Notebook : measuring devices

Results for measuring devices

Bar-graph meters

Sikha June 19, 2016

A cheap, simple kind of meter can be made using a string of light-emitting diodes (LEDs) or a liquid-crystal display (LCD) along with a digital scale, to indicate approximate levels of current, voltage or power. This type of meter has no moving parts to break, just like a digital meter. But it also offers the relative-reading feeling you get with an analog meter.

Figure below is an example of a bar-graph meter that is used to show the power output, in kilowatts, for a radio transmitter. It indicates 0.8 kW or 800 watts, approximately.

The chief disadvantage of the bar-graph meter is that it isn’t very accurate. For this reason it is not generally used in laboratory testing. The LED or LCD devices sometimes also flicker when the level is “between” two values given by the bars. This can be annoying to some people.

Bar-graph meters

Reviewed by Sikha on June 19, 2016 Rating: 5

Pen recorders

Sikha June 19, 2016

A meter movement can be equipped with a marking device, usually a pen, to keep a graphic record of the level of some quantity with respect to time. Such a device is called a pen recorder. The paper, with a calibrated scale, is taped to a rotating drum. The drum, driven by a clock motor, turns at a slow rate, such as one revolution per hour or one revolution in 24 hours. A simplified drawing of a pen recorder is shown in Fig.

A device of this kind, along with a wattmeter, might be employed to get a reading

of the power consumed by your household at various times during the day. In this way you might tell when you use the most power, and at what particular times you might be using too much.

Pen recorders

Reviewed by Sikha on June 19, 2016 Rating: 5

Light meters or illumination meter./Lux meter

Sikha June 19, 2016

Light intensity is measured by means of a light meter or illumination meter. You might think that it’s easy to make this kind of meter by connecting a milliammeter to a solar (photovoltaic) cell. And this is, in fact, a good way to construct an inexpensive light meter (Fig.).

More sophisticated devices might use dc amplifiers to enhance sensitivity and to allow for several different ranges of readings. One problem with this design is that solar cells are not sensitive to light at exactly the same wavelengths as human eyes. This can be overcome by placing a colored filter in front of the solar cell, so that the solar cell becomes sensitive to the same wavelengths, in the same proportions, as human eyes. Another problem is calibrating the meter. This must usually be done at the factory, in units such as lumens or candela. Its not important that you know the precise definitions of these units in electricity and electronics.

Light meters or illumination meter./Lux meter

Reviewed by Sikha on June 19, 2016 Rating: 5

VU and decibel meters

Sikha June 19, 2016

In high-fidelity equipment, especially the more sophisticated amplifiers (“amps”), loudness meters are sometimes used. These are calibrated in decibels, a unit that you will sometimes encounter in reference to electronic signal levels. A decibel is an increase or decrease in sound or signal level that you can just barely detect, if you are expecting the change.

Audio loudness is given in volume units (VU), and the meter that indicates it is called a VU meter. Usually, such meters have a zero marker with a red line to the right and a black line to the left, and they are calibrated in decibels (dB) above and below this zero marker (ref fig).

The meter might also be calibrated in watts rms, an expression for audio power.

As music is played through the system, or as a voice comes over it, the VU meter

needle will kick up. The amplifier volume should be kept down so that the meter doesn’t go past the zero mark and into the red range. If the meter does kick up into the red scale, it means that distortion is probably taking place within the amplifier circuit.

Sound level in general can be measured by means of a sound-level meter, calibrated in decibels (dB) and connected to the output of a precision amplifier with a microphone of known, standardized sensitivity (Fig. 3-13). You have perhaps heard that a vacuum cleaner will produce 80 dB of sound, and a large truck going by might subject your ears to 90 dB. These figures are determined by a sound-level meter. A VU meter is a special form of sound-level meter.

VU and decibel meters

Reviewed by Sikha on June 19, 2016 Rating: 5

Frequency counters

Sikha June 19, 2016

The measurement of energy used by your home is an application to which digital metering is well suited. It’s easier to read the drum type, digital kilowatt-hour meter than to read the pointer type meter. When measuring frequencies of signals, digital metering is not only more convenient, but far more accurate.

The frequency counter measures by actually counting pulses, in a manner similar

to the way the utility meter counts the number of turns of a motor. But the frequency counter works electronically, without any moving parts. It can keep track of thousands, millions or even billions of pulses per second, and it shows the rate on a digital display that is as easy to read as a digital watch. It measures frequency directly by tallying up the number of pulses in an oscillating wave, even when the number of pulses per second is huge.

The accuracy of the frequency counter is a function of the lock-in time. Lock-in is usually done in 0.1 second, 1 second or 10 seconds. Increasing the lock-in time by a factor of 10 will cause the accuracy to be good by one additional digit. Modern frequency counters are good to six, seven or eight digits; sophisticated lab devices will show frequency to nine or ten digits.

Frequency counters

Reviewed by Sikha on June 19, 2016 Rating: 5

Digital readout meters-principle and operation

Sikha June 19, 2016

Increasingly, metering devices are being designed so that they provide a direct readout, and there’s no need (or possibility) for interpolation. The number on the meter is the indication.Its that simple. Such a meter is called a digital meter.

The advantage of a digital meter is that it’s easy for anybody to read, and there is no chance for interpolation errors. This is ideal for utility meters, clocks, and so me kinds of ammeters, voltmeters and wattmeters. It works very well when the value of the quantity does not change very often or very fast.
But there are some situations in which a digital meter is a disadvantage. One good example is the signal-strength indicator in a radio receiver. This meter bounces up and down as signals fade, or as you tune the radio, or sometimes even as the signal modulates.

A digital meter would show nothing but a constantly changing, meaningless set of numerals. Digital meters require a certain length of time to “lock in” to the current, voltage, power or other quantity being measured. If this quantity never settles at any one value for a long enough time, the meter can never lock in.

Meters with a scale and pointer are known as analog meters. Their main advantages are that they allow interpolation, they give the operator a sense of the quantity relative to other possible values, and they follow along when a quantity changes. Some engineers and technicians prefer the “feel” of an analog meter, even in situations where a digital meter would work just as well.

One problem you might have with digital meters is being certain of where the decimal point goes. If you’re off by one decimal place, the error will be by a factor of 10.

Also, you need to be sure you know what the units are; for example, a frequency indicator might be reading out in megahertz, and you might forget and think it is giving you a reading in kilohertz. That’s a mistake by a factor of 1000. Of course this latter type of error can happen with an analog meter, too.

Digital readout meters-principle and operation

Reviewed by Sikha on June 19, 2016 Rating: 5

Watt-hour meters- principle and operation

Sikha June 19, 2016

The utility company is not too interested in how much power you’re using with one appliance,or even how much power a single household is drawing, at any given time. Byfar the greater concern is the total energy that is used over a day, a week, a month or ayear. Electrical energy is measured in watt hours, or, more commonly for utility purposes,in kilowatt hours (kWh). The device that indicates this is the watt-hour meteror kilowatt-hour meter.

The most often-used means of measuring electrical energy is by using a small electricmotor device, whose speed depends on the current, and thereby on the power at aconstant voltage. The number of turns of the motor shaft, in a given length of time, is directly proportional to the number of kilowatt hours consumed. The motor is placed at the point where the utility wires enter the house, apartment or building. This is usually at a point where the voltage is 234 V. This is split into some circuits with 234 V, for heavy-duty appliances such as the oven, washer and dryer, and the general household fines for lamps, clock radios and, television sets.

You’ve surely seen the little disk in the utility meter going around and around,

sometimes fast, other times slowly. Its speed depends on the power you’re using. The total number of turns of this little disk, every month, determines the size of the bill you will get—as a function also, of course, of the cost per kilowatt hour for electricity. Kilowatt-hour meters count the number of disk turns by means of geared, rotary drums or pointers. The drum type meter gives a direct digital readout. The pointer type has several scales calibrated from 0 to 9 in circles, some going clockwise and others going counterclockwise. Reading a pointer type utility meter is a little tricky, because you must think in whatever direction (clockwise or counterclockwise) the scale goes. An example of a pointer type utility meter is shown in Fig. shown below.

Read from left to right. For each little meter, take down the number that the pointer has most recently passed. Write down the rest as you go. The meter in the figure reads 3875 kWh. If you want to be really precise, you can say it reads 3875-1/2 kWh.

Watt-hour meters- principle and operation

Reviewed by Sikha on June 19, 2016 Rating: 5

Wattmeter -principle and working

Sikha June 19, 2016

The measurement of electrical power requires that voltage and current both be measured simultaneously. Remember that power is the product of the voltage and current. That is, watts (P) equals volts (E) times amperes (I), written as P = EI.

In fact, watts are sometimes called volt-amperes in a dc circuit. You might think that you can just connect a voltmeter in parallel with a circuit, thereby getting a reading of the voltage across it, and also hook up an ammeter in series to get a reading of the current through the circuit, and then multiply volts times amperes to get watts consumed by the circuit. And in fact, for practically all dc circuits,this is an excellent way to measure power (Fig.).

Quite often, however, it’s simpler than that. In many cases, the voltage from the

power supply is constant and predictable. Utility power is a good example. The effectivevoltage is always very close to 117 V. Although it’s ac, and not dc, power can be measured in the same way as with dc: by means of an ammeter connected in series with the circuit, and calibrated so that the multiplication (times 117) has already been done.

Then, rather than 1 A, the meter would show a reading of 117 W, because P= EI=117×1=117 W.

If the meter reading were 300 W, the current would be 300/117=2.56 A.An electric iron might consume 1000 W, or a current of 1000/117 _ 8.55 A. And a

large heating unit might gobble up 2000 W, requiring a current of

2000/117=17. 1A.

This might blow a fuse or breaker, since these devices are often rated for only 15 A. You’ve probably had an experience where you hooked up too many appliances to a single circuit, blowing the fuse or breaker. The reason was that the appliances, combined, drew too much current for the house wiring to safely handle, and the fuse or breaker, detecting the excess current, opened the circuit.

Specialized wattmeters are necessary for the measurement of radio-frequency

(RF) power, or for peak audio power in a high-fidelity amplifier, or for certain other specialized applications. But almost all of these meters, whatever the associated circuitry, use simple ammeters as their indicating devices.

Wattmeter -principle and working

Reviewed by Sikha on June 19, 2016 Rating: 5

FET and vacuum-tube voltmeters

Sikha June 19, 2016

It was mentioned that a good voltmeter will disturb the circuit under test as little as possible,and this requires that the meter have a high internal resistance. Besides the electrostatictype voltmeter, there is another way to get an extremely high internal resistance. This is to sample a tiny, tiny current, far too small for any meter to directly indicate, and then amplify this current so that a meter will show it. When a miniscule amount of current is drawn from a circuit, the equivalent resistance is always extremely high.

The most effective way to accomplish the amplification, while making sure that the current drawn really is tiny, is to use either a vacuum tube or a field-effect transistor (FET). You needn’t worry about how such amplifiers work right now; that subject will come much later in this book. A voltmeter that uses a vacuum tube amplifier to minimize current drain is known as a vacuum-tube voltmeter (VTVM). If an FET is used, the meter is called a FET voltmeter (FETVM). Either of these devices provide anextremely high input resistance along with good sensitivity and amplification. And they allow measurement of lower voltages, in general, than electrostatic voltmeters.

FET and vacuum-tube voltmeters

Reviewed by Sikha on June 19, 2016 Rating: 5

Multimeter- Principle and operation

Sikha June 19, 2016

In the electronics lab, a common piece of test equipment is the multimeter, in which different kinds of meters are combined into a single unit. The volt-ohm-milliammeter (VOM) is the most often used. As its name implies, it combines voltage, resistance and current measuring capabilities.

You should not have too much trouble envisioning how a single milliammeter can be used for measuring voltage, current and resistance. The preceding discussions for measurements of these quantities have all included methods in which a current meter can be used to measure the intended quantity.

Commercially available multimeters have certain limits in the values they can measure. The maximum voltage is around 1000 V; larger voltages require special leads and heavily insulated wires, as well as other safety precautions. The maximum current that a common VOM can measure is about 1 A. The maximum resistance is on the order of several mega ohms or tens of mega ohms. The lower limit of resistance indication is about an ohm.

Multimeter- Principle and operation

Reviewed by Sikha on June 19, 2016 Rating: 5

Ohmmeter- basic principle and working

Sikha June 19, 2016

You know that the current through a circuit depends on the resistance. This principle can be used to manufacture a voltmeter using an ammeter and a resistor. The larger the value of the resistance in series with the meter, the more voltage is needed to produce a reading of full scale. This has a converse, or a “flip side”: Given a constant voltage, the current through the meter will vary if the resistance varies. This provides a means for measuring resistances.

An ohmmeter is almost always constructed by means of a milliammeter or microammeter in series with a set of fixed, switchable resistances and a battery that provides a known, constant voltage (Fig.). By selecting the resistances appropriately, the meter will give indications in ohms over any desired range. Usually, zero on the meter is assigned the value of infinity ohms, meaning a perfect insulator. The full-scale value is set at a certain minimum, such as 1 Ω, 100 Ω, or 10 KΩ (10,000 Ω).

Ohmmeters must be precalibrated at the factory where they are made. A slight error in the values of the series resistors can cause gigantic errors in measured resistance. Therefore, precise tolerances are needed for these resistors. It is also necessary that the battery be exactly the right kind, and that it be reasonably fresh so that it will provide the appropriate voltage. The smallest deviation from the required voltage can cause a big error in the meter indication.

The scale of an ohmmeter is nonlinear. That is, the graduations are not the same

everywhere. Values tend to be squashed together towards the “infinity” end of the scale. It can be difficult to interpolate for high values of resistance, unless the right scale is selected.

Engineers and technicians usually connect an ohmmeter in a circuit with the

meter set for the highest resistance range first; then they switch the range until the

is in a part of the scale that is easy to read. Finally, the reading is taken, and is multiplied(or divided) by the appropriate amount as indicated on the range switch. Figure Below shows an ohmmeter reading.

The meter itself says 4.7, but the range switch says 1 KΩ. This indicates a resistance of 4.7 KΩ, or 4700 Ω.

Ohmmeters will give inaccurate readings if there is a voltage between the points

where the meter is connected. This is because such a voltage either adds to, or subtracts from, the ohmmeter battery voltage. This in effect changes the battery voltage, and the meter reading is thrown way off. Sometimes the meter might even read “more than infinity” ohms; the needle will hit the pin at the left end of the scale. Therefore, when using an ohmmeter to measure resistance, you need to be sure that there is no voltage between the points under test. The best way to do this is to switch off the equipment.

Ohmmeter- basic principle and working

Reviewed by Sikha on June 19, 2016 Rating: 5

Voltmeter- Principle and operation

Sikha June 19, 2016

Current is a flow of charge carriers. Voltage, or electromotive force (EMF), or potential difference, is the “pressure” that makes a current possible. Given a circuit whose resistance is constant, the current that will flow in the circuit is directly proportional to the voltage placed across it. Early electrical experimenters recognized that an ammeter could be used to measure voltage, since an ammeter is a form of constant-resistance circuit.

If you connect an ammeter directly across a source of voltage—a battery, say—the

meter needle will deflect. In fact, a milliammeter needle will probably be “pinned” if you do this with it, and a microammeter might well be wrecked by the force of the needle striking the pin at the top of the scale. For this reason, you should never connect milliammeters or microammeters directly across voltage sources. An ammeter, perhaps with a range of 0-10 A, might not deflect to full scale if it is placed across a battery, but it’s still a bad idea to do this, because it will rapidly drain the battery.

Some batteries, such as automotive lead-acid cells, can explode under these conditions. This is because all ammeters have low internal resistance. They are designed that way deliberately. They are meant to be connected in series with other parts of a circuit, not right across the power supply.

But if you place a large resistor in series with an ammeter, and then connect the

ammeter across a battery or other type of power supply, you no longer have a short circuit. The ammeter will give an indication that is directly proportional to the voltage of the supply. The smaller the full-scale reading of the ammeter, the larger the resistance to get a meaningful indication on the meter. Using a microammeter and a very large value of resistor in series, a voltmeter can be devised that will draw only a little current from the source.

A voltmeter can be made to have different ranges for the full-scale reading, by switching different values of resistance in series with the microammeter (Fig. 3-6). The internal resistance of the meter is large because the values of the resistors are large.

The greater the supply voltage, the larger the internal resistance of the meter, because the necessary series resistance increases as the voltage increases.

It’s always good when a voltmeter has a high internal resistance. The reason for this is that you don’t want the meter to draw much current from the power source. This current should go, as much as possible, towards working whatever circuit is hooked up to the supply, and not into just getting a reading of the voltage. Also, you might not want, or need, to have the voltmeter constantly connected in the circuit; you might need the voltmeter for testing many different circuits. You don’t want the behavior of the circuit to be affected the instant you connect the voltmeter to the supply. The less current a voltmeter draws, the less it will affect the behavior of anything that is working from the power supply.

Voltmeter- Principle and operation

Reviewed by Sikha on June 19, 2016 Rating: 5

Measuring devices for Electrical quantities..

Sikha June 19, 2016

Many measuring devices work because electric and magnetic fields produce forces proportional to the intensity of the field. By using a tension spring against which the

electric or magnetic force can pull or push, a movable needle can be constructed. The needle can then be placed in front of a calibrated scale, allowing a direct reading of thequantity to be measured. These meters work by means of electromagnetic deflection or electrostatic deflection.

Sometimes, electric current is measured by the extent of heat it produces in a resistance.

Such meters work by thermal heating principles.Some meters work by means of small motors whose speed depends on the measured quantity. The rotation rate, or the number of rotations in a given time, can be measured or counted. These are forms of rate meters. Still other kinds of meters actually count electronic pulses, sometimes in thousands,millions or billions. These are electronic counters. There are also various other metering methods.

Electromagnetic deflection

Early experimenters with electricity and magnetism noticed that an electric current produces a magnetic field. This discovery was probably an accident, but it was an accident that, given the curiosity of the scientist, was bound to happen. When a magnetic

compass is placed near a wire carrying a direct electric current, the compass doesn’t point toward magnetic north. The needle is displaced. The extent of the error depends on how close the compass is brought to the wire, and also on how much current the wire is carrying.

Scientific experimenters are like children. They like to play around with things. Most likely, when this effect was first observed, the scientist tried different arrangements to see how much the compass needle could be displaced, and how small a current could be detected. An attempt was made to obtain the greatest possible current-detecting sensitivity. Wrapping the wire in a coil around the compass resulted in a device that would indicate a tiny electric current (Fig.). This effect is known as galvanism, and the meter so devised was called a galvanometer.

Once this device was made, the scientist saw that the extent of the needle displacement increased with increasing current. Aha—a device for measuring current! Then, the only challenge was to calibrate the galvanometer somehow, and to set up some kind of standard so that a universal meter could be engineered.

You can easily make your own galvanometer. Just buy a cheap compass, about two

feet of insulated bell wire, and a six-volt lantern battery. Set it up as shown in Fig. 1. Wrap the wire around the compass four or five times, and align the compass so that the needle points right along the wire turns while the wire is disconnected from the battery. Connect one end of the wire to the minus (–) terminal of the battery. Touch the other end to the plus (+) terminal, intermittently, and watch the compass needle. Don’t leave the wire connected to the battery for any length of time unless you want to drain the battery in a hurry.

You can buy a resistor and a potentiometer at a place like Radio Shack, and set up

an experiment that shows how galvanometers measure current. For a 6-V lantern battery, the fixed resistor should have a value of at least 330 Ω at 1/4 watt, and the potentiometer should have a value of 10 KΩ (10,000 Ω) maximum. Connect the resistor and potentiometer in series between one end of the bell wire and one terminal of the battery, as shown in Fig. 2. The center contact of the potentiometer should be short-circuited to one of the end contacts, and the resulting two terminals used in the circuit. When you adjust the potentiometer, the compass needle should deflect more or less, depending on the current through the wire. Early experimenters calibrated their meters by referring to the degree scale around the perimeter of the compass.