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oldengrcook



Joined: 08 Jul 2015
Posts: 2
Location: Escondido, CA

PostPosted: Wed Jul 08, 2015 5:53 pm    Post subject: Review Reply with quote

Performance limitations of my new Max Burton Model 6400 induction hob (portable hot plate).

These differ from a resistive hot plate by developing the heat in the outer skin of the pot by inducing eddy currents and hysteresis loss in magnetic material with fat B-H curves. They only work with pots that have magnetic properties (high permeability and a lossy hysteresis loop). The ideal material is 18-0 Stainless steel, but they will also work with cast iron. The device rectifies the 60 Hz power from a dedicated 120v 15a circuit breaker protected receptacle. It uses power switches to excite a large induction coil wound with Litz wire. This forms a transformer primary winding. The secondary winding is a loosely coupled shorted turn that is described above. In the absence of a load (pot) a sensor removes the field.. It also displays an error message. There is a 15 second grace period to allow picking up a pan to swirl hot oil over the bottom and sides. After the grace period, it shuts down. It has three modes of operation: Preset power level (one of ten settings which vary the field duty cycle, using the thermal capacitance of the pan bottom and the water content of the food to filter out temperature ripples).

The second mode is closed loop temperature control. The user guide says it senses the temperature of the pan bottom. This is incorrect. It senses the temperature of the bottom of the ceramic disc that supports the pan. This separates the sensor from the heat source by an equal thermal capacitance. However the thermal resistance of the ceramic disc is about 150 times greater than aluminum. Letís be generous and say 100 X greater than the laminated bottom containing two thin sheets off SS on either side of the aluminum disc. Thermal time constant is the product of capacitance and resistance. Hence, the sensor lags the temperature of the cooking surface by a few minutes. Lag is proportional to time constant and rate of change of the heat source. Therein lies the problem. The second and third omelets come out quite well, but the first one is too dry and brown. The cooking surface will rise to over 400F before the loop starts to respond. The user guide warns against applying power to a utensil without food or water. If their closed loop control was properly designed, that warning would not be necessary.

The third mode adds a user set timer.

I bought the unit because Burton was the top pick in a test comparison at Americaís Test Kitchen. The disadvantage of using gas to saute and fry is that a gas burner heats the stainless steel sides of the pan. By means of a rising column of very hot combustion products. They scorch because SS sheet metal has low thermal capacitance and high longitudinal resistivity. Oils vaporize and then condense on the sides where they pyrolize and polymerize with the metal. They can only be removed with a strong solution of Na OH.

I also had visions of using it as a low cost sous vide cooker, where I could fill a pan of water and put in a sealed and evacuated pouch of frozen salmon filet and have it remain at a perfect temperature of 135F, followed by a very brief broil to caramelize surface oils which oxidize to generate the signature flavor of broiled salmon, leaving the flesh at melt in your mouth juicy texture. Knowing the idiosyncrasies of its design, I can probably make this happen, but it wonít be fool proof, requiring manual intervention and use of a large water bath.

Cooking appliances like this are in a highly competitive marketplace. Typically, they are designed by manufacturing engineers, as opposed to product engineer. The former are experts at lowering manufacturing costs, and design to that objective. Overlooking the incidental effects of design simplifications, like placing the sensor against the undersurface of the ceramic plate, is a typical result. True product designers start with a goal of specified temperature control maximum acceptable error, including the initialization phase. They then model a candidate design and compare the result to the requirement. Computer modeling programs are available to do this, However, in this case it is straightforward, and the large lag resulting in total loss of control during warm up is easily predictable.

The solution does add cost to the product. A thermocouple with a low mass surface face in a spring loaded carrier would protrude lightly above the top of the ceramic disc through a clearance hole is one way. Sealing the sliding mount against spills would have to be considered. Along with this hardware change, the current software does not appear to use rate of temperature change to stabilize the control loop. If the designers lack familiarity with control systems algorithmic design, temporary consultants are readily available to do this for them.

I suspect that there is a lot of similarity in present competing designs, and few, if any, were designed to control temperature from the surface of the pot. Their user admonishment to never turn it on empty is a workaround, but it eliminates some uses a creative cook could otherwise do, and burdens the cook to pay close attention to the process when a properly designed product would take care of holding temperature freeing hi or her up to do other tasks, when facing high pressure work in the kitchen.

One other serious problem exists. There is a button marked simmer, which is a default to 100 degrees F. How could this be related to simmering? This is an invitation to violate well established food safety practices and regulations. It would be only good for dough proofing, but that requires accurate temperature control, which is not there. This button implies the designers are not familiar with food preparation, and that is a red flag. They have not hears of the food safety forbidden zone of 40F to 140F.
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Dilbert



Joined: 19 Oct 2007
Posts: 1067
Location: central PA

PostPosted: Wed Jul 08, 2015 10:55 pm    Post subject: Reply with quote

interesting observations.

the thing I find most misleading is the whole idea one can set the temperature the burner/the pot to the temperature one wants the contents to maintain. what is in the pot, the heat transfer from the pot to the contents, how well/rapidly/evenly the heat is distributed throughout the contents, lid on, lid off, how high up the sides the pot heats, etc etc yadda yadda and yadda - a very very long list of variables unaccounted for - not even considered - except for the marketing hype of "turn it to dial setting for the perfect . . . "

in the end, it's all empirical.

I've read a number of people who thought it would make for a neat sous vide - the theory apparently does not stand up. those folk be prone to debate degrees by the tenth - out of reality for sorta' temperature sensing induction units.
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oldengrcook



Joined: 08 Jul 2015
Posts: 2
Location: Escondido, CA

PostPosted: Fri Jul 10, 2015 3:14 am    Post subject: Reply with quote

It is not all empirical. This very bad lag in closed loop response is very clearly caused by improperly locating the temperature sensor. If it was in contact with the bottom of the pot, and it the software followed PID algorithmic practices of control theory, it would work very well. The other factors you mention would all be swamped out by negative feedback.
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Dilbert



Joined: 19 Oct 2007
Posts: 1067
Location: central PA

PostPosted: Fri Jul 10, 2015 12:44 pm    Post subject: Reply with quote

except that it is measuring the temperature of the pot, not the contents.
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