Campbell-scientific CR800 and CR850 Measurement and Control Systems Instrukcja Użytkownika Pobierz pdf (Strona 295)

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101

Section 8. Operation

295

• Effects due to the following are not included in the specification:

o Bridge-resistor errors

o Sensor noise

o Measurement noise

The ratiometric-accuracy specification is applied to a three-wire half-bridge

measurement that uses the BrHalf() instruction as follows:

TherelationshipdefiningtheBrHalf()instructionisX=V1/Vx,whereV1

isthevoltagemeasurementandVxistheexcitationvoltage.The

estimatedaccuracyofXisdesignatedas∆X,where∆X=∆V1/Vx.∆V1is

derivedusingthefollowingme

thod.

The ratiometric-accuracy specification is applied to a four-wire full-bridge

measurement that uses the BrFull() instruction as follows:

TherelationshipdefiningtheBrFull()instructionisX=1000*V1/Vx,

whereV1isthevoltagemeasurementandVxistheexcitationvoltage.

ResultXisexpressedasmV/V.EstimatedaccuracyofXis∆X,where∆X

=1000*∆V1/Vx.∆V1isderivedusingthefollowingmethod.

∆V1isderivedusingth

eratiometric‐accuracyequation.Thederivation

isillustratedinthisexample,whichissupportedbytheassumptionthat

themeasurementisdifferentialwithinputreversal,datalogger

temperatureisbetween‐25°to50°C,analog‐inputrangeis±250mV,V1

=110mV,andexcitationisre

versedduringtheexcitationphaseofthe

measurement.Theeffecteachassumptionhasonthemagnitudeof

∆V1inthisexampleisnotedinthefollowingfigure.

Figure 91: Deriving ∆V1

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