SEE710 Instrument and Process Control

Task:

Question 1:

Explain what is meant by:

I. Active instruments

II. Passive instruments

Give examples of each and discuss the relative merits of these two classes of measuring instruments.

Question 2:

Discuss the advantages and disadvantages of null and deflection types of measuring instrument. What are null types instrument mainly used for and why?

Question 3:

I. Explain the difference between accuracy and precision in an instrument.

II. A tungsten /5% rhenium- tungsten/26% rhenium thermocouple has an output e.m.f. as shown in the following table when it is hot at the temperature shown below. Determine the sensitivity of measurement for the thermocouple in mV/

Voltage (mV) Temperature (oC)

4.37 250

8.74 500

13.11 750

17.48 1000

Question 4:

The output of a potentiometer is measured by a voltmeter having a resistance , as shown in figure below. 

R is the resistance of the total length of the potentiometer and is the resistance between the wiper and common point for a general wiper position . Show that the measurement error due to the resistance of the measuring instrument is given by: .

Hence show that maximum error occurs when is approximately equal to .