im a batch sparger too
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.. I take the approach of adding an extra handfull or 2 of grain to accommodate any efficiency loss, and have met my targets..
@Barry44, what is the start and end temps you have recorded for your mash, Im sure your covering all this but just in case..
A poor mix/dough in can leave dry pockets especially if u add the grain to the water when mixing?
if your loosing temp and starting below 67C your mash isnt sitting in optimum conditions, try an extra kettle of boiling water to preheat the tun, and when set cover the tun with old sleeping bags, coats, luandry, anything to add more insulation and help retain the heat.
2 full kettles of just off the boil water poured into my tun about 20 minutes before the hlt hits my strike temp of 72C prewarmed my coldbox tun a treat.
Also If your heating up your liquor with a controller, pid or stc1000 type box, there is a gotcha which caught me out till i caught on.
- With a static temp probe and being left a while once the strike temp is hit the water in the hlt can 'stratify' (dunno if thats the right word, but u get layers of colder water at the bottom and warmner at the top and spot on at the probe level.. many people now recirculate the hlt liquor with a small solar pump or give it a damn good stir cos of this..
Underletting (posh for filling the tun via its drain) with the grain in the tun allows the liquor to soak up evenly thru the grain helping avoid dry pockets and requiring much less effort to dough it all in
hope thats useful..