Okay - a couple of questions to help...

1. Have you set up an Equipment profile yet?
2. How many brews? and what efficiency were you set to on each brew? The same or different?
3. How much kettle trub are you leaving behind? Is it consistent?
4. Are the water levels pre-boil the same?

Recommendation for your next few batches: Now that you've done a few (diiferent?) brews I would the most basic of them and use it as a basis for consistency. I know @Nosybear would agree with this - brew the same basic recipe (typically a blonde ale) over and over to hone in your process/equipment. That's what he recommended for me and it helped tremendously.

 

Here's a couple of things to look at as maybes.......Altitude, Relative Humidity, how vigorous is your boil could be part of your "Water levels post boil is spot on, even lower sometimes. " quandary , as to the readings...are you adjusting your hygro and refracto meters for temperature? Cooling down a sample of mash wort from 150°F is much faster then for sweet wort from boiling.

Also, 5 to 7 point delta? Relax, don't worry.....and welcome to Brewer's Friend! Good first post!

 

Please expand on this.....and explain at exactly what points in the process you are taking your readings....

 

1. I usually build a new profile for a separate volume and kettle. I'd make a new profile for whichever you don't have one for.
2. I too, will have conflicting pre-boil readings and have taken to focusing on the final reading and how to improve it. - Setting yourefficiency to 60 or 62 might help bring closer to hitting the final. - Whichever you set it to, don't change it again until you can get an average from your next few batches.
3. Totally fine as far as I can see. Good call on updating the profile to reflect. This too may help...
4. solid.

Hmmm.... Switching from 4 to 2 gallon batches would partially (or mostly) explain the 58-65% gap.
Continue to focus on nailing the final volume - it sounds like your boil off rate is pretty consistent...
I can't explain why you can hit pre-boil but not post... Then again, I've had the same problem... I've even been high pre-boil but then fail to reach the post-boil... Never by much but enough to irk me. I always thought I was maybe doing something wrong...
Do you put in your temperature to allow for expansion when entering the pre-boil volume?

Anyone else have ideas? - I'll try and keep good tabs on mine over the next couple brews and see what I notice on my end.

 

These are the same points I take mine too.

 

See "Total Water Volume in Mash" and then to the far right is the "temperature correction"---- this option comes up on each brew log entry. Since water expands when it heats up you may think you have more than is there. For instance, I top up to 7 gallons for boil size but with temperature correction I typically have 6.8 gallons.

 

I also brew BIAB. My pre-boil gravities are always higher than predicted by the software (usually by 6-8 points!) but my OG’s (post-boil gravities) are usually spot-on or within a point or 2 of what was expected.

My guess is that the problem lies in your grain crush. Crush a little finer or crush twice and I bet you’ll be right where you want to be.

Cheers!

 

What's with the sparging with cool water? Is that a BIAB thing?

 

That's a reasonable approach but just a tip: Hot (170 degree F) water would get more of the sugars than cold water. You're going to boil the wort anyway so why not give it a bit of a head start?

 

That is a good point Nosy. I guess it is the same reason we use hot water to wash dishes, clothes, and brewing equipment for better cleaning.

 

I don't sparge with my BIAB. I lose about 2 litres (1/2 gallon) per 5.5 kg (12 lbs) of grist AS (after squeeze)

At the end of the mash I remove my bag and let it drain until its slowly dripping then squeeze the crap out of it. Then my boil takes another 4 L (1 gallon) out. I use the fermenter volume in the recipe builder to calculate the starting volume pre-mash (about 32 L /8.5 gal) which gives me between 24-26.5 L (6.5-7 gal) in the fermenter if I am being generous including trub. I'm usually a little low on my pre-boil gravity but usually over by a few points on my post-boil gravity but I take that into account (a little bit) when I build the recipe.

I used to spend an hour calculating out sparge volumes and final and such. In the end sparge VS no sparge...my no sparge method made no difference in the final beer quality compared to sparging so I cut out the extra time and hassle of the sparge. Saves me time and effort.

EDIT: I mill my grains fairly well and I have been hitting mid-70's % efficiency with no sparge.