If you run an iodine test on wort from the mash tun, it will only tell you if all of the starches in the liquid have been converted or not. It will not tell you if you have residual starch in the grain particles. For that you should take a sample of grains plus wort, grind the mix up in a mortar & pestle, and do an iodine test on that.
A simple way to determine if your conversion is complete is to measure the SG of the wort in the mash tun (after a good stir), and compare it to the maximum possible SG. For this you need accurate measurements of the strike water volume (room temp basis), and grain weight. From these plus the grain potentials, you can calculate the maximum possible SG in the mash tun. Your conversion efficiency is then:
(1 - Measured_SG) / (1 -Max_SG)
Kai gives an approximate chart for determining max SG based on water to grain ratio here: http://braukaiser.com/wiki/index.php?title=Understanding_Efficiency#Measuring_conversion_efficiencyYou need to be careful when comparing efficiencies batch to batch, as the accuracy of the efficiency calculations are only ± 4 - 5 percentage points, based on typical accuracies achieved in measuring water volumes, grain weight, grain potential, grain moisture content, and SG. Based on this, your two batches could have had the same efficiency (e.g. they both could actually have been 69%.)
Brew on
