Great read on weighing espresso from Paul at Marco http://marcobeveragesystems.com/weighing-espresso-how-it-works

There’s been lots of kerfuffle over the weighing of espresso and its merits. I like kerfuffle. It generally breeds positive discourse. I added to this web-noise with this tweet: ‘It works – just try it. 18g coffee 29g pour. 1.61 ratio. 29 secs. Tasted great!  Many on and off-line discussions ensued. These discussions were not whether one should or one shouldn’t weigh espresso;  they were more so how one goes about it.  So this post is how I use this weighing espresso lark.   What’s the theory? By adding the capability of weighing the espresso brewed beverage, the barista has another tangible benchmark from which to tweak the shot into loveliness. How to go about it? Start with a fixed ratio. I advise you start with a brew ratio of 1:1.5 which is the same as 66%. What does this mean? It means the weight (or mass to be accurate) of the brewed espresso is 1.5 times the weight of the dry ground coffee dose. For example, a 20g dose of coffee to yield a 30g espresso shot SO – a brew ratio of 1.5:  20 x 1.5 = 30. OR put it another way – a 66% brew ratio : 66% of of 30g is 20g. Step by step: 1. Zero (tare) your weighing scales with an empty dry portafilter. 2. Dose 20g of your chosen grind profile. 3. Position the scales underneath the group head. 4. Position the portafilter, start extraction, place espresso cup(s) on the scales and tare (zero) the scales.  (do this in a fluid movement and the scales will be zeroed before the shot comes through). The espresso machine should have an automatic shot timer – if it doesn’t, start a timer as you press the pour button.  5. Watch the shot and the weighing scales readout – when it gets to the chosen weight, stop the shot. (this takes a little practice –  dependant upon the speed of response of the scales, you may need to stop the shot at 28 to finally read a 30g shot). 6. Note the shot time. Taste the shot – is it nice? yes=good. no =bad = tweak the parameters. What to change if a tweak is required? If you want to change the brew ratio: I suggest you keep your brewed espresso weight and modify the dose. not the other way around.  You’re best to up the dose a tiny bit to bring the ratio down; similarly, drop the dose a touch to bring the ratio up from your normal shot. Here are some examples I played with : The first  shot, ratio 1:1.5 (66%) Coffee dose 18g Espresso weight 27g 27/18=1.5   To reduce the brew ratio to 1:1.6 (63%) Coffee dose 17g Espresso weight 27g 27/17=1.59 (63%)   To increase the brew ratio to 1:1.4 (70%)  Coffee dose 19g Espresso weight 27g 27/19=1.42 (70%)    If you want to keep the brew ratio as is, adjust the grinder. If your shot was a bit fast, a fine adjustment (tightening the grind) will slow the pour. If your shot was a bit slow. a coarse adjustment (loosening the grind) will speed up the pour. When you have adjusted the grind, brew again – taking care to keep the ratios the same as the original. Taste and tweak accordingly. That’s it. A simple way to benchmark your brew to help you make better espresso. No brainer.