It is important to be knowledgeable about modern instruments before you invest both time and money in your next discrete controller.

Manufacturers of DIN size (1/4, 1/8, 1/16, etc.) discrete temperature and process control instruments will all argue that they offer great products with unmatched accuracy at a fair purchase price. And, due to the large number of instrument suppliers from all corners of the world, there are literally hundreds of different products available in a variety of sizes. For this reason, it is important to be knowledgeable about modern instruments before you invest both time and money in your next discrete controller.

Start by asking three questions:
•How accurate do you need your controller to be — today and into the future?

•Which features are required? Is on/off temperature control good enough, or does your application require single or multiple PID sets, autotune or even extension into PLC-type functions?

•What is needed in terms of ease of installation, commissioning and operation of the instrument?
Manufacturers have tried to resist the commoditization of the controller industry by continuously developing new technologies such as smart and fuzzy logic algorithms along with many different feature sets. Are customers on board with this? Unfortunately, the answer is typically no, so confusion still reigns. With so many controller options, it is time consuming for controls users to rank products against each other and to understand the value/price of the various features.

Sometimes, it really is easier to just replace like for like (same manufacturer, same product) or use the natural replacement for the model from the current manufacturer. However, this is not always a successful strategy, and the incorrect choice may not provide the accuracy required by today’s standards.

Is the value of a-fraction-of-a-degree accuracy understood, required and worth the expense of possibly a higher priced, more accurate controller? To answer this question, it is useful to discuss current heat treatment industry pyrometry control specifications. After an allowable period for grandfathering in existing instrumentation, standards such as AMS2750D from SAE International actually force the oven or furnace user to look at updating their control systems to meet the prescribed standard. (Headquartered in Warrendale, Pa., SAE International is a global association of 121,000 engineers and related technical experts in the aerospace, automotive and commercial-vehicle industries. One of SAE International’s core competencies is voluntary consensus standards development.) Reliable accuracy is not a guarantee in 10- to 15-year-old devices that may have operated up to 24 hours a day, year round. The standard is intended to help ensure that parts or raw materials are heat treated in accordance with the applicable specifications to ensure proper heat treatment.

Repeatable accuracy is required, but equally important is the ability of the oven or furnace to control at setpoint with minimal or zero overshoot when ramping and minimal or no fluctuation during time at the desired temperature.

So what is the published accuracy of modern day controllers, and how does this compare with the requirements for AMS2750D? Manufacturers of instrumentation publish the accuracy of their control devices and will have typical ranges from ±0.5 to ±0.25 percent. Elite manufacturers are able to hit a tighter tolerance of ±0.1 percent or better in their high-end controllers. These higher accuracy units are required for processing at temperatures greater than 800?F (426°C) to meet the requirements of the AMS2750D specification. However, that is not the end of the story. Repeatable accuracy is required, but equally important is the ability of the oven or furnace to control at setpoint with minimal or zero overshoot when ramping and minimal or no fluctuation during time at the desired temperature. This accuracy and stability is truly put the test during Temperature Uniformity Surveys (TUS), an ongoing requirement of the AMS2750D specification.

Typically most high-end controllers have a form of autotune for setting PID constants, which means that the better the algorithm, the less manual intervention is needed to aid in getting the tuning spot-on. The manufacturers that have a handle on filtering noise also benefit from less erratic autotune results, and elite manufacturers take the lead in this area.

After taking the time to see a demonstration of the value of precision controllers, you may be surprised that the difference in price point between precision controllers and standard controllers can generally be absorbed by the savings from eliminating unnecessary setup and commissioning time.

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