Refractometer errors due to hop components?

[Mr. Fuggles]

I have been using a refractometer for a while now but have noticed that it can give a different reading in relation to the hydrometer .

1. All my readings in Brix are converted to SG using accurate tables (http://www.fermsoft.com/gravbrix.php)
2. I regularly calibrate my refractometer with distilled water to get the zero reading and also against my hydrometer with wort to check they are reading the same (usually they are).
3. The errors seemed to be coming up with the gravity readings taken immediately after the final hop additions. This observation seems to be reinforced by the fact that I recently brewed an American I.P.A and got very different readings from the refractometer and the hydrometer just after the final hop additions (15.2 degrees Brix and 1.066 S.G).

Of course it could be that the hops affect the hydrometer reading. I cannot find anything in the literature about this problem and don't know if it is a documented effect or if I have some unknown error in my procedure.

Any ideas?

[brewinhard]

Is your refractometer ATC (automatic temp conversion?)? If so, what about when you are reading your gravity with your hydrometer? Do you adjust for any temperature differences? I would try to avoid getting any hop material or hot break in your refractometer reading as that could probably mess up your reading as well.

[Mr. Fuggles]

Yes, refractometer is ATC and I always correct my hydrometer readings for temperature of sample.

I try not to let bits of hop material and trub get on the plate when using the refractometer, but even so I don't believe that it is critical to have a sample completely free of ALL suspended particles. (When taking samples from grapes in the field, I know that juice is just squeezed onto the plate of the refractometer and can't believe that NO bits of stuff get in!)

[spiderwrangler]

Does the refractometer always read lower than your hydrometer during the conditions you describe?