CHAPTER 3 TOOLS AND TEST EQUIPMENT

CHAPTER 3

TOOLS AND TEST EQUIPMENT1Learning Outcomesi.Understand measurement properties

ii.Understand Electrical measuring equipment

MeasurementMeasurement is the act of measuring physical quantities to obtain data that are transmitted to recording/display devices and/or to control devices.

Measuring physical quantitiesRecording the dataSource of Errors in MeasurementAccuracyResolutionRepeatabilityZero Error or Zero ShiftResponse TimeHysterisisLinearitySensibility or Scale Factor

AccuracyA descriptive term meaning how close the measured value of a quantity is to its actual value.

Accuracy is expressed usually as a maximum error.

For example, if a length of about 30 cm is measured with an ordinary wooden ruler then the error maybe up to 1 mm too high or too low.

This is quoted as an accuracy of 1mm.

This may also be expressed as a percentage which in this case would be 0.33%.

If your instrument measures in "1"sthen any value between6and7is measured as"7

If your instrument measures in "2"sthen any value between7and9is measured as"8

That the micrometer screw gauge is more accurate than the other measuring instruments.

ResolutionThe fineness with which a measurement can be made.

This must be distinguished from accuracy.

If a quality steel ruler were made to a very high standard but only had markings or graduations of one per centimetre it would have a low resolution even though the graduations were very accurate.

RepeatabilityThe closeness of agreement of the readings when a number of consecutive measurements are taken of a chosen value during full range traverses of the measuring.

If a 5 kg set of weighing scales was increased from zero to 5 kg in 1 kg steps a number of times, then the spread of readings is the repeatability.

It is often expressed as a percentage of full scale.

Zero Error or Zero ShiftThe displacement of a reading from zero when no reading should be apparent.

An analogue electrical test meter, for example, often has some form of adjustment to zero the needle.

1. A zero error arises when the measuring instrument does not start from exactly zero.2. Zero errors are consistently present in every reading of a measurement.3. The zero error can be positive or negative.

(NO ZERO ERROR: The pointer of the ammeter place on zero when no current flow through it.)

(NEGATIVE ZERO ERROR: The pointer of the ammeter does not place on zero but a negative value when no current flow through it.)

(POSITIVE ZERO ERROR: The pointer of the ammeter does not place on zero but a negative value when no current flow through it.)

Response TimeThe time taken by the output of a system to respond to a change in the input.

A system measuring engine oil pressure needs a faster response time than a fuel tank quantity system.

Errors in the output will be apparent if the measurement is taken quicker than the response time.

HysterisisThe difference between the indications of a measuring instrument when the same value of the measured quantity is reached by increasing or by decreasing that quantity.

Symptoms: The pointer does not return to zero when the load has been removedCauses: The presence of dry friction The properties of elastic elements (the presence of internal stresses)

Hysteresis Error:The difference in position between the loading and unloading curvesLinearityOutput from a measuring system can be linear or non-linear

Calibration of a linear system is easy. (Requires 2 or 3 points)

Calibration of a non-linear system is very cumbersome and time-consuming

Sensibility or Scale FactorA measure of the incremental change in output for a given change in the input quantity.

Sensitivity is quoted effectively as the slope of a graph in the linear region.

A figure of 0.1 V/ C for example, would indicate that a system would increase its output by 0.1V for every 1 C increase in temperature of the input.

Electrical Measuring EquipmentBasic Test MeterAn essential tool for working on vehicle electrical and electronic systems is a good digital multimeter.Digital meters are most suitable for accuracy of reading as well as their available facilities.A way of determining the quality of a meter as well as by the facilities provided is to consider the following:accuracy,loading effect of the meter,protection circuits.

The Use of MultimeterThe method shown here is a measurement of frequency, about 34.7 Hz at the cam or phase sensor (Figure 3.5).

Measuring temperature is easy if a simple probe is used. This should be touched to a metal component near where the measurement is required the head nest to the temperature sensor, for example. The engine shown here had only just been started (Figure 3.7).

Injectors can be tested by measuring resistance (often about 16) or by checking the duty cycle as a percentage. The injector here was tested at idle speed and had a duty cycle of just 0.7% i.e. the reverse of the reading shown (Figure 3.8).

Voltage tests can be carried out at any point in a circuit. The supply voltage to a component should not normally be less than 95% of the battery voltage.

A simple battery voltage test is shown here (engine running), showing the charging voltage at idle speed (Figure 3.9).

To measure current the circuit must either be broken and remade with the meter or an inductive clamp used. In this case a clamp is used showing the current flowing into the battery from the alternator.Note that the meter for these tests is often set to mV and that the sensitivity varies. (Figure 3.10).

Testing RPM involves either connecting to the coil negative terminal or using a clamp on a plug lead.

In this case the test is being carried out on a distributor less ignition system, which because of the lost spark and no distributor means the reading should be doubled (Figure 3.11).

OscilloscopeTwo types of oscilloscope are available, these are eitheranalogue or digital.

AnalogueAn analogue oscilloscope works by directly applying a voltage being measured to an electron beam moving across the oscilloscope screen.

Digital

A digital oscilloscope samples the waveform and uses an analog-to-digital converter (or ADC) to convert the voltage being measured into digital information.

Purpose of using OscilloscopeOscilloscopes are used in the sciences, medicine, engineering, and telecommunications industry.

General-purpose instruments are used for maintenance of electronic equipment and laboratory work.

Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system, or to display the waveform of the heartbeat as anelectrocardiogram.