Melting Chocolate Very Accurately

During my time in Sweden I decided to help out IYPT Team Sweden with building scientific apparatus to help with solving the problems set for the tournament in July 2014.

First a little background on IYPT: The International Young Physicist Tournament is a competition aimed at senior secondary school (aged 15 to 18) students. A set of physics problems is issued approximately 12 months before the tournament, students then have to conduct experiments and form some kind of understanding of the problem. The competition a debate in the form of a presentation, opposition, and review by three individual teams.

My next few blog posts will go into a little detail of some of the rigs that Felicia and I helped design and build for Sweden. The rigs have been designed and built to gather accurate experimental data from the results of experiments.

Melting Chocolate Very Very Accurately: And then watching it go solid again.
The name of this problem was "Chocolate Hysteresis" in that when chocolate melts at a certain temperature, it often doesn't solidify until it cools to a temperature noticeably lower than the melting point. That is a hysteresis in the melting temperature.

This problem proved to be kind of funky: The most difficult parts about it were measuring the temperature of the chocolate while it was being melted, and attempting to tell whether it was "solid" or "liquid"

Chocolate Rig V1:
The first attempt we built was a simple peltier heated and cooled calorimeter. The temperature was controlled using an Arduino and a motor shield took care of four quadrant driving the peltier.
The melting pot was machined from aluminium and the other side of the peltier was stuck to an ex CPU heatsink.

Melting pot sans insulation.

Melting pot with stirrer and insulation.
Rotating saddle rig in the background.

We measured the temperature of the chocolate itself by stirring it with a temperature probe. The temperature itself was measured with a DS18B20 temperature sensor embedded into the base of the aluminium calorimeter. Temperature control was done by a PID sketch running on the Arduino with a gui written in processing.

Processing GUI for the PID controller.

This setup didn't work all that well, it was a bit slow to melt and difficult to ascertain whether the chocolate had melted or not.
Around this time I had done some background reading on rotary viscometry as a possible means of determining whether the chocolate was liquid, solid, or somewhere in between.
Rotary viscometry works by measuring the shear force in a liquid by rotating a cylindrical or spherical object in it.

Chocolate Rig V2:
We then set about designing a new rig that would incorporate some kind of rotary viscometer and also allow the chocolate to be heated more quickly.
The idea was an aluminium tube mounted on a large ball bearing, this allowed the tube to move with minimal friction. The vessel containing the chocolate was mounted to a turntable allowing it to rotate.

Ball bearing and water tubes.

This rig was heated slightly differently: The aluminium tube or "probe" was filled with temperature controlled water... This proved a little easier said than done as nothing could touch the inside of the probe and there had to be temperature sensors inside it.

We heated the water using an element salvaged from a dead drip coffee machine, thanks to Bill Hammack for Inspiration. 

Does this element still work?

Element liberated from a dead coffee machine.

The element was controlled by a solid state relay driven by the same Arduino with slightly different PID code.

Coffee maker element
and Solid State Relay.

Arduino and Screwshield

The DS18B20 temperature sensor was cable tied to one of the water tubes, along with a Vernier Temperature Probe

Suction tube with temperature sensors.
Ball bearing and viscosity
probe in foreground.

Pump held in a clamp stand.

The water was circulated around the system by a small pump intended for supplying drinking water on boats. We never ran it on more than about 6v... Anything more and it was too powerful.

At the top of the probe where there had to be two hoses and two temperature sensors that did not touch anything. Space was getting a little tight. Lacking any other suitable materials to hand, I dusted off my glass blowing skills and made a custom angled glass nozzle to return the heated water to the probe.

Well dis ordered chaos.

Around about this time everything was cobbled together with a huge number of lab clamp stands and duct tape.

The shear force is measured in the chocolate by wrapping a string around the probe which is then connected to a Vernier Force Sensor (called a load cell in my profession) 

Force Sensor connected to the shear probe.

This whole design centred around having the chocolate container rotating, for both stirring the chocolate as it melted and to allow some kind of rotary viscometry. I found in the collection, a very nice gearhead motor with a drill chuck. This was pressed into service to rotate the chocolate container and all its insulation.

Motor and rotary motion
sensor assembly.

In order to have some idea of what the viscosity of the chocolate is, the angular velocity of the chocolate container was also measured. This was done with a Vernier Rotary Motion Sensor belt driven from a pulley behind the drill chuck on the motor.
All of the sensors were connected to a Vernier LabQuest Mini to allow data collection in Logger Pro.

The data gathering brain of the operation.

After all that work this creation still didn't work very well... As soon as the chocolate stated to melt it was pushed away from the probe and re solidified.

Chocolate Rig V3
The failure of V2 brought about the design of V3 pretty quickly. The main change was to make the waterbath on the outside of the chocolate container. Easier said than done as everything is spinning!

The video below should make things a little clearer. We made a melting container and waterbath out of acrylic so you can see what is going on inside.



The layout was more or less the same as V2 however the viscosity measuring probe was changed to plastic in order to reduce heat losses.
We made a new water bath and melting pot assembly out of a plastic bowl and an empty tin can.

Mmm Chocolatey.

The tin can was simply glued to the bottom of the plastic bowl with silicone aquarium adhesive.
This version of the machine was finally able to melt chocolate in a reasonable time period and we were able to start gathering some sensible data. Overview of the machine running below:

Circuit diagram of the water circulation system.

The general procedure for gathering data went something along the lines of heat the chocolate to whatever temperature was desired and then cool it again. The cooling rate could be controlled by adding ice to a water bath around a length of the tubing.

Ice bath for rapid
cooling of the chocolate.

The cat decided that the ice water was very interesting.

The data recorded by logger pro proved to be quite interesting. One can see the viscosity of the chocolate increase as it transitions through a liquid to solid change.

Below shows a typical graph of data, traces are as follows:
Red = Water bath temperature.
Orange = Chocolate temperature. (not always in the chocolate)
Blue = Shear force in the chocolate when rotating.
Green= Angular velocity of the chocolate container.

There are some pretty interesting kinks in the chocolate temperature graph as it solidifies... These only seem to appear when the chocolate is being stirred. When time permits I would like to spend some more time investigating this phenomenon.

References:

Rotary viscometer: http://www.pavementinteractive.org/article/rotational-viscometer/
Heston Blumenthal on Chocolate: https://www.youtube.com/watch?v=ovr1yXqIX8Q
PID control with Arduino: http://playground.arduino.cc/Code/PIDLibrary

Available Vernier sensors: http://www.vernier.com/products/sensors/