Draw-Off Temp Troubles

[GotSap?]

My finishing temps have been all over the place this year. My first draw of the year came at 219.3F, proper density on the hydro. 30 minutes later I'm at 224.7F and not even close to done. This happens all the time, temps all over the place. I've used a brand new syrup thermometer, as well as the digital one pictured below, and the problem has been the same. I'm thinking the issue is that not enough of the probe is submerged, but it's as close to the bottom of the pan as it can be without touching. Anyone else run across this, and if so, how did you fix it?

1801718018

[Haynes Forest Products]

Look at the tip of your probe for a small welded area. Most good probes have the sensing only at the very tip and that is indicated by the uneven weld.

[DrTimPerkins]

How deep are you running in the front (syrup) pan? Raise the probe 1/4-1/2" off the bottom of the pan. You may have bubbles or niter interfering with the temperature sensing.

Check the probe for correct temp readings. Boil some water with the probe in it. Should be close to 212 deg F (factor in elevation/barometric pressure to be sure). Then put it in a cup filled with ice and a little water and see if it reads 32 deg F.

Does the temperature bounce around a lot? Might indicate a short somewhere, faulty connection, or poor probe grounding.

Lastly, is that a thermistor based thermometer or a thermocouple-based thermometer? The connector looks like a yellow, two-spade plug (one plug slightly wider than the other). That would indicate it is a Type-K thermocouple thermometer. Thermisters thermometers tend to be very tight error limits (+/- 0.1 deg F). Thermocouples have error limits that are far higher, often +/- 2 deg C (+/- 3.5 deg F at syrup temperature). To get really technical, the hand-held unit is probably OK (+/- 0.4 deg C or so), but the WIRE is usually only rated for +/- 2 deg C unless you have SLE (special limits or error) wire/probes. The overall system accuracy is dictated by the lowest component accuracy (probably the wire). Thermocouples also operate in the microvolt range, so it doesn't take much of any voltage (from any source) to throw them out of whack.

If that is the case (it is a thermocouple), then it'll get you somewhere in the ballpark, but probably isn't suitable for determining stable draw-off temperatures.

[bowhunter]

I would suspect the probe is too close to the pan and may be touching occasionally if you have this problem with both bimetallic thermometer and digital types. The surface of the pan will be several degrees higher than the bulk liquid temperature. I just use an inexpensive kitchen digital thermometer and it becomes erratic when the prob is going bad. They only last about one season for me.

[Haynes Forest Products]

Hey Bowhunter would you go hunting with a bow with sights that were erratic at best. :lol:

[bowhunter]

Don't use sights on my recurve.... On a serious note temperature probes, even expensive ones, are pretty unreliable. You're working with a very thin layer of boiler sap and it is very easy to touch the pan.

[DrTimPerkins]

On a serious note temperature probes, even expensive ones, are pretty unreliable. You're working with a very thin layer of boiler sap and it is very easy to touch the pan.

On the contrary, under the conditions in which they operate, well-made temperature probes operate quite reliably and will remain accurate over many years. The issue here (based upon the original post) is likely to be trying to measure too close to the pan level, which is not necessary (the probe should be at least 1/4-1/2" off the bottom of the pan) OR a temperature monitoring system that is not up to the task at hand.

[johnallin]

I have a temp monitor system from Marchland. One probe at the draw-off, the second is about 20" away at the far end of that channel, and one in the stack.
They are very consistent and made things much easier for me. I watch the draw off probe - make note of when we start to draw syrup and stay with that throughout the day. The second one tells me whats coming down the pike. If we're pulling syrup at 219.5 and the second probe is holding at 219.5, I know it's going to be a long draw. When it drops to 118 +/- it's time to stop.

[GotSap?]

The probe (K type) is about 1/4" from the pan, and the sap level is about 1-1/4" deep. I swapped to a brand new probe this weekend and the problem persisted. I tested the old and new probe in both ice water, and boiling water. In both cases they were +/- 2F. I also swapped to a Marcland digital thermometer that I have, using the same probes. The temps were way off. Finally, I decided to go back to the old Marcland syrup thermometer that I originally bought for this evaporator. I couldn't even calibrate it. Just moving it a little in boiling water causes the needle to jump all over. Consistency is the big problem. Every time I draw off, the temp is different. Makes it a real pain to have to constantly check with hydrometer.