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-   -   Hydrometer reading vs Refractometer reading (http://www.homebrewtalk.com/f128/hydrometer-reading-vs-refractometer-reading-260055/)

Mpavlik22 07-31-2011 09:08 PM

Hydrometer reading vs Refractometer reading
 
Whats up all?

I just got done brewing a Pale Ale (all grain). I just got a refractometer and decided to try it out. I still used my hydrometer to compare. I got completely different reading pre-boil & post-boil.

Before you ask, i did calibrate the refractometer with distilled water. Also it is a ATC (auto temp correction) unit. regardless since i take my hydro reading at 60 deg F, I cooled to 60 deg and took my hydro reading and refrac reading at the same time and temp.

Anyways my readings:
Pre Boil:
Target - 1.042
Hydrometer - 1.041
Refractometer - 1.033

Post Boil:
Target - 1.054
Hydrometer - 1.051
Refractometer - 1.043

So can anyone explain why my readings are so different between the hydrometer and refractometer?

Thanks

jd3 07-31-2011 09:57 PM

Is your hydrometer calibrated?

Mpavlik22 07-31-2011 09:58 PM

Yes hydrometer is also calibrated.

jd3 07-31-2011 10:06 PM

The only reading I take anymore is a OG and a FG. Why are you taking pre and post boil readings. I'd worry less about them and more about results.

Yooper 07-31-2011 10:10 PM

Quote:

Originally Posted by jd3 (Post 3132433)
The only reading I take anymore is a OG and a FG. Why are you taking pre and post boil readings. I'd worry less about them and more about results.

A preboil reading is very useful. If you are way over or under on your target OG, you can fix it right then. You can also get your post boil OG just from doing some simple math from the preboil reading (If you have a preboil gravity of 1.060 with 7 gallons, for example, you KNOW what your gravity will be with 5.25 gallons in the fermenter), and adjust any hopping rates accordingly. You can also figure your efficiency from the preboil reading. Once you have your system completely locked in, maybe a preboil reading is less useful, but I wouldn't dismiss it.

jd3 07-31-2011 10:14 PM

While I know there is a simple formula to get the reading post boil, I think it makes more sense to just take the reading.

I suspect the refractometer is either poorly calibrated, or just general poor quality and giving a bad reading. I use them in fish tanks and if they are the cheaper variety I can get different readings back to back.

Obviously one of the items is wrong. While I'd hope that the hydrometer is right since it is closest to the predicted readings, you would be a better judge of how things went.

I'd try it on some other known samples and go from there....

Sithdad 07-31-2011 11:29 PM

Get some distilled water and test both your hydrometer and your refractometer. Your refractometer may need adjusting. Also, some refractometers need to have their results adjusted based on a comparable hydrometer result. For example, after adjusting your refractometer you test a wort sample with your hydrometer and your refractometer. If, your temperature adjusted, hydrometer reads 1.040 and your refractometer reads 1.044 your refractometer has and adjusted wort value of 0.004. This would mean that all of your readings, from your refractometer, would require an additional 0.004 being added.

Mpavlik22 07-31-2011 11:35 PM

As already stated I have calibrated both with distiller water

iaefebs 07-31-2011 11:53 PM

Quote:

Originally Posted by Mpavlik22 (Post 3132566)
As already stated I have calibrated both with distiller water

I'm not sure what kind of an answer you are looking for. It seems people have trouble with refractometers. I don't know if it is because the newer cheaper ones have defects or if they are not getting the proper temp for a reading. I take a small sample of pre boil wort and place it in my ferment chamber for a few minutes then take a reading. I do the same post boil. When I sample during ferment I just insert a straw through the airlock hole to grab a sample. I hardly ever use a hydrometer untill it gives me an excuse to taste a sample. My refractometer is 5 years old.... maybe they made them better back then. I think you are experiencing temperature problems.

ajdelange 08-01-2011 01:49 AM

Quote:

Originally Posted by Mpavlik22 (Post 3132345)
So can anyone explain why my readings are so different between the hydrometer and refractometer?

Thanks

Refractometers are calibrated for sucrose solutions in water. Wort is not a sucrose solution. It contains a complex spectrum of sugars including some, but not very much, sucrose. A hydrometer is also calibrated against a sucrose solution. Thus neither instrument reads the true extract of the wort but density measurements track solution strength better than measurements based on refractive index i.e. a hydrometer reading will compare much more favorably to a true density reading made with an oscillating U-tube meter or pycnometer than a refractometer reading will. Refractometers really should not be relied on for anything other than monitoring the decline of extract as time progresses in sparging but home brewers insist on using them for a purpose to which they are not really suited. It must be said that the agreement between hydrometer and refractometer with wort is usually better than what you observed. In fact the agreement is often quite good but it is not unusual at all for them to disagree by a couple P as in your case. The problem is you have no way of knowing whether the agreement is good or bad unless you make a comparative measurement in which case there is no point in taking the refractometer reading as you have to take a hydrometer reading to verify it. The exception to this is in commercial brewing where a calibration curve between refractometer and densitometer reading are made and used for a particular beer. When that same beer is brewed again a refractometer reading can be entered into the calibration curve and a good estimate of the extract taken out.

ATC may be partially responsible in your case. The ATC in a refractometer is based on the shift in refractive index of a pure sucrose solution relative to 20 C (68 F). Wort's refractive index does not shift at this same rate so all measurements with a refractometer should be taken with ATC turned off at a temperature of 20 C. Not saying that this explains all your discrepancy but it might explain some of it. Another thought is turbidity. In the instruments where the light actually passes through the sample (the kind you look into) scattering may make the delineation between the dark and light parts of the field fuzzy. In the electronic instruments where the light never actually enters the sample you have better immunity to this effect.


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