A helpful guideline for coffee-bed depth was established by Ted Lingle, author of The Coffee Brewing Handbook. He recommends a coffee-bed depth of between ≈ 2.5 and 5 centimetres (between 1 and 2 inches). If you go shallower than 2.5 cm, the risk of channelling increases (see Lesson 3.05). If you go deeper than 5 cm, your contact time is likely to become excessively slow.
The largest cone-shaped filter you are likely to find is an 03 size. You rarely find an 04 size because beyond this point, manufacturers tend to shift to a flat-bed funnel shape instead of a cone shape. Rather than encouraging baristas to continue to pile up more grinds into a deeper bed as batch sizes scale up, the solution is for filter cones to become wider instead of taller. The dose size that represents the cutoff point here is around the 75-g (2.6-oz) level. There are batch-brewing machines on the market capable of brewing 40-litre (10.5-gallon) batches! If you are contemplating the size of brewer your cafe needs, consider this wisdom from prominent coffee educator and author Scott Rao:
‘Brew batches no larger than 2 litres… if you’re serving fewer than 500 cups of filter coffee per day, two litres per batch is sufficient. If you’re really busy, i.e., you serve more than 50 cups (12-oz or 350-ml) of batch brew per hour, it may be best to brew into a rotation of three two-litre carafes during those busy hours. That system will keep the coffee fresher and tasting better than will brewing fewer, but larger, batches.’