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Ignition is only a SCADA software package that acquires its data from I/O using OPC. I assume that there is some type of PLC or remote I/O multiplexer somewhere. It would be helpful if the Analog Input cards of the I/O were enabled for HART. Unless this is the case, then the HART data is not...

The hot/cold water mixing control loop is a classic example of two highly interacting control loops. It is difficult and nearly impossible to tune, even when using two independent PID controllers. You are mixing two streams that have variable temperature and variable flow rates. Whenever you...

I teach this in my CAP exam preparation course. Theoretically, you need proportional-only control and no reset. Practically, most users prefer just a little Integral Control to reduce the offset. It may not eliminate it. Too much integral causes the unstable oscillation that you have seen. You...

You are experiencing a common problem in level control. Level is an integrating process, and you are seeing the problem of using an integral time constant that is not zero or close to zero. Do not add integrating control to an integrating process. You need proportional-only control for level...

The intrinsic safety barrier has a current limiting resistor that imposes significant resistance to the 4-20ma loop. I found an excellent discussion in this on-line article: <b>www.omega.com/temperature/Z/pdf/z131-148.pdf</b> You will probably not have 24v appear at the transmitter; only...

All AC Variable Frequency Drives (VFD) have an electrical output that is a variable frequency and voltage that when passed through the coils of an electric motor "simulate" a sine wave. If you try to measure this with an AC VOM, you will not get a proper RMS measurement. The drive itself is a...

The HART data is a digital FSK signal that you will find on pins of the AD5700 chip. You do not need to worry about separating the analog signal from the HART FSK - that is the job of the AD5700 chip. In order to route this signal to your DAC, you should consult the wiring diagram on the AD5700...

Remote operated valves are great and should be used when necessary for remote operation of a process. However, hand operated shutoff or blocking valves are required for maintenance to prevent exposure of maintenance personnel to the process fluid while they have removed a piece of equipment...

The SMAR converter previously mentioned is a 3-channel device. It takes up to 3 different 4-20mA analog signals and converts them to Foundation Fieldbus H1. Each channel can be configured with all of the function blocks appropriate for an AI device. The AI block takes care of linearization...

> Can i say an i/p transducer to positioner? No, you cannot equate an I/P to a positioner. An I/P converter is a very simple device that converts a 4-20mA signal to a 3-15psi air signal. It is often used with pneumatic control valves but it has no logic or feedback. You can say that you "use...

I see that a history lesson is required. I include this history when I teach the ISA CAP Exam Preparation course, EC00. The RESET term and its definition as REPEATS comes from the fact that the basic Proportional Control equation is expressed like this: M = Kp * error + bias error is...

I agree that a typical process control network has both time critical and non-time critical requirements. However, we always knew that and most process control network protocols can handle both types of transmissions quite well. For example, Foundation Fieldbus both H1 and HSE handle...

As long ago as the original 1990's when the ISA SP50 standards committee was writing the User Layer that became the basis for Foundation Fieldbus function blocks, we identified "attributes" that became Device Descriptions. We assumed that these would be read-only data items and always stored on...

The flame detector has an ultraviolet light sensor that is activated when it "sees" flame. This causes the detector to appear as a closed circuit when sensed by the PLC I/O. There is a classic problem in the industry that tempts boiler operators to bypass (short circuit across the terminals)...

The Digital Input card for this type of fault-detecting contact input is really a low accuracy A/D converter. It supplies typically 5-12volts DC power to the contact. There is a known resistor placed across the field contact switch, and another across the terminals of the DI card. The current...