15 Loop Diagram Questions

Loop Diagram helps us to read the wiring connections from field instruments to system cabinet. Find the top 15 Loop Diagram Questions here.

Read: What are Loop DIagrams?

Examine this loop diagram, and answer the following questions:

Loop Diagram

Click on the image to zoom

Question 1:

What type of control loop is represented in this diagram? In other words, what is the process variable, and how is this process variable manipulated?

Question 2:

What is the calibrated range of the sensing instrument?

Question 3:

How are physical locations for wire connection points declared in this diagram?

Question 4:

Assuming the resistor inside the DCS input card is 250 ohms, calculate the amount of voltage between terminals 23 and 24 at a transmitter signal value of 50%.

Answer:  At a 50% signal (12 mA in a 4-20 mA range), the voltage dropped between terminals 23 and 24 will be 21 volts.

Question 5:

Identify where wires are part of a larger, multi-conductor cable, and identify how those wires are distinguished from all the others in that cable.

Question 6:

Identify the convention used to label wire pairs for each field instrument. In other words, how can a person tell whether a certain wire pair is going out to the transmitter, the indicator, or the control valve?

Question 7:

Identify at least two different ways you could measure the transmitter’s signal without interrupting the 4-20 mA current signal to the flow controller.

Question 8:

Explain why interrupting the loop’s continuity is a bad thing if the control system is operating, controlling a live process.

Question 9:

What do the letters “FI” and “FO” stand for? Are these labels ISA-standard?

Question 10:

Is FT-733 self-powered or loop-powered? How can you tell?

Question 11:

Sketch arrows showing the direction of electric current in each wire (using conventional flow notation), identifying each component as being either a source or a load.

Question 12:

Identify all the effects of pair 4 within cable ISOM-18 failing open and failing shorted.

Question 13:

Identify all the effects of cable FI-733 failing open.

Question 14:

Identify all the effects of pair 5 within cable ISOM-18 failing open and failing shorted.

Question 15:

When we use loop diagrams for troubleshooting? Explain with some examples.

Share your answers with us through comments.

Read Next:

• HART Communicator Analysis
• Instrumentation documents
• What are Instrument Errors?
• Configure a Smart Transmitter
• What is NAMUR NE 43?

Credits: Tony R. Kuphaldt