Refractometer repeatability/accuracy

[triangulum33]

I took several measurements pre-boil and postboil with my new refractometer and the results varied wildly.

Preboil I measured 15.2, 14.8 and 14.3. All of these measurements allowed ~5 minutes for the sample on refractometer to drop to near room temp.
Post boil I measured 18 and 17.2 with ~10 minutes each time for temp to drop.

The last measurement I did Post and preboil ended up being very close to my hydrometer.


Is there some technique that I could be messing up?

 

[badbrew]

Maybe the wort solids are settling out in between samples?

 

[triangulum33]

I did a little test with grapefruit juice heated/cooled in a shot glass with different temps measured 3 times each:

45 degrees
11.4
11
11.5

100 degrees
11.6
11.7
11.7

185 degrees (simulate a preboil sample)
14.2
15.2
16.5

~75 degrees (allowed to cool from 185 in pipette)
12.1
12.2
12.2

The part that confuses me is that I let the 45 and 185 degree tests stabilize for a few minutes each in the refractometer and the values did not change. The 185 degree read very high right when I put the drops on and did not change as the sample sat in the refractometer for several minutes. I would've expected all the tests to converge to a common reading as they sat and stabilized.

 

[ayoungrad]

Does your refractometer have ATC?

 

[triangulum33]

Does your refractometer have ATC?

Yup. Its model RF-15 and has a "ATC" sticker on the body.

 

[ayoungrad]

I have never taken a measurement over mash-out temp (168) but my measurements up to that temperature have been within a few points. Which, for me is accurate enough.

Why do you need measurements at 185?

As far as the variability for your pre-boil, I think similarly to others. That it is likely from draff, solids or from other causes of inhomogeneity of your samples.

 

[triangulum33]

Why do you need measurements at 185?

I want to measure my pre-boil gravity to figure mash efficiency. I picked 185 because that is what my microwave happened to heat the juice to.