Thu Jan 21, 2010 5:45 am
There are lots of options for control. The simplest is to wire a diode in series with the element and put a switch across it. With the line switch off, the element is off. With the line switch on and the diode switch off (open) the heater is at half power and with both switches closed it is at full power.
Another approach is a Variac - a manually adjustable autotransformer which allows control of the power anywhere from 0 to about 110% of full.
As you mention a probe I assume you want automatic control which means an automatic controller. There are several types of these. The simplest (differential type) closes a switch when the temperature gets below a set point and then opens when it rises a certain number of degrees (that you can usually adjust) above that. These are fine for freezers and fermenters (where the action is reversed relative to the way I described it) but can't control any better than the width of the differential (deadband).
The most capable and generic controllers are of the PID (Proportional, Integral, Differential) variety and the more modern fuzzy logic ones. They hold temperature and respond to changes in set point and load rapidly without overshoot if tuned properly. Most controllers today are equipped with an autotune feature which is a plus in the sense that you don't have to know how to tune a controller and bad news in the sense that the autotune features don't always give the best "tune set" but you can usually arrive at a good tune set by tweaking the one the device comes up with. Setting up a PID controller is a bit of an art.
Another variable with PID control is the form of the output. This can be a continuous signal from 0 - 100% (either a 0 - 5 Vdc voltage or 4 - 20 ma current) going to a solid state device which controls the firing of SCRs (for 10% power they conduct for 1 half cycle out of 10, for 50% on every other half cycle etc.) These setups are expensive but with all the stuff on e-bay you might pick one up at reasonable cost. For the typical brewing application on-off (proportional) control should be OK. Here if the controller wants 10% power it sets a control signal (usually TTL logic level i.e. 5 VDC but the output may be just a pair of relay contacts) high for 10% of a cycle time e.g. 3 out of 30 seconds. For 20 percent it is high 6 seconds and off for 24 and so on. The control signal operates a relay (solid state is fine and they don't have contacts which can weld) and you will definitely need this unless your controller is equipped with a relay with contacts hefty enough to carry the load which is unlikely. Note that the continuous system is effectively the same except that the basic unit of condution is the 8.33 ms of a half cycle of the line. As a final note: most modern PID controllers will have a differential mode as well.
Then there is the matter of the sensor. First as to the placement - the sensor should be located at the point at which you want to control the temperature. At the outlet of the heater tube may seem an obvious choice but for time proportional control I would expect problems unless the flow rate is so fast that the rise through the heater tube is very small. A more sensible placement would seem to be where the flow enters the heater tube - similar to the concept of installing a house thermostat close to the air return duct inlet. Another possibility would be where the fluid leaves the reservoir (mash tun, I assume) or elsewhere in the reservoir itself.
Still on probe there is the question of the type. Most controllers accept RTD, thermister and thermocouple input. For brewing RTD's are, IMO, the most practical though certainly the other types can be made to serve.
As to where to get this stuff: sources abound. Cole Parmer, Omega Engineering, Watlow, Eurotherm, Cal..... As you probably don't want to be shelling out for new Ebay seems a good place to search. The problem you will face is a surfeit of choices. As this is obviously a fairly complex area I'd suggest imitating someone else's success. Give the readers additional detail on what you are trying to do. Someone on this forum probably has a similar system and may be able to suggest sources for components.
