A overview to Resistance MeasurementWith resistance measurement,precision is everything.This guide is what we knowabout achieving the highestquality measurements possible.
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The measurement of very big or an extremely smallquantities is constantly difficult, and also resistancemeasurement is no exception. Values over 1GΩand values listed below 1Ω both existing measurementproblems.
Cropico is a human being leader inlow resistance measurement;we develop a comprehensiverange of low resistance ohmmeters andaccessories which cover many measurementapplications. This handbook gives an introduction oflow resistance measure techniques, explainscommon causes of errors and also how to stop them.We have also included advantageous tables of cable andcable characteristics, temperature coefficientsand miscellaneous formulas to ensure you make the bestpossible choice when selecting your measuringinstrument and measurement technique.We hope you will uncover this guide a valuableaddition to your toolkit.
Manufacturers the componentsResistors, inductors and also chokes all have toverify the their product meets the specifiedresistance tolerance, end of manufacturing line andquality manage testing.
Manufacturers of switches,relays & connectorsVerification the the call resistance is belowpre specified boundaries is required. This have the right to beachieved at finish of manufacturing line testing,ensuring high quality control.
Cable manufacturersMust measure the resistance that the copperwires castle produce, resistance also high meansthat the present carrying ability of the cableis reduced; resistance too low means that themanufacturer is being too generous on thecable diameter using much more copper than heneeds to, which deserve to be really expensive.
Installation & maintenance of powercables, switchgear & voltage insanity changersThese require the cable joints and also switchcontacts to it is in of the lowest possible resistancethus staying clear of the joint or call frombecoming excessively hot, a negative cable joint orswitch call will soon fail because of this heatingeffect. Routine preventative maintain withregular resistance check ensures the bestpossible life performances.
Electric engine & generator manufacturersThere is a requirement to determine themaximum temperature reached under full load.To determine this temperature, the temperaturecoefficient of the copper winding is used. Theresistance is an initial measured with the motor orgenerator cold i.e. In ~ ambient temperature, theunit is then run at complete load because that a stated periodand the resistance measure again. Indigenous thechange in resistance value, the internalmotor/generator temperature have the right to bedetermined. Ours ohmmeters are additionally used tomeasure the individual coils the a engine winding,to certain there room no brief or open circuitturns and that every coil is balanced.
The automotive industryRequirement to measure up the resistance the robotwelding cables to ensure the the weld qualitydoes no deteriorate, i.e. Battery lead crimpconnectors, air bag detonator resistance,resistance of wiring harness, and quality ofcrimp connectors on components.
Fuse manufacturersFor high quality control, resistance bondingmeasurements top top aircraft and also military vehicles, itis essential to ensure the all equipmentinstalled in plane is electrically connectedto the waiting frame, including galley equipment.Tanks and other military vehicles have thesame requirements. Producers and also users oflarge electrical currents all should measuredistribution of share resistance, busbars, andconnectors to electrodes because that electroplating.
Railway utilitiesIncluding trams and also underground railways(Metro) - because that the measurement of powerdistribution cable joints, consisting of the resistanceof rail track joints, because the rails are oftenused because that signalling information.
Ohm’s law V = i x R (Volts = present xResistance). The Ohm (Ω) is a unit the electricalresistance equal to that of a conductor in i m sorry acurrent of one ampere is created by a potential ofone volt across its terminals.Ohm"s law, named after that discoverer the Germanphysicist Georg Ohm, is among the most important,basic regulations of electricity. It defines the relationshipbetween the three fundamental electrical quantities:current, voltage and also resistance. When a voltage isapplied to a circuit containing just resistiveelements, existing flows follow to Ohm"s Law,which is presented below.
4.Principles the resistance measurement
Ammeter Voltmeter methodThis method goes right ago to basics. If us usea battery as our voltage source, a voltmeter tomeasure the voltage and an ammeter to measurethe present in the circuit, we have the right to calculate theresistance with reasonable accuracy.Whilst this an approach can administer goodmeasurement results, the is not a practicalsolution to everyday measurement needs.
Kelvin dual BridgeThe Kelvin leg is a variation of theWheatstone bridge which enables lowresistances to be measured. Themeasurement selection would typically be 1mΩto 1kΩ v the smallest resolution of 1μΩ.The constraints of the Kelvin bridge are:-requires manual balancingsensitive null detector or galvanometer isrequired come detect balance conditionmeasurement present needs to be sensibly high to accomplish sufficient sensitivity
The Kelvin double Bridge has normally beenreplaced through digital ohmmeters.
DMM - Two-wire ConnectionA an easy digital multimeter deserve to be offered forhigher worths of resistance. They rental the 2wire technique of measurement and also are onlysuitable because that measuring values over 100Ω andwhere high accuracy is no required.
When measure up the resistance of acomponent (Rx) a test present is forcedthrough the component and the test metermeasures the voltage at its terminals. Themeter climate calculates and also displays theresulting resistance and also is well-known as a twowiremeasurement. It need to be listed that themeter steps the voltage in ~ its terminalsand not throughout the component. Together a an outcome ofthis, the voltage drop across the connectionleads is additionally included in the resistancecalculation. An excellent quality test leads will have actually aresistance of around 0.02Ω per meter.In enhancement to the resistance of the leads, theresistance that the lead link will additionally beincluded in the measurement and this have the right to beas high as or even higher in worth than theleads themselves.
When measuring larger resistance worths thisadditional lead resistance error have the right to beignored, yet as you can see indigenous the chartbelow, the error becomes significantly higheras the measured worth decreases, and totallyinappropriate below 10Ω.
Examples of possible measurement errors
|RX||Test lead resistance R1 + R2||Connection resistance R3 + R4||Rx measured in ~ DMM terminals = Rx + R1 + R2 + R3 + R4||Error||Error %|
|100 mΩ||0.04Ω||0.04Ω||180 mΩ||0.08Ω||80|
|10 mΩ||0.04Ω||0.04Ω||90 mΩ||0.08Ω||800|
|1 mΩ||0.04Ω||0.04Ω||81 mΩ||0.08Ω||8000|
To measure up true DC, resistance ohmmeterstypically usage 4 wire measurement. DC present ispassed with the Rx and through theohmmeter’s interior standard. The voltage acrossthe Rx and the inner standard is then measuredand the proportion of the two readings is supplied tocalculate the resistance. With this an approach thecurrent only requirements to be secure for the fewmilliseconds required for the ohmmeter come makeboth readings, yet it requires two measurementcircuits. The voltage measured is very little and aμV measure up sensitivity is commonly required.
Alternatively a constant current source is used topass a current through the Rx. The volt dropacross the Rx is climate measured and also the resistancecalculated. This an approach requires only onemeasurement circuit yet the current generator hasto be secure under all measurement conditions.
Four cable connectionThe 4 wire (Kelvin) method of measurement ispreferred for resistance values listed below 100Ω, and also allsmashville247.net milliohmmeters and microhmmeters usethis method. These dimensions are do using4 separate wires. 2 wires lug the current, knownas the resource or present leads and also pass currentthrough the Rx. The other 2 wires recognized as thesense or potential leads, are used to feeling thevoltage drop throughout Rx. Whilst some small currentwill flow in the sense leads, it is negligible and also canbe ignored. The volt drop throughout the ohmmeter’ssense terminals is as such virtually the very same as thevolt drop throughout Rx. This technique of measurementwill create accurate and consistent outcomes whenmeasuring resistances listed below 100Ω.
From a measurement allude of check out this is thebest kind of link with 4 different wires;2 existing (C and also C1) and 2 potential (P andP1). The existing wires must constantly be placedoutside the potential back the exactplacement is no critical. The potential wiresmust be linked exactly at the clues youwant to measure between. The measuredvalue will be in between the potential points.Whilst this gives the ideal measurement resultsit is frequently not practical. Us live in a no perfectworld and sometimes tiny compromises haveto be made, Cropico can offer a number ofpractical measurement solutions.
5.Methods the 4 terminal connections
Kelvin clipsKelvin clips are comparable to crocodile (Alligator)clips however with each jaw insulated native theother. The present lead is associated to one jawand the potential lead to the other. Kelvin clipsoffer a very practical systems to making a fourterminal connection to wires, busbars,plates etc.
Duplex HandspikesHandspikes offer another really practicalconnection solution specifically for sheetmaterial, busbars and also where accessibility can be aproblem. The handspike is composed of two sprungspikes attached in a handle. One spike is gift connection and the other is the potentialor feeling connection.
Stacked command connectionSometimes the only handy solution to makinga link to the Rx is to use stacking leads.The existing lead is pushed right into the back of thepotential lead. This technique will give tiny errorsbecause the measurement allude will be wherethe potential lead connects to the present lead.For measure up of awkward-to-reach samples,this can be the best compromise solution.
When measure up cables during manufacture,and for quality regulate purposes, the is necessaryto maintain regular measuring conditions. Thelength the the cable sample is usually 1 metreand to ensure that exact 1 metre lengths aremeasured, a cable clamp need to be used.Cropico market a variety of cable clamps i m sorry willaccommodate most cable sizes. The cable come bemeasured is placed in the clamp and the end ofthe cable are clamped in the existing terminals.The potential connection points are usually inthe form of knife edge contacts which space exactly1 metre apart.
Jigs and fixturesWhen measuring other components such asresistors, fuses, move contacts, rivets etc. Theimportance of utilizing a check jig to organize thecomponent can not be emphasised enough. Thiswill ensure the the measure up conditions, i.e.position of measure leads, space the same foreach component which will result in consistent,reliable and meaningful measurements. Jigsoften have to be specially design to suit theapplication.
6.Possible measurement errors
There space several feasible sources ofmeasurement error associated with lowresistance measurements. The most commonones are explained below.
Dirty connectionsAs through all measurements, that is important to ensurethat the machine you are connecting is clean and also freefrom oxides and dirt. High resistance connectionswill cause reading errors and may preventmeasurements. It should also be noted that somecoatings and oxides on materials are goodinsulators. Anodising has actually a very high resistance andis a classic example. Be sure to clean off the coatingat the link points. Cropico ohmmetersincorporate a command error warning which will indicateif the relationships are as well high in resistance.
Resistance the leads also highWhilst in theory the 4 terminal method ofmeasurement is unaffected by command length, caremust be required to ensure the the leads space not toohigh in resistance. The potential leads space not criticaland can usually be approximately 1kΩ there is no affecting themeasurement accuracy, but the existing leads arecritical. If the existing leads room too high in resistancethen the voltage drop across them will result ininsufficient voltage throughout the DUT (Device UnderTest) to do a sensible reading. Cropicoohmmeters check this compliance voltage acrossthe DUT and prevent a measurement native beingmade if it falls too low. A warning screen is alsoprovided; avoiding the reading, ensuring the falsemeasurements room not brought out. If you need touse long measuring leads, then increase thediameter of the cables to mitigate their resistance.
Measurement NoiseAs with any form of short voltage measurement, noisecan be a problem. Noise is created within check leadswhen they room in the affect of a magnetic fieldwhich is changing or the leader are moving withinthat field. To minimise this effect, leads have to bekept as quick as is practical, preserved still and also ideallyshielded. Cropico realises that there space manypractical limit on achieving this ideal, andhave because of this designed the circuits within theirohmmeters to minimise and also eliminate this effects.Thermal emfThermal emf in the DUT is most likely the biggestcause the errors in short resistance measurements.We must first understand what we typical bythermal emf, and how it is generated. Thermalemfs are small voltages which space generatedwhen two dissimilar metals are joined together,forming what is known as a thermocouplejunction. A thermocouple will generate one emfdepending ~ above the materials used in ~ thejunction and the temperature difference betweenthe hot and the reference, or cold, junction.
This thermocouple impact will present errorsinto the measurement if procedures are no takento compensate and eliminate this thermalemfs. Cropico microhmmeters andmilliohmmeters remove this impact byoffering an automatic typical mode because that themeasurement, sometimes called theswitched DC or median method. Ameasurement is made with the currentflowing in the forward direction climate asecond measure up is made with thecurrent in the reverse direction. The valuedisplayed is the average of these twomeasurements. Any thermal emf in themeasuring device will include to the firstmeasurement and be subtracted from thesecond; the resulting average valuedisplayed eliminates or cancels the thermalemf from the measurement. This methodgives the ideal results for resistive tons butis not an ideal for inductive samples suchas motor or transformer windings. In thesecases the ohmmeter is likely to switchcurrent direction prior to the inductance isfully saturated and the exactly measuredvalue will certainly not it is in achieved.
Measurement of joint resistance the 2 busbars
Wrong test CurrentConsideration should constantly be provided to theeffect the measurement existing will have onthe DUT. Tools with a little mass orconstructed with materials that have actually a hightemperature coefficient, such together thin strandsof copper wire, will need to be measuredwith the minimum current accessible to avoidheating. In these cases a solitary pulse ofcurrent may be appropriate to cause the veryminimum of heating. Need to the DUT besubject to the influences of thermal emf thenthe switched current technique describedearlier is appropriate. The Cropico DO5000series of ohmmeters have actually selectablecurrents indigenous 10% come 100% in 1% steps,plus a solitary pulse mode and consequentlymay be configured to suit many applications.
Temperature influencesIt is important to be aware that the resistanceof most materials will be affected by theirtemperature. It might be necessary, dependingupon the accuracy of measurement required,to control the environment in which themeasurement is made, for this reason keeping theambient temperature constant. This wouldbe the case when measure up resistancereference requirements which are measured in acontrolled laboratory at one of two people 20°C or 23°C.For dimensions where controlling theambient temperature is not possible, the ATC(automatic temperature compensation)facility have the right to be used. A temperature probe,connected come the ohmmeter, senses theambient temperature and the resistancereading is repair to a referencetemperature the 20°C. 2 of the mostcommon materials measured are copper andaluminium and also their temperature coefficientsare depicted opposite.
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The Temperature Coefficient of Copper (nearroom temperature) is +0.393 % per°C. Thismeans if the temperature boosts 1°C theresistance will increase 0.393%. Aluminiumis +0.4100 % per°C.