when a keg empties, so foam and gas cannot fill the line between the keg and the tap.
Where should a foam on beer detector be fitted?
In the beer line close to the kegs, usually on a wall in the cold room, so most of the line stays full of beer when a keg runs dry.
Do short draught systems need FOBs?
They help less. With only a metre or two of line, the beer lost at a keg change is small. FOBs pay back best on long draws.
