Catastrophic Pump Meltdown

April 4, 2011 · 10 comments

Melting, What a World...

My desire for precision and accuracy occasionally leads to trouble. It wasn’t enough to have constructed my own immersion circulator, I had to test it to make sure it would be accurate through the range of temperatures (60°C – 85°C) I would need for sous vide cooking.

My plan was simple: I’d set the circulator at various temperatures, starting at 50°C, stepping through 5°C increments, and ending at 90°C. I would plot the value reported by the circulator thermocouple (green LEDs in this photo) against the actual temperature recorded by my calibrated digital probe thermometer (Thermapen). If the numbers lay along a straight line, I would be able to program an offset into the controller.

If only it had been that simple. The first thing I had to do was change the pre-set upper temperature limit in the controller from 40°C to 100°C, which would give me the testing range required. I immediately noticed a huge discrepancy between the set temperature (orange LEDs), reported value, and actual temperature. After a bit of fiddling with sub-menus (a process not improved by an interface that uses only four buttons) I programmed an offset of -28°C – a negative temperature correction that seemed to resolve the discrepancy at 60°C.

As I began the stepwise testing, I realized that the offset would have to change, and dusted off my algebra knowledge to calculate a slope (y = mx + b, remember?). Then it occurred to me to check all of the settings for the thermocouple, which is when I discovered that the reported value was being stated in °F instead of °C. Once I fixed that, I was ready to begin in earnest.

I stared at 55°C and stepped up, but when I got to around 75°C my readings seemed to level off. As I stared at the circulator, watching vapor escape from the tub of water, it hit me: I had reached the equilibrium point where evaporative cooling was perfectly balanced with the heat input. I solved that problem by floating about 100 ping pong balls ($12 for a gross at Amazon) on the surface of the water bath. They prevented further heat loss, and could easily be moved aside to add or remove cooking bags.

I got the water up to 75°C, then 80, and 85°C, when I noticed that the pump was making a lot of noise. I reached 90°C, shut off the pump, and let the whole rig cool down overnight. The next day, when I turned on the circulator, I didn’t hear the pump, and couldn’t see the water circulating. That’s when I noticed it had melted, as seen above. The cover was completely ruined and the case had begun to warp. Much to my surprise, the pump still worked with the cover removed, but I didn’t want to risk having it crack open and dump raw current into the ungrounded water bath.

After a bit of online research I learned that most of these pumps were rated only to 85°C. I bought a replacement, and instaled it this past weekend:

It’s larger and sturdier, but I decided to seal the snap-on cover to the rest of the unit with silicone cement. It runs quietly and survived another test run, but as added insurance I reprogrammed the upper limit to not exceed 86°C. I’m satisfied enough with the performance of the circulator that I have removed the electrical tape securing the front lid and permanently sealed it with more silicone.

What about the calibration curve that caused all of this trouble in the first place? It’s accurate to a tenth of a degree, with no offset required in the controller. Science: it may get you in trouble at first, but it always works out in the end.

10 comments

Kimbo April 4, 2011 at 4:28 pm

I was going to play around with my own ghetto sous vide rig – a cooler, some hot water and plastic bags – but you have inspired me to put one of these together too. I’m looking forward to seeing what you end up making with it.

David April 4, 2011 at 4:44 pm

Give it a try, As I mentioned, cutting the plastic box is the hardest part. If decide to go ahead with it, feel free to contact me with any questions you may have.

Chris Shaw April 4, 2011 at 4:37 pm

Instead of risking electric shock, couldn’t you just run PVC tubing to and from the water bath for circulation? It’s a lot easier and cheaper to replace the tubing.

David April 4, 2011 at 4:46 pm

You mean like using an aquarium pump? There are lots of solutions like that, but they al involve having to set up another box and plugging it in. The thing I built is all-in-one, which is what I was looking for.

Dennis April 5, 2011 at 10:16 pm

What happened to good ole’ over engineering? I guess when the manufacturer said “85C”, he really meant it…! Nice job on the calibration work, though-Dennis

David April 5, 2011 at 10:51 pm

That pump was over-engineered, it was meant to curculate water in a small fountain. How many fountains have you seen that require 85°C water?

Chris April 8, 2011 at 7:41 am

Sorry if this is a dumb question, but what would you be cooking at that temp anyway? What temp are the bags you are cooking in rated? I homebrew and know most plastics want to start leaching chemicals after 160F (71.1C)

David April 8, 2011 at 3:41 pm

Vegetables need 85°C to cook. Bags designed for sous vide are safe and don’t leach at high temps.

Chris April 11, 2011 at 5:30 pm

With a little 12v dc power source in the box you could use something like this. http://cgi.ebay.com/Mini-Water-Coffee-Pump-24-GPH-P-25A-DC-12V-/250722722777?pt=Small_Kitchen_Appliances_US&hash=item3a603d23d9

Based on that I’m curious about the guts of a coffee pot. Do you still have the packaging on the immersion heaters? I’m trying to find out what the wattage is on one of those.

David April 11, 2011 at 6:25 pm

I saw that pump, but didn’t know how it would be hooked up to the rest f the components for power. The fountain pump runs off of 120V AC, so it can be wired together with the immersion heaters in the same circuit.

The immersion heaters are Norpro 559, rated at 300 watts.

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