Editing Multimeter (section)

== Characteristics ==

=== Resolution ===
The resolution of a multimeter is the smallest part of the scale which can be shown, which is scale dependent. On some digital multimeters it can be configured, with higher resolution measurements taking longer to complete. For example, a multimeter that has a 1&nbsp;mV resolution on a 10&nbsp;V scale can show changes in measurements in 1&nbsp;mV increments. Absolute accuracy is the error of the measurement compared to a perfect measurement. Relative accuracy is the error of the measurement compared to the device used to calibrate the multimeter. Most multimeter datasheets provide relative accuracy. To compute the absolute accuracy from the relative accuracy of a multimeter add the absolute accuracy of the device used to calibrate the multimeter to the relative accuracy of the multimeter.<ref name="Keithley Instruments">{{cite web|title=Model 2002 Multimeter Specifications|url=http://www.keithley.com/data?asset=5799|publisher=Keithley Instruments}}</ref>

The resolution of a multimeter is often specified in the number of decimal [[Numerical digit|digits]] [[Sensor resolution|resolved]] and displayed. If the most significant digit cannot take all values from 0 to 9 it is generally, and confusingly, termed a fractional digit. For example, a multimeter which can read up to 19999 (plus an embedded decimal point) is said to read {{frac|4|1|2}} digits. By convention, if the most significant digit can be either 0 or 1, it is termed a half-digit; if it can take higher values without reaching 9 (often 3 or 5), it may be called three-quarters of a digit. A {{frac|5|1|2}}-digit multimeter would display one "half digit" that could only display 0 or 1, followed by five digits taking all values from 0 to 9.<ref>{{cite web|url=http://zone.ni.com/devzone/cda/tut/p/id/3295|access-date=2008-01-26|title=Digital Multimeter Measurement Fundamentals|publisher=National Instruments}}</ref> Such a meter could show positive or negative values from 0 to 199999. A {{frac|3|3|4}}-digit meter can display a quantity from 0 to 3999 or 5999, depending on the manufacturer. While a digital display can easily be extended in [[display resolution|resolution]], 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 (i.e., high-accuracy) measurements require a good understanding of the instrument specifications, good control of the measurement conditions, and traceability of the calibration of the instrument. However, even if its resolution exceeds the [[accuracy and precision|accuracy]], a meter can be useful for comparing measurements. For example, a meter reading {{frac|5|1|2}} stable digits may indicate that one nominally 100&nbsp;kΩ resistor is about 7&nbsp;Ω greater than another, although the error of each measurement is 0.2% of reading plus 0.05% of full-scale value. Specifying "display counts" is another way to specify the resolution. Display counts give the largest number, or the largest number plus one (to include the display of all zeros) the multimeter's display can show, ignoring the [[decimal separator]]. For example, a {{frac|5|1|2}}-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. The accuracy of a digital multimeter may be stated in a two-term form, such as "±1% of reading +2 counts", reflecting the different sources of error in the instrument.<ref>Stephen A. Dyer, ''Wiley Survey of Instrumentation and Measurement'', John Wiley & Sons, 2004 {{ISBN|0471221651}}, p. 290</ref>

[[File:Multimeter-4269.jpg|thumb|Display face of an analog multimeter]]

Analog meters are older designs, but despite being technically surpassed by digital meters with bar graphs, may still be preferred{{according to whom|date=March 2020}} by engineers{{which|date=March 2020}} and troubleshooters.{{original research inline|date=March 2020}} One reason given is that analog meters are more sensitive (or responsive) to changes in the circuit that is being measured.{{citation needed|date=March 2020}} A digital multimeter samples the quantity being measured over time, and then displays it. Analog multimeters continuously read the test value. If there are slight changes in readings, the needle of an analog multimeter will attempt to track it, as opposed to the digital meter having to wait until the next sample, giving delays between each discontinuous reading (plus the digital meter may additionally require settling time to converge on the value). The digital display value as opposed to an analog display is subjectively more difficult to read. This continuous tracking feature becomes important when testing capacitors or coils, for example. A properly functioning capacitor should allow current to flow when voltage is applied, then the current slowly decreases to zero and this "signature" is easy to see on an analog multimeter but not on a digital multimeter. This is similar when testing a coil, except the current starts low and increases. Resistance measurements on an analog meter, in particular, can be of low precision due to the typical resistance measurement circuit which compresses the scale heavily at the higher resistance values. Inexpensive analog meters may have only a single resistance scale, seriously restricting the range of precise measurements. Typically, an analog meter will have a panel adjustment to set the zero-ohms calibration of the meter, to compensate for the varying voltage of the meter battery, and the resistance of the meter's test leads.

=== Accuracy ===
Digital multimeters generally take measurements with [[accuracy and precision|accuracy]] superior to their analog counterparts. Standard analog multimeters measure with typically ±3% accuracy,<ref>{{cite book|publisher=McGraw-Hill|title=Handbook of electronics calculations for engineers and technicians|author=Milton Kaufman}}</ref> though instruments of higher accuracy are made. Standard portable digital multimeters are specified to have an accuracy of typically ±0.5% on the DC voltage ranges. Mainstream bench-top multimeters are available with specified accuracy of better than ±0.01%. Laboratory grade instruments can have accuracies of a few [[parts per million]].<ref>{{cite web|url=http://literature.cdn.keysight.com/litweb/pdf/5965-4971E.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://literature.cdn.keysight.com/litweb/pdf/5965-4971E.pdf |archive-date=2022-10-09 |url-status=live|title=Keysight 3458A Digital Multimeter Data Sheet|last=Keysight Technologies|website=Keysight Technologies|access-date=2014-07-31}}</ref>

Accuracy figures need to be interpreted with care. The accuracy of an analog instrument usually refers to full-scale deflection; a measurement of 30&nbsp;V on the 100&nbsp;V scale of a 3% meter is subject to an error of 3&nbsp;V, 10% of the reading. Digital meters usually specify accuracy as a percentage of reading plus a percentage of full-scale value, sometimes expressed in counts rather than percentage terms.

Quoted accuracy is specified as being that of the lower millivolt (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. AC measurements only meet specified accuracy within a specified range of [[frequencies]].

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, for example, the US [[National Institute of Standards and Technology]] (NIST), or other national [[standards organization]].

Test equipment tends to drift out of calibration over time, and the specified accuracy cannot be relied upon indefinitely. For more expensive equipment, manufacturers and third parties provide calibration services so that older equipment may be recalibrated and recertified. The cost of such services is disproportionate for inexpensive equipment; however extreme accuracy is not required for most routine testing. Multimeters used for critical measurements may be part of a [[metrology]] program to assure calibration.

A multimeter can be assumed to be "average responding" to AC waveforms unless stated as being a "true RMS" type. An average responding multimeter will only meet its specified accuracy on AC volts and amps for purely sinusoidal waveforms. A True RMS responding multimeter on the other hand will meet its specified accuracy on AC volts and current with any waveform type up to a specified [[crest factor]];  RMS performance is sometimes claimed for meters which report accurate RMS readings only at certain frequencies (usually low) and with certain waveforms (essentially always sine waves).

A meter's AC voltage and current accuracy may have different specifications at different frequencies.

=== Sensitivity and input impedance ===
When used for measuring voltage, the input impedance of the multimeter must be very high compared to the impedance of the circuit being measured; otherwise circuit operation may be affected and the reading will be inaccurate. Meters with electronic amplifiers (all digital multimeters and some analog meters) have a fixed input impedance that is high enough not to disturb most circuits. This is often either one or ten [[megohm]]s; the [[standardization]] of the input resistance allows the use of external high-resistance [[Test probe|probes]] which form a [[voltage divider]] with the input resistance to extend voltage range up to tens of thousands of volts.  High-end multimeters generally provide an input impedance greater than 10&nbsp;GΩ for ranges less than or equal to 10&nbsp;V.  Some high-end multimeters provide >10&nbsp;Gigaohms of impedance to ranges greater than 10&nbsp;V.<ref name="Keithley Instruments"/> Most analog multimeters of the moving-pointer type are [[Buffer amplifier|unbuffered]], and draw current from the circuit under test to deflect the meter pointer. The [[Electrical impedance|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&nbsp;Ω/V sensitivity will have an input resistance of 2&nbsp;MΩ on the 100&nbsp;V range (100&nbsp;V × 20,000&nbsp;Ω/V = 2,000,000&nbsp;Ω). On every range, at full-scale voltage of the range, the full current required to deflect the meter movement is taken from the circuit under test. Lower sensitivity meter movements are acceptable for testing in circuits where source impedances are low compared to the meter impedance, for example, power circuits; these meters are more rugged mechanically. Some measurements in signal circuits require higher sensitivity movements so as not to load the circuit under test with the meter impedance.<ref>{{cite book|publisher=[[McGraw-Hill]]/TAB Electronics|title=How to Test Almost Everything Electronic|first=Delton|last=Horn|pages=4–6|year=1993|isbn=0-8306-4127-0}}</ref><ref name=":0">{{Cite book|url=https://books.google.com/books?id=Us6vnQEACAAJ|title=Electrical circuits|last=Siskind|first=Charles S.|date=1956|language=en}}</ref>

Sensitivity should not be confused with [[Sensor resolution|resolution]] of a meter, which is defined as the lowest signal change (voltage, current, resistance and so on) that can change the observed reading.<ref name=":0" />

For general-purpose digital multimeters, the lowest voltage range is typically several hundred millivolts AC or DC, but the lowest current range may be several hundred microamperes, although instruments with greater current sensitivity are available. Multimeters designed for (mains) "electrical" use instead of general [[electronics engineering]] use will typically forego the microamps current ranges. Measurement of low resistance requires lead resistance (measured by touching the test probes together) to be subtracted for best accuracy. This can be done with the "delta", "zero", or "null" feature of many digital multimeters. Contact pressure to the device under test and cleanliness of the surfaces can affect measurements of very low resistances. Some meters offer a four wire test where two probes supply the source voltage and the others take measurement. Using a very high impedance allows for very low voltage drop in the probes and resistance of the source probes is ignored resulting in very accurate results. The upper end of multimeter measurement ranges varies considerably; measurements over perhaps 600&nbsp;volts, 10&nbsp;amperes, or 100&nbsp;[[Ohm|megohms]] may require a specialized test instrument.

=== Burden voltage ===
Every inline series-connected ammeter, including a multimeter in a current range, has a certain resistance. Most multimeters inherently measure voltage, and pass a current to be measured through a [[shunt resistance]], measuring the voltage developed across it. The voltage drop is known as the burden voltage, specified in volts per ampere. The value can change depending on the range the meter sets, since different ranges usually use different shunt resistors.<ref>{{cite web |url=http://us.fluke.com/fluke/usen/community/fluke+plus/articlecategories/electrical/burdenvoltage.htm |title=Explanation of burden voltage by multimeter manufacturer Fluke |publisher=[[Fluke Corporation|Fluke]] |access-date=2010-11-02}}</ref> The burden voltage can be significant in very low-voltage circuit areas. To check for its effect on accuracy and on external circuit operation the meter can be switched to different ranges; the current reading should be the same and circuit operation should not be affected if burden voltage is not a problem. If this voltage is significant it can be reduced (also reducing the inherent accuracy and precision of the measurement) by using a higher current range.

=== Alternating current sensing ===
Since the basic indicator system in either an analog or digital meter responds to DC only, a multimeter includes an AC to DC conversion circuit for making alternating current measurements. Basic meters utilize a [[Rectifier|rectifier circuit]] to measure the average or peak absolute value of the voltage, but are calibrated to show the calculated [[root mean square]] (RMS) value for a [[Sine wave|sinusoidal]] [[waveform]]; this will give correct readings for alternating current as used in power distribution. User guides for some such meters give [[correction factor]]s for some simple non-[[Sine wave|sinusoidal]] [[waveform]]s, to allow the correct [[root mean square]] (RMS) equivalent value to be calculated. More expensive multimeters include an AC to DC converter that measures the true RMS value of the waveform within certain limits; the user manual for the meter may indicate the limits of the [[crest factor]] and frequency for which the meter calibration is valid. RMS sensing is necessary for measurements on non-sinusoidal [[Period (physics)|periodic]] waveforms, such as found in audio signals and [[variable-frequency drive]]s.