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ETM TERM ASSIGNMENT
Q1) Discuss accuracy class indicated on analogue instruments
Accuracy classes are defined and used in IEC and ANSI standards. These classes
are denoted by either a letter or percentage. For example, class B is a temperature accuracy from IEC-751 that requires accuracy of +-0.15 degree Celsius. Class 0. is an ANSI C12.20 accuracy class for electric meters with absolute accuracy better than +-0.5% of the nominal full-scale reading.
Typically, a class specifies accuracy at a number of points, with the absolute accuracy at lower values being better than the nominal “percentage of full scale” accuracy.
Accuracy classes such as IEC’s 0.15s are special high accuracy class. According to IEC standards, with the following classes pf accuracy: 0.05 - 0.1 - 0.2 - 0.3 - 0.4 - 0.5 - 1- 1.5 - 2.5 – 5. This means, that an analogue instrument of class 1 shows relative percentage error within +-1% for all values in the scale. The class parameter is very important in order to identify the scope of the instrument, for example:
- Classes 0.05-0.1 instruments: they are used as standard equipment in the laboratory
- classes 0.2-0.3-0.5: they are used as equipment in laboratory
- classes 1-1.5: they are used to control systems and to calibrate the panel instrument
- classes 2.5-5: they are used with panel instruments
Values 2.5 and 1.5 showing the accuracy class
Q2) Based on your observations, enlist, classify and write a short note on various analogue meters available on the dashboard of your vehicle Some of the commonly found Analog meters available on the dashboard of a vehicle are:
- (^) Odometer: It is classified as integrating type of instrument An odometer, also known as a milometer, is a noun derived from Greek words hodos(path) and metron(measure). It is an instrument used to measure the distance travelled by the vehicle, such as a bike or car. The device can be electrical or mechanical or a combination of both. Working: ▲ Mechanical odometers are turned by a flexible cable made from a tightly wound spring. ▲ On a bicycle, a little wheel rolling against the bike wheel turns the cable, and the gear ratio on the odometer has to be calibrated to the size of this small wheel. ▲ On a car, a gear engages the output shaft of the transmission, turning the cable. ▲ The cable snakes its way up to the instrument panel, where it is connected to the input shaft of the odometer. Accuracy: Most of the odometers work by counting wheel rotations assume that the distance travelled is the number of rotation times the circumference of the wheel(2pir). If nonstandard or underinflated or worn out tyres are used there can be errors in the readings of odometer. DISTANCE TRAVEL= (FINAL READING -INITIAL READING)(ACTUAL DIAMETER) (STANDARD TIRE DIAMETER)*
It is common for odometers to be off by several percent which is typically proportional to speedometer error.
- Speedometer: It is classified as indicating instrument
An image of speedometer along with odometer
An open thermostat can cause the reading on the dial to show cold leading to failure of engine when overheating takes place.
- Fuel gauge: It is classified as indicating instrument. In automobile and aircrafts engineering a fuel gauge or gas gauge is an instrument used to indicate the amount of fuel in fuel tank.
As used in vehicle the gauge consists of two parts:
- (^) the sensing unit – in the tank
- the indicator – on the dashboard Working: ▲ The sending unit is located in the fuel of the car. It consists of a float, usually made of foam, connected to a thin, metal rod. ▲ The end of the rod is mounted to a variable resistor. The more resistance there is, the less current will flow. ▲ In a fuel tank, the variable resistor consists of a strip of resistive material connected on one side to the ground. ▲ A wiper connected to the gauge slides along this strip of material, conducting the current from the gauge to the resistor. If the wiper is close to the grounded side of the strip, there is less resistive material ▲ in the path of the current, so the resistance is small. ▲ If the wiper is at the other end of the strip, there is more resistive material in the current's path, so the resistance is large. ▲ When the float is near the top of the tank, the wiper on the variable resistor rests close to the grounded (negative) side, which means that the resistance is small and a relatively large amount of current passes through the sending unit
an image of temperature and fuel gauge F: full tank E: empty tank H: high temp C: low temperature
back to the fuel gauge. As the level in the tank drops, the float sinks, the wiper moves, the resistance increases and the amount of current sent back to the gauge decreases. ERRORS: If an electrical fault opens the circuit shows the tank is empty rather than full. Corrosion or wear of potentiometer can also show erroneous reading of the fuel level. Sending current through a tank which is in contact of the fuel can cause a fire hazard. Also, these resistance sensors show increased failure due to addition of alcohol in fuel tanks which increases the corrosion rate of potentiometer.
- (^) Tachometer: It is classified as indicating instrument. A tachometer is an instrument measuring the rotation speed of the shaft or disk as in a motor or other machine. The device usually displays the revolution per minute on a calibrated analogue dial. Essentially both speedometer and a tachometer are used for measuring speeds but the initial is one is used for the measurement of the speed of the vehicle and later for the speed of engine. Working ▲ Tachometers depended on a geared rotating mechanical drive taken from some moving part of the engine, such as the flywheel, camshaft, fan pulley, etc. ▲ The drive rotated a magnet, thus inducing eddy currents in an aluminum disc, much like a speedometer, but the scale was marked in rpm.
A tachometer showing the revolution per min
