A multimeter or a multitester , also known as a volt/ohm meter or VOM , is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter may include features such as the ability to measure voltage, current and resistance. There are two categories of multimeters, analog multimeters and digital multimeters (often abbreviated DMM or DVOM .)
A multimeter can be a hand-held device useful for basic fault finding and field service work or a bench instrument which can measure to a very high degree of accuracy. They can be used to troubleshoot electrical problems in a wide array of industrial and household devices such as batteries, motor controls, appliances, power supplies, and wiring systems.
Multimeters are available in a wide ranges of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5000.
History
Scientists originally used galvanometers to measure current. A galvanometer may be wired to measure resistance (given a known voltage source) or voltage (given a fixed resistance). While appropriate for primitive lab use, switching from one setup to another is inconvenient in the field.
Multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. The invention of the first multimeter is attributed to Post Office engineer Donald Macadie, who became dissatisfied with having to carry many separate instruments required for the maintenance of the telecommunication circuits. Macadie invented an instrument which could measure amps, volts and ohms, so the multifunctional meter was then named Avometer. The meter comprised a galvanometer, voltage and resistance references, and a switch to select the appropriate circuit for the input under test.
Macadie took his idea to the Automatic Coil Winder and Electrical Equipment Company (ACWEEC, founded probably in 1923). The first AVO was put on sale in 1923, and although it was initially a DC-only instrument many of its features remained almost unaltered right through to the last Model 8.
As modern systems become more complicated, the multimeter is becoming more complex or may be supplemented by more specialized equipment in a technician's toolkit. For example, where a general-purpose multimeter might only test for short-circuits, conductor resistance and some coarse measure of insulation quality, a modern technician may use a hand-held analyzer to test several parameters in order to validate the performance of a network cable.
Quantities measured
Contemporary multimeters can measure many quantities. The common ones are:
- Voltage in volts.
- Current in amperes.
- Resistance in ohms.
Additionally, multimeters may also measure:
- Capacitance in farads.
- Conductance in siemens.
- Decibels.
- Duty cycle as a percentage.
- Frequency in hertz
- Inductance in henrys
- Temperature in degrees Celsius or Fahrenheit.
Digital multimeters may also include circuits for:
- Continuity that beeps when a circuit conducts.
- Diodes and Transistors
Various sensors can be attached to multimeters to take measurements such as:
- Light level
- Acidity/Alkalinity(pH)
- Wind speed
- Relative humidity
Resolution
Digital
The resolution of a multimeter is often specified in "digits" of resolution. For example, the term 5½ digits refers to the number of digits displayed on the readout of a multimeter.
By convention, a half digit can display either a zero or a one, while a three-quarters digit can display a numeral higher than a one but not nine. Commonly, a three-quarters digit refers to a maximum value of 3 or 5. The fractional digit is always the most significant digit in the displayed value. A 5½ digit multimeter would have five full digits that display values from 0 to 9 and one half digit that could only display 0 or 1. Such a meter could show positive or negative values from 0 to 199,999. A 3¾ digit meter can display a quantity from 0 to 3,999 or 5,999, depending on the manufacturer.
While a digital display can easily be extended in precision, the extra digits are of no value if not accompanied by care in the design and calibration of the analog portions of the multimeter. Meaningful high-resolution measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument.
Specifying "display counts" is another way to specify the resolution. Display counts give the largest number, or the largest number plus one (so the count number looks nicer) the multimeter' display can show, ignoring a decimal separator. For example, a 5½ digit multimeter can also be specified as a 199999 display count or 200000 display count multimeter.
Often the display count is just called the count in multimeter specifications. In some designs the underlying analog-to-digital converter mechanism may have more or less digits of precision than displayed.
Analog
Resolution of analog multimeters is limited by the width of the scale pointer, vibration of the pointer, the accuracy of printing of scales, zero calibration, number of ranges, and errors due to non-horizontal use of the mechanical display. Accuracy of readings obtained is also often compromised by miscounting division markings, errors in mental arithmetic, parallax observation errors, and less than perfect eyesight. Mirrored scales and larger meter movements are used to improve resolution; two and a half to three digits equivalent resolution is usual (and may be adequate for the limited precision actually necessary for most measurements).
Resistance measurements, in particular, are of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values.
Accuracy
Digital multimeters generally take measurements with accuracy superior to their analog counterparts. Analog multimeters typically measure with about three percent accuracy. Standard portable digital multimeters claim to be capable of taking measurements with an accuracy of 0.5% on the DC voltage ranges. Mainstream bench-top multimeters make claims to have an accuracy of better than ±0.01%. Laboratory grade instruments can have accuracies in the parts per million figures.
A multimeter's quoted accuracy is specified as being that of the lower (mV) DC range, and is known as the "basic DC volts accuracy" figure. Higher DC voltage ranges, current, resistance, AC and other ranges will usually have a lower accuracy than the basic DC volts figure.
Manufacturers can provide calibration services so that new meters may be purchased with a certificate of calibration indicating the meter has been adjusted to standards traceable to the National Institute of Standards and Technology. Such manufacturers usually provide calibration services after sales, as well, so that older equipment may be recertified. Multimeters used for critical measurements may be part of a metrology program to assure calibration.
Sensitivity and input impedance
The current load, or how much current is drawn from the circuit being tested may affect a multimeter's accuracy. A smaller current draw usually will result in more precise measurements. With improper usage or too much current load, a multimeter may be damaged therefore rendering its measurements unreliable and substandard.
Meters with electronic amplifiers in them, such as all digital multimeters and analog meters using a transistor for amplification, have an input impedance that is usually considered high enough not to disturb the circuit tested. This is often one million ohms, or ten million ohms. The standard input impedance allows use of external probes to extend the direct-current measuring range up to tens of thousands of volts.
Most analog multimeters of the moving pointer type are unbuffered, and draw current from the circuit under test to deflect the meter pointer. The impedance of the meter varies depending on the basic sensitivity of the meter movement and the range which is selected. For example, a meter with a typical 20,000 ohms/volt sensitivity will have an input resistance of two million ohms on the 100 volt range (100 V * 20,000 ohms/volt = 2,000,000 ohms). Lower sensitivity meters are useful for general purpose testing especially in power circuits, where source impedances are low compared to the meter impedance. Some measurements in signal circuits require higher sensitivity so as not to load down the circuit under test with the meter impedance.
Sometime sensitivity is confused with resolution of a meter, which is defined as measure of the lowest voltage, current or resistance that can change measurement reading. For general-purpose digital multimeters, a full-scale range of several hundred millivolts AC or DC is common, but the minimum full-scale current range may be several hundred milliamps. Since general-purpose
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