When to Select a Limit, Versus an Alarm Controller
Watlow
In Watlow catalogs, brochures and web pages there is a category of controllers listed as "Limits and Alarms". It is not always clear to everyone when to use these controllers or which type of controller to use. This document is an attempt to clear up some of these issues.
Even though limits and alarm controllers are put together in our documentation, in terms of application, there is no relationship between a limit controller and an alarm controller. The primary purpose of each is completely different. They are together in the catalog simply to highlight the fact that they are not PID controllers. Now lets define what is each controllers purpose.
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Limit Controller
The primary purpose of a limit controller is to act as part of a redundant control system that prevents runaway conditions in the event the primary control system fails. Its job may be to prevent an over temperature condition, an under temperature condition, or sometimes both conditions. A controller designated as a limit controller means that it meets approval agency standards for a limit purpose. Underwriters Laboratories (UL) and Factory Mutual (FM) are two approval agencies that Limit controllers are typically submitted to for approval.
Circumstances such as the application, industry standards, regulations, and local laws will determine whether a Limit controller is needed as well as which agency approvals or features are required. In general, use a limit controller as part of a redundant control system any time a failure of the main control system could result in a hazardous condition. See Watlow Over/under temperature protection bulletin for more details.
In order to obtain agency approvals, additional restrictions on the controller are implemented that do not apply to PID controllers. These restrictions often make a limit controller unsuitable for alternate applications that might at first glance seem appropriate. Here are a couple of the most significant limitations imposed on limit controllers:
Circumstances such as the application, industry standards, regulations, and local laws will determine whether a Limit controller is needed as well as which agency approvals or features are required. In general, use a limit controller as part of a redundant control system any time a failure of the main control system could result in a hazardous condition. See Watlow Over/under temperature protection bulletin for more details.
In order to obtain agency approvals, additional restrictions on the controller are implemented that do not apply to PID controllers. These restrictions often make a limit controller unsuitable for alternate applications that might at first glance seem appropriate. Here are a couple of the most significant limitations imposed on limit controllers:
- Once the controller trips on a limit condition, it must stay tripped until the temperature has dropped back into the safe region and the reset button is pressed or power is removed from the limit. Unlike an alarm output, this output is always latching and cannot be programmed otherwise. This means this controller will not work for control, such as ON/OFF controller because once the output deactivates it does reactivate again. Limit controllers are available with "Auto Reset on Power Loss", but this refers to how the controller responds when it is initially powered up. It always must be manually reset to recover from its limit condition.
- The limit output must be a mechanical relay. It cannot be a solid state relay or a Switched DC control signal. To obtain a non-latching alarm function with a limit controller, order a secondary output, if available. For alarm only applications, the limits primary output would still have to be a mechanical relay output, which means extra money for an output that is not utilized. A better solution for a single loop alarm only controller would be a low cost PID controller with an alarm output option.
- The limit trip point is not readily available for casual adjustment by operators of the equipment. Lets say you do find an application where a controller that goes up to temperature and then shuts off until reset is appropriate. The next consideration about a limit controller, is that the set point is intentional made less convenient to access. This is because of the important job that limit controllers perform. Again a low cost PID controller might be a better solution for alarms. By using a deviation alarm, the trip points are readily adjustable.
Watlow's limit controllers that are presently available are; Series L, Series SD Limit, Series 94, Series 97, and the TLM multi-loop limit.
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Alarm Controller
Watlow manufactures one controller, the CAS200 alarm scanner that falls under the category of an alarm controller. An alarm controller is a controller that doesn't control a process but instead provides additional or auxiliary information about the process. It provides the information either through alarm outputs or a serial interface. Temperature measurements from various locations from within the process, or a signal that indicates to the main control system that a particular area of the system is within the desired temperature range are available. The CAS200 does not meet the special criteria of a limit controller that was listed above:
- CAS200 outputs are Switched DC signals used to control solid state relays. Limits require mechanical relay outputs.
- CAS200 Outputs can be programmed as latching or non-latching. Limits must have latching outputs.
The CAS200 is a 16-channel controller with up to 34 outputs available.
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When to Consider a Low Cost PID Controller?
There are many applications that one might try to fit to a Limit controller or an alarm controller that is best performed by a low cost PID controller. It more directly meets the application it at a lower cost. All current Watlow PID controllers have the capability to disable PID control and operate in an ON/OFF control mode with hysteresis. Available options for alarm outputs are flexible in programming. Here are some ideas for using a PID controller for some very basic applications:
- Product Protection from a cost standpoint. Its desired to stop a process if the temperature gets out of range because being out of temperature range increases the amount of reject product. However, to use a latched limit causes the system to shutdown unnecessarily for extended periods of time until someone can reset the system. A redundant controller using ON/OFF temperature controller or an alarm output will stop the process when it deviates out of range, yet allow it to recover automatically if temperature returns to normal. This system may also be in addition to the limit control system but is not considered part of a safety system. It would trip at temperatures that reduce the efficiency of the process, not at temperatures that are considered hazardous. (Caution: See Watlow's over/under temperature control bulletin.)
- Notification. Its desired to add the ability to trigger a light or warning buzzer to an existing system. Adding a PID controller with an alarm output to the system could perform this function. With the Series SD controller, the primary output can be configured as an alarm. Other controllers could use a secondary output for the alarm. (The primary output is ordered as a "No Charge" Switch DC output.) The alarm output would then allow utilization of such features as alarm silencing. Alarm silencing will allow the alarm to be disabled during startup, so that the alarm doesn't activate until process value is achieved and then deviates from a normal process range.
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