Purchasing A Temperature Controller, Part 1
In previous articles, I have described controller features in the context of various processes and applications. Here, I will review some of the features and specifications of commonly used discrete panel-mounted controllers. A review of all makes and features is impossible here, so to supplement this column, my best advice is, extend your reading to catalogs, operation manuals, FAQs and web sites of the top manufacturers. Technology helplines are so overloaded that they become impenetrable and direct you to existing sources of help. Rightly so - but be prepared to quarry your way through some hard to read material. With product knowledge in your brain and an eye on your process you can make a sound and economical controller choice.
DimensionsThe most common controller sizes by bezel format are 96 x 96mm (1/4DIN), 96 x 48 mm (1/8DIN) 48 x 48 mm (1/16 DIN) and 48 x 24 mm (1/32 DIN).
DIN means Deutsches Institut fur Normung (German Institute for Standardization). It has pioneered many industrial standards, and those for rectangular panel instruments have gained worldwide adoption, aiding interchangeability and reducing panel fabrication costs.
While DIN is useful shorthand, one DIN (192 x 192 mm) can be sliced many different ways in increments of 12 mm, and non-square models can be oriented as landscape or portrait. I prefer to specify width and height.
Panel Cut-OutsPanel cut-outs conform to body width and height dimension and include a small clearance for insertion. Front-to-back depth dimension depends on the packing density of the internal electronics. Consider depth when you want a space-saving and cost effective enclosure.
Entry-Level ControllersFigure 1 shows the wiring of a simple entry level 48 x 24 mm controller. Its sensor is shown connected to the input terminals. Line voltage can be any value from 85 to 264 V. The controller's switching power supply self-adjusts to accommodate this range. This design avoids the winding burnouts often suffered by misconnections on dual tapped transformer power supplies.
Logic output is shown triggering a solid-state contactor with fast-cycling DC pulses. The contactor has internal diagnostics for load malfunction. A fault condition can be transmitted back on the logic wires to operate the relay provided that it has not been configured for cooling or some other alarm duty. It is common practice to wire the relay for shutdown or audible or visible alarm.
With a 2-wire RTD connection the temperature will read high by about 1 degrees C per 0.4 olhms of cable resistance. You can trim this out by using the offset adjustment.
The internal input filter reduces noise on input signals. Its effect is equivalent to a single RC low pass filter with the RC product expressed in seconds.
On entry-level instruments, it is common to have only one display and to bump the process temperature temporarily in order to display the parameters one by one as you are setting or observing them.
In my next column, I will continue to discuss controller specifications and process wiring of discrete panel-mounted controllers.