Drop in specific gravity from post boil to fermenter?

[Kepler]

Hi all, so I just finished brewing a session ale. I use a recirculating biab system. At the end of the boil I took a gravity sample and was pleasantly surprised that it was right on the expected 1.043.

I transferred to my fermenter via a plate chiller, aerated for 15 mins using an aquarium pump and took another gravity sample. My gravity had dropped to 1.040. I took another sample to confirm and it was the same.

I use a refractometer to take samples.

Any ideas what might have caused the gravity to drop between post boil and fermenter? I can’t think of anything that would have diluted the wort…

 

[Radboud]

Was there water/star san in the plate chiller and hoses? Did the refractometer itself significantly warm up/cool down between measurements?

 

[Kepler]

Was there water/star san in the plate chiller and hoses? Did the refractometer itself significantly warm up/cool down between measurements?

Thanks but no and no... I run the hot wort through the plate chiller and back into the kettle in the last 5 minutes of the boil so cant be significantly different in gravity. My refractometer has temp correction and is left just sitting on a table the whole time.

I thought there might have been starsan in the pipette used to take the sample. That's why I did a second check.

 

[Radboud]

My (cheap Chinese) refractometer can do ATC from 50-86F, which means if you have it laying around outside on a cold day and do a measurement, then hold it in your hand for a while and take another measurement you are outside the ATC ranges and the measurements might differ. Has bitten me once or twice.

I hope you will be able to determine the cause of this!

 

[balrog]

I have temp issues with my ATC refrac as well, notably the measurement of BIAB pre-boil. I pull the bag and let it drip whilst I heat to boil (about 12 min) and my measurements during that time consistently climb as the wort approaches boil, even exceeding the theoretical max calculated using Kaiser's equations. The measurements that are "real" and match hydrometer check, are the ones IMMEDIATELY after pulling the bag prior to adding much heat. It is strange, but consistent.

 

[cyberbackpacker]

Another thing to consider is that you may have had stratification in the kettle where you read a higher sg and once in the fermenter, the wort was fully blended, giving you your true sg.

Just a thought