Analog Loops

Question 1

What is a live zero offset and why do we use them?

Answer 1

When the LRV does not begin at zero.

• To distinguish between a zero signal and no signal.
• Pneumatics work better with an offset zero

Question 2

Are 2 wire TX’s powered by loop current?

Answer 2

No, they are powered by power supply and require a minimum voltage to operate. They regulate the loop current.

Question 3

What are the two types of signal transmission?

Answer 3

• Current

- Voltage.

Question 4

True/False.

When you have line resistance you want to have a high receiver resistance for transmitting voltage signal.

Answer 4

True, higher receiver resistance is required for accurate transmission of voltage signal. You are susceptible to noise!

Question 5

What are two styles of flow transmitters that require 4-wire design?

Answer 5

• Magnetic

- Mass

Question 6

What is the voltage measured at 4mA and 20mA on an I/P with an internal resistance of 176 ohms

Answer 6

V=IxR
.004Ax176ohms=.7V
.02Ax176ohms=3.52V

Question 7

What is Kirchhoff’s voltage law?

Answer 7

Sum of voltages around a loop must equal zero.

Meaning all supplied voltage is used somewhere in a loop.

Question 8

What is NAMUR NE43’s Hi/Lo fail alarms for current?

Answer 8

Hi: 22.5mA
Lo: 3.6mA