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UNIVERSITY OF TECHNOLOGY, JAMAICA
SCHOOL OF ENGINEERING
Experiment # 7
IMPROVING THE RANGE OF AN INSTRUMENT
1. Objective:
The objective of this experiment is to introduce the student to the principle of improving the
range of the single function instrument, such as a voltmeter or an ammeter. This is sometimes
necessary and as an electrical technician you must be able to make the necessary adjustments in
your daily routine maintenance.
In this exercise students should learn how to conduct passive and active tests on a measuring
instrument and/or systems and components.
2. Apparatus:
A single function instrument
Multi-meter
Solder iron.
Insulated wire
Resistors
Solder sucker
Basic hand tools
100-200Ω variable resistor
12V DC supply
3. Theory:
A measuring instrument is a device for measuring a physical quantity. In the physical sciences,
quality assurance, and engineering, measurement is the activity of obtaining and comparing
physical quantities of real-world objects and events.
Established standard objects and events are used as units, and the process of measurement gives
a number relating the item under study and the referenced unit of measurement. Measuring
instruments, and formal test methods which define the instrument's use, are the means by which
these relations of numbers are obtained. All measuring instruments are subject to varying degrees
of instrument error and measurement uncertainty.
The type of instrument to be purchased for general use in the electro technical industries is a
difficult choice because there are so many different types on the market and every
manufacturer’s representative is convinced that his company’s product is the best.
However, most instruments can be broadly grouped under two general headings: those having
analogue and those with digital displays.
These meters have a pointer moving across a calibrated scale. They are the only choice when a
general trend or variation in value is to be observed. Hi-fi equipment often uses analogue
displays to indicate how power levels vary with time, which is more informative than a specific
value. Red or danger zones can be indicated on industrial instruments. The fuel gauge on a motor
car often indicates full, half full or danger on an analogue display which is much more
informative than an indication of the exact number of litres of petrol remaining in the tank.
These meters are only accurate when used in the calibrated position – usually horizontally. Most
meters using an analogue scale incorporate a mirror to eliminate parallax error.
Sometime it is necessary to change the range of a single function instrument, for example I have
in my possession a 2A Amp-meter but I need one to measure 10A. This instrument can be
modified so as to read accurately the 10A. In fact this is the principle used in designing multirange instruments.
4.0 PRE – LAB
1) Draw a labeled diagram of the Amp-meter and explain how it may be used for measuring
current in a load.
2) Explain how two meters may be used to measure power in a DC circuit, what are the
instruments and how are they connected in the circuit.
3) Explain the two possible connection of measuring resistance in a circuit using the volt
meter and an ammeter.
5.0 Procedure:
5.1
Conduct a passive test on the given single function instrument to determine the full scale
deflecting current and the internal resistance of the instrument.
5.2
Use the supply voltage, the variable resistor and the multi-meter to conduct an active on
the TEST instrument as shown below, to verify/establish the full scale current and the
resistance of the given instrument.
U2
+
5
0.000
V1
12 V
+
0.000
200Ω
5%
Key=A
3
DC 1e-009 W
4
R2
A
-
V
U1
DC 10M W
+
0.000
-
V
TEST
DC 10M W
1
5.3
Once the parameters of the TEST instrument is determined, the use this information to
design a multi-range volt meter to measure, 25V DC, 150VDC, and 200VDC.
5.4
Use the resistor values calculated to modify the TEST instrument.
5.5
Get circuit checked and then proceed to test and calibrate your design.
6. REPORT
Your report should include your pre-lab and final design, also explain any difficulty that arise
and how you went about solving the problem