PID vs normal roaster?

[travieso2989]

Hello, I'm considering buying an electric roaster. I've found one that has a 2 lb capacity with PID for roughly the same price as another that holds 5 lbs, but does not have PID. Is PID that great? Does it allow for more precise roasting?

 

[peterjschmidt]

The PID can take control of the roast, but in my world, I'd rather learn how to roast and do the controlling myself. If you want a roaster that you can set and forget, or have inexperienced people operating the machine, the a PID might be something to consider.

The question is, what will the PID be controlling? Only heat? In that case you'll still have to control airflow, which makes it less desirable since you'll still have to attend the roast.

 

[MillCityRoasters]

The short answer is no. Most temperature controllers are PID, but when installed in coffee roasters, most are set for on/off control only. This means you might have a PID controller, but it's used as a high limit controller only.

Beyond that, I'd need to know which roasters you are comparing.

 

[ellatas]

It depends on how you roast. When comparing gas vs electric, just like a stove top, gas has the potential for more "control-ability" in that it can turn on and off quicker than an electric element. There are faster reacting electric elements, but they can be brittle and less robust. That said, for control-ability on the gas, you will need a burner with high turndown ratio and a modulating valve on the gas train. The modulating valve can be adjusted manually (as per Peter's comments) or via a PLC.
For control on the PLC, an effective "profile roasting" system does not need to utilize a PID control. Rather the PLC control can be considered a "step" control; that is when the coffee temperature reaches various temperature milestones, the burner is adjusted to levels for that particular roast profile recipe. This creates a profile roast control that repeatable and follows time temperature curves quite accurately. The airflow and drum speed is kept constant.
A more sophisticated control is "curve following". This is a PID control that looks at the error between the actual bean temperature and the constantly moving set temperature as it moves along the curve. For smaller roasters, it still only modulates the burner and the airflow and drum speeds are kept constant. There are cost effective options available with this control, but the benefits over the "step" control are small and depend on your own perceptions on quality.
On larger recirculating roasters, PID control can be used on the burner output, the fan speed, and the drum pressure. As discussed in other posts, roasting coffee on most drum roasters is primarily done through convection. Therefore, adjusting the air temperature entering the drum as well as the volume of air impacts the heat applied to the coffee. However, making adjustments on more than one variable at the same time can get complicated very quickly. Consequently, it is best to limit the adjusting to the burner output (to affect the inlet air temperature) and keep the airflow constant...or at specific settings throughout the roast.

 

[MadMaximal]

Any PID system is fairly easy in setup and programming. The tuning is where the magic happens and it is the difference between an incredible system and a shitty one.


Pid works like this, it consists of 3 factors, proportional, integral and derivative controls. Proportional is the difference between the setpoint and the current value (error) multiplied by a factor (tuning).


Integral is the history of the error, this builds up if even a small error exists. This removes the steady state error.


The derivative is the change of the error over time, and tuning this prevents over- or undershooting.
A friend of mine is using this PID controller.