Hydrometer mystery...

[jrgagne99]

Here's one for you all that I can't seem to figure out.

I had about 3 gallons of syrup in my canner at 60F. I stirred it up and took a hydrometer reading. It was reading spot-on syrup for that temperature according to the table (syrup is 36.0 Baume at 60F). Great! Just heat it up and bottle it, right?

So I heated it up to 193F, stirred it up, and on a whim, I checked the density again. This time, the hydrometer read 33.5, one full Baume past syrup (according to the table, syrup is 32.5 Baume at 193F). A second and third reading confirmed that the syrup was one full Baume too heavy. The hydropmeter cup was plenty warm by time of the third reading. Anyway, I diluted with water to 32.5 Baume and bottled it.

But this bothers me. What am I doing wrong? Why did the hydrometer say "syrup" at 60F yet "past syrup" at 193F?

[PerryW]

two things come to mind....

1) Syrup is very viscous at 60 deg F and the hydrometer seems to be very sluggish. Also, air bubbles seem to cling to the hydrometer at this temp that may affect the reading.

2) You will increase density during the heating process by evaporation, especially if not covered

I would always trust the reading at the higher temp.

[jrgagne99]

Bubbles are a possibility. A clinging bubble would tend to make the hydrometer float higher than it otherwise would, resulting in an erroneously heavy reading at 60F. But what I had was an erroneously light reading (it read "syrup" when it was actually "past syrup").

On the other hand non-clinging bubbles would tend to make the hydrometer sink lower, giving an erroneously light reading that could explain my situation.

As for evaporation, I could understand a small amount of evaporating during heating, but not a whole Baume's worth. Especially since I have a cover on my canner, and the orlon filter and pre-filteres were in place.

I have 2.5 gallons in the canner right now. I'll repeat this experiment tonight and report my findings.

[jrgagne99]

I got similar results last night.

Last time, the hydrometer showed "spot-on syrup" at 40F and "past-syrup" at 193F. This time, the hydrometer showed "light syrup" at 60F and "spot-on syrup" at 193F. Although these sound like different situations, the basics are essentially the same, in that the hydrometer is reading lower than expected at the near-room temperatures.

I used the same clean, calibrated (May, 2012) hydrometer and repeated each measurement twice. I'm using a standard size hydrometer cup (2" diameter), stir the batch prior to each measurement, and I have confirmed the accuracy of my temperature gauge.

I just don't get it. I wonder if the temperature/density relationship for syrup made from red maples is somehow different than sugar maples?

My next step is to open a bottle of room temperature syrup and measure the density. If conditions hold, I expect the hydrometer to read about 1 Baume light at room temperature. This doesn't make sense, since I have never had any spoilage in 6-years of sugarmaking, and often times, even get sugar crystals at the bottom of the jugs, indicating that my syrup is slightly too heavy.

[jmayerl]

If you.........

[happy thoughts]

I tend to lean toward the evaporation theory. I reheated some syrup last night for bottling and happened to notice how much moisture collected on the lid. But one thing you haven't mentioned is the accuracy of your thermometer. Have you tested it? Maybe your thermometer is off.

[happy thoughts]

Here's one way to test if it's evaporation. Do a third test on the same syrup that's been reheated and cooled to room temp. See if the readings from the second and third test agree.

[jrgagne99]

But one thing you haven't mentioned is the accuracy of your thermometer. Have you tested it? Maybe your thermometer is off.

As I said above, I checked the thermometer. I compared it against two others of different types, and all were within 3 degrees at both room temp and 193F. Besides, even it the thermometer is off by say 10 degrees, that only makes a 1/4 Baume difference on the chart. I was off by a whole Baume.


I reheated some syrup last night for bottling and happened to notice how much moisture collected on the lid.

I too noted how much condensation was present on the bottom side of the cover. But I dripped that back into the syrup and stirred before measuring, to try to eliminate that as a variable...



Here's one way to test if it's evaporation. Do a third test on the same syrup that's been reheated and cooled to room temp. See if the readings from the second and third test agree.

That was my next plan: Open a bottle of syrup at room temperature, measure. Then heat to 193 (covered) and measure again. Then cool back to room temperature (covered) and measure again.

[happy thoughts]

That was my next plan: Open a bottle of syrup at room temperature, measure. Then heat to 193 (covered) and measure again. Then cool back to room temperature (covered) and measure again.

If that's what you end up doing, set aside the first drawn off bottle for testing. That will be the one that should be closest to the hot reading. I've read that by the bottom of the batch enough water can have evaporated off to increase density significantly.

good luck!

[jrgagne99]

It is possible that during my room-temperature measurements, syrup was sticking to the hyrdometer and weighting it down more, causing it to read light. I'll try to confirm this the next time I have cold syrup in the canner.

[jrgagne99]

I have my answer. My technique has been wrong. I was pushing the hydrometer down and letting it bob up. When the sap is cold, (50-60F), its high viscosity causes syrup to stick to the hydrometer and weigh it down. This causes a lighter reading that it actually is. Depending on how much syrup sticks to the hydrometer, it can be light by a whole Baume or more during the cold test. At 190F, the effect was much less pronounced , because the viscosity is significantly reduced, and hardly any syrup sticks to the hydrometer.

So, the lesson learned is that you should manually lower the hydrometer to a point that is slightly higher than where you expect it to float. Then slowly release it and let it settle to its final level without sinking lower first. I could have saved myself a lot of time with one simple suggestion- RTM (Read the Manual).

Getting back to question of whether it could have been evaporation during heating... I have determined that this could not have been the cause. I had three gallons in my canner. Based on the dilution tables, for three gallons of syrup to change from "spot-on" syrup to syrup that is 1 Baume heavy, it would have needed to evaporate about 16 ounces of water (2 cups) during the heating process (20 minutes). No way that could happen even if the canner was left uncovered. With the cover on, there was at most, a tablespoon of condensate on the bottom side. I suspect that folks who have attributed thickening syrup to "evaporation during heating" probably have made the same mistake as me with the hydrometer technique. Due to viscosity and surface tension, cold-test measurements are very difficult to make reliably.