Measuring Instruments--part 2

Reading a Meter

Learning to read the scale of a multimeter takes time and practice. Most people use meters every day without thinking about it. A common type of meter used daily by most people is shown below. The meter illustrated is a speedometer similar to those seen in automobiles. This meter is designed to measure speed. It’s calibrated in miles per hour (mph). The speedometer has a full-scale value of 80 miles per hour. If the pointer is positioned, most people would know instantly that the speed of the automobile is 55 miles per hour.

+++++9 illustrates another common meter used by most people. This meter is used to measure the amount of fuel in the tank of the automobile.

Most people can glance at the pointer of the meter and know that the meter is indicating that there is one quarter of a tank of fuel remaining. Now assume that the tank has a capacity of 20 gallons. The meter is indicating that = gallons of fuel remain in the tank.

+++++10 Typical multimeter scale.

Learning to read the scale of a multimeter is similar to learning to read a speedometer or fuel gauge. The meter scale has several scales used to measure different quantities and values. The top of the scale is used to measure resistance, or ohms. Notice that the scale begins on the left at infinity and ends at zero on the right. Ohmmeters are covered later in this unit.

The second scale is labeled AC-DC and is used to measure voltage. Notice that this scale has three different full-scale values. The top scale is 0-300, the second scale is 0-60, and the third scale is 0-12. The scale used is determined by the set ting of the range control switch. The third set of scales is labeled "AC amperes." This scale is used with a clamp-on ammeter attachment that can be used with some meters. The last scale is labeled "dBm," which is used to measure decibels.

+++++11 The meter indicates a value of 250.

Reading a Voltmeter:

Notice that the three voltmeter scales use the primary numbers 3, 6, and 12 and are in multiples of 10 of these numbers. Because the numbers are in multiples of 10, it’s easy to multiply or divide the readings in your head by moving a decimal point. Remember that any number can be multiplied by 10 by moving the decimal point one place to the right, and any number can be divided by 10 by moving the decimal point one place to the left. For example, if the selector switch were set to permit the meter to indicate a voltage of 3 volts full scale, the 300-volt scale would be used, and the reading would be divided by 100. The reading can be divided by 100 by moving the decimal point two places to the left. ---, the pointer is indicating a value of 250. If the selector switch is set for 3 volts full scale, moving the decimal point two places to the left will give a reading of 2.5 volts. If the selector switch were set for a full-scale value of 30 volts, the meter would be indicating a value of 25 volts. That reading is obtained by dividing the scale by 10 and moving the decimal point one place to the left.

Now assume that the meter has been set to have a full-scale value of 600 volts. The pointer is indicating a value of 44. Because the full-scale value of the meter is set for 600 volts, use the 60-volt range and multiply the reading on the meter by 10 by moving the decimal point one place to the right. The correct reading becomes 440 volts.

Three distinct steps should be followed when reading a meter. These steps are especially helpful for someone who has not had a great deal of experience reading a multimeter. The steps are as follows:

1. Determine what the meter indicates. Is the meter set to read a value of DC voltage, DC current, AC voltage, AC current, or ohms? It’s impossible to read a meter if you don’t know what the meter is used to measure.

2. Determine the full-scale value of the meter. The advantage of a multimeter is that it can measure a wide range of values and quantities. After it has been determined what quantity the meter is set to measure, it must then be determined what the range of the meter is.

There is a great deal of difference in reading when the meter is set to indicate a value of 600 volts full scale and when it’s set for 30 volts full scale.

3. Read the meter. The last step is to determine what the meter is indicating. It may be necessary to determine the value of the hash marks on the meter face for the range for which the selector switch is set. If the meter is set for 300 volts full scale, each hash mark has a value of = volts. If the full-scale value of the meter is 60 volts, however, each hash mark has a value of 1 volt.

+++++13 An ammeter connects in series with the load.

+++++14 In-line ammeter.

The Ammeter

An ammeter has a typical impedance of less than 0.1 ohm. If this meter is connected in parallel with the power supply, the impedance of the ammeter is the only thing to limit the amount of current flow in the circuit. Assume that an am meter with a resistance of 0.1 ohm is connected across a 240-volt AC line. The current flow in this circuit would be 2400 amperes (240 V/0.1 ohm = 2400 A). A blinding l ash of light would be followed by the destruction of the ammeter.

Ammeters, connected directly into the circuit, are referred to as in-line ammeters.

Ammeter Shunts

DC ammeters are constructed by connecting a common moving-coil type of meter across a shunt. An ammeter shunt is a low-resistance device used to conduct most of the circuit current away from the meter movement. Because the meter movement is connected in parallel with the shunt, the voltage drop across the shunt is the voltage applied to the meter. Most ammeter shunts are manufactured to have a voltage drop of 50 millivolts (mv). If a 50-millivolt meter movement is connected across the shunt, the pointer will move to the full-scale value when the rated current of the shunt is flowing. In the example, the ammeter shunt is rated to have a 50-millivolt drop when a 10-ampere current is flowing in the circuit. Because the meter movement has a full-scale voltage of 50 millivolts, it will indicate the full-scale value when 10 amperes of current are flowing through the shunt.

+++++15 A shunt is used to set the value of the ammeter. Load Shunt; Battery; DC ammeter

+++++16 Ammeter shunt.

Ammeter shunts can be purchased to indicate different values. If the same 50-millivolt movement is connected across a shunt designed to drop 50 millivolts when 100 amperes of current flow through it, the meter will have a full-scale value of 100 amperes.

The resistance of an ammeter shunt can be calculated using Ohm's law.

The resistance of a shunt designed to have a voltage drop of 50 milli-ohm.

In the preceding problem, no consideration was given to the electrical values of the meter movement. The reason is that the amount of current needed to operate the meter movement is so small compared with the 100-ampere circuit current it could have no meaningful effect on the resistance value of the shunt. When calculating the value for a low-current shunt, however, the meter values must be taken into consideration. For example, assume the meter has a voltage drop of 50 millivolts (0.050 V) and requires a current of 1 milliampere (0.001A) to deflect the meter full scale. Using Ohm's law, it can be found that the meter has an internal resistance of 50 ohms (0.050 V/0.001 A = 50 ohm). Now assume that a shunt is to be constructed that will permit the meter to have a full-scale value of 10 milliamperes. If a total of 10 milliamperes is to flow through the circuit and 1 milliampere must flow through the meter, then 9 milliamperes must flow through the shunt. Because the shunt must have a voltage drop of 50 millivolts when 9 milliamperes of current are flowing through it, its resistance must be 5.555 ohms (0.050 V/0.009A = 5.555 ohm).

+++++18 A make-before-break switch is used to change meter shunts. DC ammeter Shunt; Make-before-break switch.

+++++17 The total current is divided between the meter and the shunt. DC ammeter.

Multirange Ammeters

Many ammeters, called multirange ammeters, are designed to operate on more than one range. This is done by connecting the meter movement to different shunts. When a multirange meter is used, care must be taken that the shunt is never disconnected from the meter. Disconnection would cause the meter movement to be inserted in series with the circuit, and full- circuit current would flow through the meter. Two basic methods are used for connecting shunts to a meter movement. One method is to use a make before-break switch. This type of switch is designed so that it will make contact with the next shunt before it breaks connection with the shunt to which it’s connected. This method does, however, present a problem- contact resistance. Notice that the rotary switch is in series with the shunt resistors. This arrangement causes the contact resistance to be added to the shunt resistance and can cause inaccuracy in the meter reading.

The Ayrton Shunt

The second method of connecting a shunt to a meter movement is to use an Ayrton shunt. In this type of circuit, connection is made to different parts of the shunt, and the meter movement is never disconnected from the shunt. Also notice that the switch connections are made external to the shunt and meter. This arrangement prevents contact resistance from affecting the accuracy of the meter.

+++++19 An Ayrton shunt. DC ammeter

Calculating the Resistor Values for an Ayrton Shunt:

When an Ayrton shunt is used, the resistors are connected in parallel with the meter on some ranges and in series with the meter for other ranges. In this ex ample, the meter movement has full-scale values of 50 millivolts, 1 milliampere, and 50 ohms of resistance. The shunt will permit the meter to have full-scale current values of 100 milliamperes, 500 milliamperes, and 1 ampere.

To find the resistor values, first calculate the resistance of the shunt when the range switch is set to permit a full-scale current of 100 milliampere. When the range switch is set in this position, all three shunt resistors are connected in series across the meter movement. The formula for finding this resistance is, where:

Rs = resistance of the shunt

Im = current of the meter movement

Rm = resistance of the meter movement

I = total circuit current

Rs = 0.5 ohm

+++++20 The meter is in parallel with all shunt resistors.

Next, find the resistance of RSH1, which is the shunt resistor used to produce a full-scale current of 1 ampere. When the selector switch is set in this position, RSH1 is connected in parallel with the meter and with RSH2 and RSH3. RSH2 and RSH3, however, are connected in series with the meter movement. To calculate the value of this resistor, a variation of the previous formula is used.

The new formula is, where:

RSH1 = the resistance of shunt 1

I_m = current of the meter movement

RSUM = the sum of all the resistance in the circuit.

Note that this is not the sum of the series-parallel combination. It’s the sum of all the resistance. In this instance it will be 50.5 ohm=(50 ohm [meter] + 0.5 ohm [shunt]).

I_T = total circuit current

+++++21 Current path through shunt and meter for a full-scale value of 1 ampere.

When the selector switch is changed to the 500-milliampere position, RSH1 and RSH2 are connected in series with each other and in parallel with the meter movement and RSH3. The combined resistance value for RSH1 and RSH2 can be found using the formula ... Now that the total resistance for the sum of RSH1 and RSH2 is known, the value of R can be found by subtracting it from the value of R . ....The value of RSH3 can be found by subtracting the total shunt resistance from the values of RSH1 and RSH2.

The preceding procedure can be used to find the value of any number of shunt resistors for any value of current desired. Note, however, that this type of shunt is not used for large current values because of the problem of switching contacts and contact size. The Ayrton shunt is seldom used for currents above 10 amperes. An ammeter with an Ayrton shunt is shown below. The Ayrton shunt with all resistor values is given below.

+++++22 Current path through the meter and shunt for a full-scale value of 0.5 ampere.

+++++23 DC ammeter with an Ayrton shunt.

+++++24 The Ayrton shunt with resistor values.

Next: part 3  ---  Prev: part 1

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