Hydrometer discrepancy?

[Daniel Ashman]

I took a sample of a cider that has been fermenting for a couple of weeks. I spun and bounced the hydrometer a few times to make sure there were no bubbles and it wasn't catching on the sides of the sample container. The reading came out at 1.013 SG at about 24degC. I came back about an hour later and it was now reading 1.001 SG. The temperature may have risen a degree or 2.

Any ideas what may be causing this discrepancy? Which reading do I trust? The second reading is closer to what I was expecting it to be. I'll take another reading in a day or 2 and see what it comes out at.

Could soaking the hydrometer for a while cause issues with the reading?

[Druid]

I would trust the first reading, where you know the sample temperature (although at 24*c, you should use the adjustment chart that came with the hydrometer to adjust for temperature - most hydrometers are accurate at 20*c).

I notice you are in Auckland - with the recent warm weather, your sample could well have risen way higher, especially if left in the sun or a warm room.

Having said that, I guess there's a possibility that it degassed while it was standing too, despite spinning it etc.

[Daniel Ashman]

Other than temperature is there anything else that could be happening over an hour to cause the reading to change so dramatically?

I'll take another reading in the next few days and see what it comes out as. I also have a refractometer but from my research refractometer readings are inaccurate once there is alcohol in the sample.

[Druid]

In my experience, there's not really anything else which could affect the gravity beside temp (and further fermentation) - you've noted the only other possibility (gas bubbles sticking to the hydrometer bulb - but 12 points is perhaps too big a difference to be explained by that).
You could try shaking your sample to degas it and let it stand for a while so that it goes completely flat. Then when you measure the gravity, also get the temperature and adjust for it if you need to.

[Aarad Russell]

I've heard of the paper scale slipping inside the hydrometer.
You might want to try it in water (as close to the hydrometers rated temperature as you can) which will hopefully give you a result around 1.000 SG. You should be able to add/subtract any major difference there to your cider reading, provided that you account for any temperature differences.

[Daniel Ashman]

Good idea. I will try this. Based on my calculations the difference in the 2 readings is too high to be accounted for due to temperature calibration difference alone.