BeerBorg

I was always around 75%+
Last 3 batches I'm at barely 50%
Havent changed anything

Maybe your readings are off? How are you testing? Is the grain the same? Any possibility that water quality changed?

Buy grain in bulk? Maybe a batch got malted funkily.

How about the grain crush? Has that changed at all? Do you crush your own or does the LHBS crush for you? If they crush for you, you might want to ask them to check their mill to see if the settings are off. Or ask them to run it through twice and see what happens.

Also....are you checking your mash temps to make sure they are where you want them?

Double-check calibration on hydrometer/refractometer?
+1 on the crush suggestion.

Holy crap, that's a huge drop. I think you've got a nice list to start with to try and figure out what is going on.
Sorry to hear about the trouble.

I assume you're brewing beers of similar gravity, and haven't moved on to "bigger" beers in the past few batches? Efficiency tends to go down as gravity points go up. Not THAT much, but....
Not likely the explanation, I realize, but just figured I'd throw that out there as something else to consider.

Hmmm...

Could be a combination of things...

All good suggestions so far.

To add to each of the individual things to check...

How's the calibration on your thermometer? Maybe you are mashing higher than you think...

Also, how about the gap on your mill... Has it widened? If your LHBS grinds for you, has theirs changed?

BigPapaG wrote:How's the calibration on your thermometer? Maybe you are mashing higher than you think...

Unless I am nuts, and unless we are talking temps outside of enzyme range, IE > 160, this should not effect your efficiency as far as OG by very much. You still get sugars, just different kinds. It will effect your FG much more then your OG. Now if it's the other way - in the mid/low 140s or so, it might take a lot longer to convert as the reactions start to slow down - and seem like bad efficiency if you don't wait it out.

Yup, I was going for higher as that would make the mash less fermentable... Edit: and if it's really off, you could be higher than 160°F !

Mashani is right too, mashing lower would slow conversion and could be a bigger culprit, especially if your mash time was shortened at all... You could be reading 148-152°F and really be down in the low 140's...

I usually see thermometers off be a couple of degrees, not usually as much as 10° but it's possible...

(Note: I just pulled an oven thermometer out that was room temp in my hand and reading 425°F...
That was a spring type so it would not be uncommon for that to be way off, but I digress)

Then, maybe there are a couple of things just not quite right... They could add up...

Ive actually doubted my hydrometer since I got this one ... and I don't know for sure but it may be all of these "low" batches were done since getting this hydrometer
I have added DME to a couple of them to raise OG,
But then it also seems FG has been getting a little lower than expected. And the only one that been drank so far certainly seemed to be higher ABV than they should have ben based on OG readings
So I'm kinda thinking it may be my hydrometer, and guess what, I broke it this morning
I guess I have to get a new one tomorrow
Hopefully that has been my problem

Was having mash / brewhouse performance problems a while back as well.
Hydrometer was the culprit.

Defect traced to the paper with the various scales shifting inside the neck.
There were two red marks on the glass where the top of the paper should align.

Good tip Herb. I just recently noticed that mine is .0002 off, and that it is just a rolled up paper in there.

You can easily see if the hydrometer is wrong, all you need is distilled water, which should read 1.0000