HART Transmitter Calibration Application Note · 3 Fluke Corporation Hart Transmitter Calibration It’s easy to calibrate and maintain HART instrumentation with one powerful tool

Application Note

HART® TransmitterCalibration

Why use “smart”instrumentation?Like most process plants, yourorganization is probably facing thedual challenges of maximizingproductivity while minimizingmaintenance costs. “Smart” digitaltransmitters offer superior perfor-mance and reliability, while savingtime and effort in maintenance andcalibration. Manufacturers of fieldinstruments have helped acceleratethe changeover by offering smarttransmitters at prices nearly as lowas analog units. As digitalinstruments using the HART protocolquickly become the standard,communicators and calibrators arebecoming essential everyday tools.

What is HART?HART, the Highway AddressableRemote Transducer protocol, uses a1200 baud Frequency Shift Keying(FSK) signal to superimpose digitalinformation on the conventional4-20 mA analog signal.

Why use the HARTprotocol?HART is an industry standarddeveloped to define thecommunications protocol betweenintelligent field devices and acontrol system, HART is the mostwidely used digital communicationprotocol in the process industry.More than five million HART fieldinstruments are installed in morethan 100,000 plants worldwide.The HART protocol:• Is supported by all of the major

suppliers of process fieldinstruments supported by theHART Communication Foundation,

New

Sof

twar

e

Vers

ion

2.5

2 Fluke Corporation Hart Transmitter Calibration

an industry-wide non-profitorganization. See the Web siteHYPERLINK http://www.hartcomm.org for informationon the HART standard.

• Preserves present controlstrategies.

• Allows traditional 4-20 mAsignals and digital communicationto share the same two-wire loops.

• Provides important information forinstallation and maintenance: TagIDs, measured values, range andspan data, product informationand diagnostics.

• Reduces operation costs bymaking it easier to manage andfully utilize “smart” instrumentnetworks.

HART calibrationis required!A common misconception is that theaccuracy and stability of HART in-struments eliminate the need forcalibration.Another misconception is that cali-bration can be accomplished by re-ranging field instruments using onlya HART communicator. Still anothermisconception is that the controlsystem can remotely calibrate smartinstruments. These are not true. Allinstruments drift. Re-ranging withjust a communicator is notcalibration. A precision calibrator orstandard is required. Regular perfor-mance verification with a calibratortraceable to national standards isnecessary due to:1. Shifts in performance of electronic

instruments over time, due toexposure of the electronics andthe primary sensing element totemperature, humidity, pollutants,vibration, and other field environ-mental factors.

2. Regulations governing occupa-tional safety, consumer safety,and environmental protection.

3. Quality programs such asISO 9000 standards for all instru-ments that impact product quality.

4. Commercial requirements such asweights, measures, and custodytransfer.

Regular calibration is also prudentsince performance checks will oftenuncover problems not directlycaused by the instrumentation, suchas solidified or congealed pressurelines, installation of an incorrectthermocouple type, or other errorsand faults.A calibration procedure consists of averification (As Found) test,adjustment to within acceptabletolerance if necessary, and a finalverification (As Left) test if an adjust-ment has been made. Data from thecalibration are collected and used tocomplete a report of calibration,documenting instrument perfor-mance over time.All instruments, even HARTinstruments, must be calibrated on aregular, preventive maintenanceschedule. The calibration intervalshould be set short enough to insurethat an instrument never drifts out oftolerance, yet long enough to avoidunnecessary calibrations. Alterna-tively, the interval may bedetermined by critical processrequirements, e.g., calibration beforeeach batch.

How are HART instrumentsproperly calibrated?To calibrate a HART instrumentconsistent with its application, it isvery helpful to understand thefunctional structure of a typical HARTtransmitter. The article in AppendixA, by Kenneth L. Holladay ofSouthwest Research Institute,describes a typical HART instrumentand defines both proper and im-proper calibration practices.Originally published in Intech, May1996, it is reprinted with permissionof the author.

Note: If you are unfamiliar with HARTcalibration or need a review, this is anexcellent point to stop and read the articlein Appendix A. It covers the basics of HARTinstrumentation and addresses issues critical toinstrument maintenance.

HART instruments consist of threedistinct sections (see Figure 1).Proper HART calibration may involveeither or both sensor trim and outputtrim. Adjusting range values (LRVand URV) without a calibrator is notcalibration. Performing an outputtrim while ignoring the input sectionis not proper calibration. Adjustingrange values with a calibrator maybe a practical calibration alternativefor instruments operated in 4-20 mAanalog mode, provided that the PVand PVAO are not used for processcontrol.

Figure 1


It’s easy to calibrate and maintain HART instrumentation with onepowerful tool.

With the 744 DPC, you can:

• Generate precision electrical, temperature, orpressure signals for analog stimulus or sensorsimulation.

• Simultaneously measure electrical, temperature,or pressure signals from transmitter output.

• Determine type, manufacturer, model, tag ID byinterrogating HART devices.

• Read HART PV function and smart transmitter digitaloutput while measuring analog mA output.

• Read and write HART configuration functions to makefield adjustments to PV range points, damping, andother top-level configuration settings.

• Change sensor configuration on supported temperaturetransmitters.

• Re-label smart transmitters by reading and writingHART tag and message fields.

• Clone additional transmitters by reading and storingbasic HART configurations.

• Perform automated HART sensor trim and output trimfor selected devices in conjunction with As Found/AsLeft tests.

• Perform loop test with simultaneous analog and digitalmA readout.

• Address new, fast, pulsed-excitation smart transmittersand PLCs.

• Control Hart Scientific Dry Block Calibrator.• No need for separate HART communicator.

744 upgrades available

Fluke periodically releases new internal software for theFluke 744. These upgrades include:• New revisions of previously supported instruments.• Device-specific command support for new instruments.• New HART communication capability.

The upgrade can be easily loaded from a PC to a 744.


Model Number

PV(Primary Variable)

PVAO(Digital representation

of the Primary VariableAnalog Output)

Analog Measure Value

Analog Source Value

Tag ID

PV LRV(Primary VariableLower Range Value)

PV URV(Primary VariableUpper Range Value)


Is there still a role for thecommunicator?Commissioning a HART instrument ormodifying HART variables notsupported by the 744 requires theuse of a communicator. The 744 isdesigned to perform the vastmajority of day-to-day operationsyou normally perform with aseparate communicator. The HARTcapability of the 744 is comparableto that of the model 275 HART com-municator, with the exception of theDD interpreter. While the DDinterpreter enables the 275communicator to read command setlibraries from any HART supplier, itoffers capabilities far beyond thosegenerally required for daily HARTinstrument maintenance.

HART calibrationapplicationsThe following examples demonstratehow the 744 makes HARTcalibration an efficient operation.The 744 enables easy hookup usingits HART cable, fast access to themost important HART data, automaticbranching to appropriate adjustmentchoices, automatic completion of testtemplates, and automatic fetchingand sending of analog readingsduring trim.

Versatile HART protocolsupportThe Fluke 744 supports thecommands contained in HARTprotocol version 5.7. With 2 MB ofmemory, the 744 supports asubstantial set of HART instructions:• Universal commands — provide

functions that are implemented inall field devices, for example, readmanufacturer and devicetype, read primary variable (PV), orread current output and percent ofspan

• Common practice commands —provide functions that are commonto many but not all field devices,for example read multiple vari-ables, set damping time, or performloop test

• Device-specific commands—provide functions that are uniqueto a particular field device, forexample sensor trim. The 744Version 2.0 supports these devices:

1Sensor Trim not supported

HART operating modessupported

• For Point to Point operation, themost commonly used mode,connects the 744 to a single HARTdevice in a 4-20 mA loop.

• In Multi-Drop mode, severalHART instruments can be bussedtogether. The 744 searches foreach, identifies addresses in use,and allows you to select theinstrument for calibration andrelated operations.

• In Burst Mode, the HARTinstrument transmits bursts ofdata without waiting to beinterrogated by a master unit. The744 can take transmitters out ofburst mode during test orcalibration, then later restorethem to burst mode.

Table 1

What’s new in Version 2.5

• Device-specific calibrationsupport for new instruments:• Micro Motion 2000, 2000 IS, 9701, 9712 and 9739 coriolis flow transmitters

• Fuji FCX and FCXA2 pressure and FRC temperature transmitter

• New features:• Support for New Hart Scientific dry blocks: 7103, 9007, 9011, 9023, 9103, 9105, 9107, 9122, 9127, 9132, 9133 and 9150

• Enhanced dry block delay setting for temperature switch testing

• Switch test without reset

Hart Applications


Example 1

Calibration of a Rosemount3051 HART Pressure Trans-mitterBasic connectionsThis example assumes thatthe transmitter is isolated from theprocess and is not electricallyconnected to a loop power supply.Make basic connections to the 3051per the diagram in Figure 4. Polarityof the HART communicationconnection is not important.A separate 250 ohm resistor is notnecessary because the 744 incorpo-rates a resistor in series with the24V loop supply through its mAjacks. The 3051 in this example isconfigured for psi units.

Figure 4

Figure 5

Figure 6

Procedure1. Power on the Fluke 744

Calibrator. Press the red keyfollowed by the Loop Powersoftkey and the 744 will displaythe basic HART information forthe 3051 (Figure 5).

2. Press the key again and youare prompted to select the 744configuration (Figure 6). SelectingMEAS mA, SOURCE psi willconfigure the calibrator to mea-sure the analog mA output andthe pressure being applied simul-taneously to the transmitter inputand the pressure module.(Selecting MEAS PV, SOURCE psiwill configure the 744 to evaluatethe digital PV output from thetransmitter.) Press ENTER to select.

ENTER


3. Vent the pressure line and pressCLEAR

(ZERO) to zero the pressure module.Press the As Found softkey, andthen press ENTER to select Instru-ment for a linear transmittercalibration. (If the 3051 isconfigured for square root output,select Instrument.) Notice thatthe calibration template is auto-matically completed with theexception of Tolerance. Fill in theappropriate test tolerance andpress Done.

4. Press the Manual Test softkey tobegin calibration. Apply the inputpressures as instructed in theSOURCE screen. Press the AcceptPoint softkey when the correctpressure is applied for each point.When the test is complete, theerror summary table is displayed(Figure 7). Test errors exceedingthe tolerance are highlighted.When done viewing the table,press the Done softkey. PressDone again to accept, or ENTER tochange the tag, serial number orID fields.

Figure 7

Figure 8

5. If the As Found test failed (i.e.,there were highlighted errors inthe error summary table),adjustment is necessary. Press theAdjust softkey. Select SensorTrim and press ENTER . (Do notselect Pressure Zero Trim. It isthe same as trimming the lowersensor point at zero, which isuseful for pressure transmittersthat do not offer Sensor Trim.)The 744 screen should look likeFigure 8.

Figure 9

7. Select Output Trim and pressENTER . The value of the primaryvariable (PVAO) is in the upperright corner of the display. This isnormally a4 mA signal. The mA value, asconstantly measured by the Fluke744, is in the center of thedisplay. Press the Fetch softkey toload the measured mA value.Press Send to send the value tothe 3051 to trim the outputsection for the 4 mA value. PressContinue for the 20 mA trim andrepeat this step.

8. After completing Output Trim,press the Done softkey andproceed with the As Leftverification test. Press the As Leftsoftkey. Press Done and thenpress Manual Test. Apply therequested pressures and pressAccept Point when the readingsare stable. On completion an errorsummary table is displayed. Ifnone of the errors are highlighted(Figure 9), the 3051 passes thecalibration test. If errors are high-lighted, the test has failed andfurther adjustment is required.Return to step 5 for adjustment ofthe 3051.

6. Select Perform user trim – bothand press ENTER . Zero the pressuremodule (vented to atmosphere) bypressing CLEAR

(ZERO) . Press the Continuesoftkey and you are prompted forthe Lower Trim value. For bestresults, apply the LRV pressureand press Fetch to load the valuebeing measured by the pressuremodule. Press Trim. Then pressContinue to move to the UpperTrim. As before, apply the URVpressure, press Fetch, and pressTrim. If the 3051 is used with thedigital PV output, skip to step 8and perform the As Left test. If the4-20 mA analog output is used inthe process, continue on to step 7.


Example 2

Calibration of a Rosemount3144 HART TemperatureTransmitterBasic connectionsThis example assumes that thetransmitter is isolated from theprocess and is not electricallyconnected to a loop power supply.Make basic connections to the 3144per the diagram in Figure 10.Polarity of the HART communicationconnection is not important.A separate 250 ohm resistor is notnecessary because the 744 incorpo-rates a resistor in series with the24V loop supply through its mAjacks. The 3144 in this example isconfigured for a type K thermo-couple sensor with a span of0-300 °C.

Figure 10

Procedure1. Power on the Fluke 744 Calibra-

tor. Press the red keyfollowed by the Loop Powersoftkey. Press ENTER to bypass thewarning screens and the 744 willdisplay the basic HART informa-tion for the 3144 (Figure 11).

Figure 11

2. Press the key again and youare prompted to select the 744configuration (Figure 12).Selecting MEAS mA, SOURCET/C typ K configures thecalibrator to measure the analogmA output of the transmitter andsource the correct temperaturestimulus at the 3144 input.(Selecting MEAS PV, SOURCET/C typ K will configure the 744to evaluate the digital PV outputfrom the transmitter.) Press ENTER

to select.

Figure 12


3. Press the As Found softkey, andthen press ENTER to selectInstrument for a linear transmit-ter calibration. Notice that thecalibration template isautomatically completed withthe exception of the Tolerance.Fill in the appropriate test toler-ance and press the Done softkey.

4. Press the Auto Test softkey tobegin calibration. Once the testis complete, an error summarytable is displayed (Figure 13).Test errors exceeding thetolerance are highlighted. Whendone viewing the table, pressthe Done softkey. Press Doneagain to accept, or ENTER tochange the tag, serial number orID fields.

5. If the As Found test failed (i.e.,there were highlighted errors inthe error summary table),adjustment is necessary. Pressthe Adjust softkey. Select SensorTrim and press ENTER . Select Per-form user trim – both and pressENTER . The 744 screen should looklike Figure 14.

Figure 13

6. For best results, press LRV toapply the LRV for the Lower Trimvalue. Press Trim and thenContinue to move to the UpperTrim. Press URV, press Trim, andthen press Done. If the 3144 isused with the digital PV output,skip to step 8 and perform the AsLeft test. If the analog 4-20 mAoutput is used in the process,continue on to step 7.

7. Select Output Trim and pressENTER . The value of the primaryvariable (PVAO) is in the upperright corner of the display. (Figure18). This is normally a 4 mAsignal. The mA value, asconstantly measured by the Fluke744, is in the center of the dis-play. Press the Fetch softkey toload the measured mA value.Press Send to send the value tothe 3144 to trim the outputsection for the 4 mA value. PressContinue for the 20 mA trim andrepeat this step.

Figure 14

8. After completing Output Trim,press the Done softkey andproceed with the As Leftverification test. Press the As Leftsoftkey. Press Done and thenpress Auto Test. On completion,an error summary table isdisplayed. If errors are high-lighted, the test has failed andfurther adjustment is required.Return to step 5 for adjustment ofthe 3144.

Figure 15

Figure 16


Example 3

Calibration of HARTinstruments usinguniversal commandsThe 744 supports a majority of theinstalled workload of HARTtransmitters – see Table 1 – bysupporting sensor trim, whichemploys device-specific commandsthat are unique to a particular in-strument. So how can you calibrateinstruments that are not supportedby the 744?The short answer is that the 744supports a substantial set of theuniversal HART commands and thecommon practice HART commands.The 744 can communicate withvirtually any HART instrument and,in most cases, can complete a cali-bration procedure (except for sensortrim for unsupported instruments).This example applies to instru-ments used in analog mode (4-20mA). If the instrument is operated indigital mode, i.e., its PV is the outputvariable that is used for control, acalibration of the Input Section is allthat is needed. Adjustment willrequire a Sensor Trim, (see Figure17) which means that for instru-ments not supported by the 744 youwill need to use both a 744 (toperform the As Found and As Lefttests and record the results) and acommunicator (to perform sensor trim).For instruments used in analogmode, i.e., where the 4-20 mAanalog output is used for control,the 744 can be used for calibration.After performing an As Found and

Figure 17

determining that adjustment isrequired, this example first performsan Output Trim to bring the instru-ment within tolerance. Failing that,the example performs an adjustmentto the Lower and Upper RangeValues (LRV and URV) to compensatefor input section error.

Note: Appendix A explains that these adjust-ments do not constitute a proper HART calibra-tion. While this is true, these adjustments are apractical calibration alternative forinstruments operated in 4-20 mA analog modeif error corrections are not large

How to determine digitalor analog?The transmitter is in digital mode ifits HART Poll Address is set between1 to 15. An address of 0 (zero) sets itto 4-20 mA analog output mode. The744 will automatically connect to adevice at address 0; if a device is notfound at 0 the 744 will begin polling

addresses 1 to 15. The 744 alsodisplays a non-zero address with thebasic HART information.

Basic connectionsThis example assumes that thetransmitter is isolated from theprocess and is not electricallyconnected to a loop power supply.Make basic connections to the trans-mitter per the diagram in Figure 18.Polarity of the HART communicationconnection is not important. A sepa-rate 250 ohm resistor is not neces-sary because the 744 incorporates aresistor in series with the 24V loopsupply through its mA jacks. Thisexample assumes a type K thermo-couple transmitter with an inputrange of 0-100 °C, 4-20 mA output,and a 0.25% test tolerance.

Figure 18


Procedure1. Power on the Fluke 744 Calibra-

tor. Press the key and theLoop Power softkey (if looppower is not already supplied).Press ENTER until any device warn-ings are cleared and the basicHART information is displayed(Figure 19).

2. Press the key again and youare prompted to select the 744configuration (Figure 20). Movethe cursor to MEAS mA, SOURCET/C typ K, and press ENTER . (If youwere verifying the digital PVinstead of the mA output, i.e., thetransmitter has a non-zero HARTpoll address, you would selectMEAS PV, SOURCE T/C typ Kinstead.)

Figure 19

Figure 20

3. Press the As Found softkey andpress ENTER to selectInstrument calibration. Move thecursor to Tolerance and ENTERthe appropriate test tolerance(0.25% in this example). Verifythat the 0% Value and 100%Value are the proper, nominaloperating values for the transmit-ter (0.0 °C and 100.0 °C in thisexample, Figure 21). If the Lower(0%) and Upper (100%) RangeValues (LRV and URV) have beenpreviously modified for calibrationpurposes, you will need to ENTERthe nominal values. For example,if a previous calibration modifiedthe URV to 100.2 °C, you need tomanually ENTER the nominalvalue of 100.0 °C for the 100%Value. Entering nominal zero andspan values ensures that errorsare calculated correctly.

Figure 21

4. Press Done and then press AutoTest. Once the test is complete,an error summary table isdisplayed (Figure 22). Test errorsexceeding the tolerance are high-lighted. If the test passed, i.e., ifno errors are highlighted, adjust-ment is not required.

If errors are highlighted, adjust-ment is necessary by performingan Output Trim. Press Done toleave the results screen, edit thetag, serial number or ID fields asnecessary, and press Done again.

5. Press the Adjust softkey, selectOutput Trim and press ENTER . Thevalue of the primary variable(PVAO) is in the upper right cornerof the display (Figure 23). This isnormally a 4 mA signal. The real-time mA value as measured bythe Fluke 744, is in the center ofthe display. Press the Fetchsoftkey to load the measured mAvalue. Press the Send softkey tosend the value to the transmitterto trim the output section for the 4mA value. Press Continue for the20 mA adjustment and repeat thisstep.

Figure 22

Figure 23


6. Now perform an As Left test.Press As Left, press Done, andthen press Auto Test. Oncompletion the error summarytable is displayed. If errors arehighlighted, the test has failedand further adjustment isrequired.

Note: If the failure error is large, sensor trimadjustment with a communicator may benecessary. Often, however, adjustment canbe accomplished with a 744 by modifyingthe LRV (Lower Range Value) and URV(Upper Range Value) to compensate forInput Section error.

7. In the case of a pressure trans-mitter that has on-board Zeroand Span adjustment buttons,calibration is easy. Simply applya calibrated source at the LRVand URV values and press therespective Zero and Spanbuttons on the transmitter. Thenverify the condition of thetransmitter by completing an AsLeft test as in step 6. Many HARTtransmitters do not have physicaladjustments and need either acommunicator or a Fluke 744 toadjust the LRV and URV values.For those cases, proceed to step8.

8. The error summary table(displayed from step 6) providesthe data necessary to make LRVand URV changes. Write downthe ERROR % values for thefailed 0% and 100% test points.(If the error summary table is nolonger displayed, you can usethe Review Memory softkey torecall the As Left data.)Return the 744 to the normalMeasure/Source screen display-ing the As Left softkey bypressing the Done softkey 3times.

9. Calculate the new LRV or URV bymultiplying the span by the errorin percent and adding that to theold value. If our example has thefollowing nominal Source values,errors, and old LRV and URVvalues:

Source Error% Old LRV/URV

0% Value 0.0 ºC 0.84% 0.4 ºC

100%Value 100.0 ºC -2.41% 102.0 ºC

Calculate the new LRV and URV asfollows:LRVnew = LRVold + (Span ××××× Error0%)

LRVnew = 0.4 ºC + (100.0 ºC × 0.84%)

LRVnew = 0.4 ºC + (100.0 ºC × 0.0084)

LRVnew = 0.4 ºC + 0.8 ºC

LRVnew = 1.2 ºC

URVnew = URVold + (Span ××××× Error100%)

URVnew = 102.0 ºC + (100.0 ºC ×–2.41%)

URVnew = 102.0 ºC + (100.0 ºC ×–0.0241)

URVnew = 102.0 ºC – 2.4 ºC

URVnew

= 99.6 ºC

10.Press and then press theSetup softkey. Select Basic fromthe menu and press ENTER to displaythe basic setup parameters shownin Figure 24. To ENTER the newLRV, move the cursor to LowerRange Value and press ENTER . Typethe new LRV and press ENTER . Alsotype in the new URV and pressENTER . Press the Send softkey.

Table 211.Now press Done and then press

Abort 3 times. Perform a new AsFound test by pressing As Found.(Remember to make sure that theoriginal, nominal zero and spanvalues are shown as the 0%Value and 100% Value.) PressDone and then press Auto Test.On completion, the error summarytable is displayed.If errors are highlighted, the testhas failed – repeat the adjustmentor trim sensor section with acommunicator.

Figure 24


Appendix A

Calibrating HARTTransmittersBy Kenneth L. Holladay, P.E.

Calibrating a conventionalinstrumentFor a conventional 4-20 mAinstrument, a multiple point test thatstimulates the input and measuresthe output is sufficient to haracterizethe overall accuracy of the transmit-ter. The normal calibrationadjustment involves setting only thezero value and the span value, sincethere is effectively only oneadjustable operation between theinput and output as illustratedbelow.

This procedure is often referred to asa Zero and Span Calibration. If therelationship between the input andoutput range of the instrument is notlinear, then you must know thetransfer function before you cancalculate expected outputs for eachinput value. Without knowing theexpected output values, you cannotcalculate the performance errors.

Calibrating a HARTinstrumentFor a HART instrument, a multiplepoint test between input and outputdoes not provide an accurate repre-sentation of the transmitter’s opera-tion. Just like a conventionaltransmitter, the measurementprocess begins with a technologythat converts a physical quantity intoan electrical signal. However, the

similarity ends there. Instead of apurely mechanical or electrical pathbetween the input and the resulting4-20 mA output signal, a HARTtransmitter has a microprocessor thatmanipulates the input data. Asshown in Figure A2, there aretypically three calculation sectionsinvolved, and each of these sectionsmay be individually tested andadjusted.Just prior to the first box, theinstrument’s microprocessormeasures some electrical propertythat is affected by the processvariable of interest. The measuredvalue may be millivolts, capacitance,reluctance, inductance, frequency,or some other property. However,before it can be used by the micro-processor, it must be transformedto a digital count by an analog todigital (A/D) converter.In the first box, the microprocessormust rely upon some form ofequation or table to relate the rawcount value of the electricalmeasurement to the actual property(PV) of interest such as temperature,pressure, or flow. The principle formof this table is usually established bythe manufacturer, but most HARTinstruments include commands toperform field adjustments. This isoften referred to as a sensor trim.The output of the first box is a digitalrepresentation of the process vari-able. When you read the processvariable using a communicator,this is the value that you see.The second box is strictly a

Figure A2HART Transmitter Block Diagram

mathematical conversion from theprocess variable to the equivalentmilliamp representation. The rangevalues of the instrument (related tothe zero and span values) are usedin conjunction with the transferfunction to calculate this value.Although a linear transfer function isthe most common, pressuretransmitters often have a square rootoption. Other special instrumentsmay implement commonmathematical transformations or userdefined break point tables. Theoutput of the second block is adigital representation of the desiredinstrument output. When you readthe loop current using a communica-tor, this is the value that you see.Many HART instruments support acommand which puts the instrumentinto a fixed output test mode. Thisoverrides the normal output of thesecond block and substitutes aspecified output value.The third box is the output sectionwhere the calculated output value isconverted to a count value that canbe loaded into a digital to analogconverter. This produces the actualanalog electrical signal. Once againthe microprocessor must rely onsome internal calibration factors toget the output correct. Adjustingthese factors is often referred to as acurrent loop trim or 4-20 mA trim.

Figure A1Conventional Transmitter Block Diagram


HART calibrationrequirementsBased on this analysis, you can seewhy a proper calibration procedurefor a HART instrument is significantlydifferent than for a conventionalinstrument. The specific calibrationrequirements depend upon theapplication.If the application uses the digitalrepresentation of the processvariable for monitoring or control,then the sensor input section mustbe explicitly tested and adjusted.Note that this reading is completelyindependent of the milliamp output,and has nothing to do with the zeroor span settings. The PV as read viaHART communication continues tobe accurate even when it is outsidethe assigned output range. For ex-ample, a range 2 Rosemount 3051chas sensor limits of -250 to +250inches of water. If you set the rangeto 0 - 100 inches of water, and thenapply a pressure of 150 inches ofwater, the analog output will saturateat just above 20 milliamps. However,a communicator can still read thecorrect pressure. If the current loop output is not used(that is the transmitter is used as adigital only device), then the inputsection calibration is all that is re-quired. If the application uses themilliamp output, then the outputsection must be explicitly tested andcalibrated. Note that this calibrationis independent of the input section,and again, has nothing to do withthe zero and span settings.

Calibrating the inputsectionThe same basic multiple point testand adjust technique is employed,but with a new definition for output.To run a test, use a calibrator tomeasure the applied input, but readthe associated output (PV) with acommunicator. Error calculations aresimpler since there is

always a linear relationship betweenthe input and output, and both arerecorded in the same engineeringunits. In general, the desiredaccuracy for this test will be themanufacturer’s accuracy specification.If the test does not pass, then followthe manufacturer’s recommendedprocedure for trimming the inputsection. This may be called a sensortrim and typically involves one ortwo trim points. Pressure transmit-ters also often have a zero trim,where the input calculation isadjusted to read exactly zero (notlow range). Do not confuse a trimwith any form of re-ranging or anyprocedure that involves using zeroand span buttons.

Calibrating the outputsectionAgain, the same basic multiple pointtest and adjust technique isemployed, but with a new definitionfor input. To run a test, use a com-municator to put the transmitter intoa fixed current output mode. Theinput value for the test is the mAvalue that you instruct the transmit-ter to produce. The output value isobtained using a calibrator to mea-sure the resulting current. This testalso implies a linear relationshipbetween the input and output, andboth are recorded in the same engi-neering units (milliamps). The de-sired accuracy for this test shouldalso reflect the manufacturer’s accu-racy specification.If the test does not pass, then followthe manufacturer’s recommendedprocedure for trimming the outputsection. This may be called a 4-20mA trim, a current loop trim, or a D/Atrim. The trim procedure shouldrequire two trim points close to orjust outside of 4 and 20 mA. Do notconfuse this with any form of re-ranging or any procedure thatinvolves using zero and spanbuttons.

Testing overall performanceAfter calibrating both the Input andOutput sections, a HART transmittershould operate correctly. The middleblock in Figure A2 only involvescomputations. That is why you canchange the range, units, and transferfunction without necessarilyaffecting the calibration. Notice alsothat even if the instrument has anunusual transfer function, it onlyoperates in the conversion of theinput value to a milliamp outputvalue, and therefore is not involvedin the testing or calibration of eitherthe input or output sections.If there is a desire to validate theoverall performance of a HARTtransmitter, run a Zero and Span testjust like a conventional instrument.As you will see in a moment, how-ever, passing this test does not nec-essarily indicate that the transmitteris operating correctly.

Effect of damping on testperformanceMany HART instruments support aparameter called damping. If this isnot set to zero, it can have anadverse effect on tests and adjust-ments. Damping induces a delaybetween a change in the instrumentinput and the detection of thatchange in the digital value for theinstrument input reading and thecorresponding instrument outputvalue. This damping induced delaymay exceed the settling time used inthe test or calibration. The settlingtime is the amount of time the test orcalibration waits between settingthe input and reading the resultingoutput. It is advisable to adjust theinstrument’s damping value to zeroprior to performing tests oradjustments. After calibration, besure to return the damping constantto its required value.


Operations that are NOTproper calibrationsDigital range changeThere is a common misconceptionthat changing the range of a HARTinstrument by using a communicatorsomehow calibrates the instrument.Remember that a true calibrationrequires a reference standard,usually in the form of one or morepieces of calibration equipment toprovide an input and measure theresulting output. Therefore, since arange change does not referenceany external calibration standards, itis really a configuration change, nota calibration. Notice that in the HARTtransmitter block diagram (Figure 2),changing the range only affects thesecond block. It has no effect on thedigital process variable as read by acommunicator.

Zero and span adjustmentUsing only the zero and spanadjustments to calibrate a HARTtransmitter (the standard practiceassociated with conventionaltransmitters) often corrupts theinternal digital readings. You maynot have noticed this if you neveruse a communicator to read therange or digital process data. Asshown in Figure 2, there is morethan one output to consider. Thedigital PV and milliamp values readby a communicator are also outputs,just like the analog current loop.Consider what happens when usingthe external zero and span buttonsto adjust a HART instrument.Suppose that an instrument technicianinstalls and tests a differentialpressure transmitter that was set atthe factory for a range of 0 to 100inches of water. Testing thetransmitter reveals that it now has a1 inch of water zero shift. Thus with

both ports vented (zero), its output is4.16 mA instead of 4.00 mA, andwhen applying 100 inches of water,the output is 20.16 mA instead of20.00 mA. To fix this he vents bothports and presses the zero button onthe transmitter. The output goes to4.00 mA, so it appears that the ad-justment was successful.However, if he now checks thetransmitter with a communicator, hewill find that the range is 1 to 101inches of water, and the PV is 1 inchof water instead of 0. The zero andspan buttons changed the range (thesecond block). This is the only actionthat the instrument can take underthese conditions since it does notknow the actual value of the refer-ence input. Only by using a digitalcommand which conveys thereference value can the instrumentmake the appropriate internal ad-justments.The proper way to correct a zeroshift condition is to use a zero trim.This adjusts the instrument inputblock so that the digital PV agreeswith the calibration standard. If youintend to use the digital processvalues for trending, statistical calcu-lations, or maintenance tracking,then you should disable the externalzero and span buttons and avoidusing them entirely.

Loop current adjustmentAnother observed practice amonginstrument technicians is to use ahand-held communicator to adjustthe current loop so that an accurateinput to the instrument agrees withsome display device on the loop. Ifyou are using a Rosemount model268 communicator, this is a “currentloop trim using other scale.” Referagain to the zero drift example justbefore pressing the zero button.

Suppose there is also a digital indi-cator in the loop that displays 0.0 at4 mA, and 100.0 at 20 mA. Duringtesting, it read 1.0 with both portsvented, and it read 101.0 with 100inches of water applied. Using thecommunicator, the technician per-forms a current loop trim so that thedisplay reads correctly at 0 and 100,essentially correcting the output tobe 4 and 20 mA respectively.While this also appears to besuccessful, there is a fundamentalproblem with this procedure. Tobegin with, the communicator willshow that the PV still reads 1 and101 inches of water at the testpoints, and the digital reading of themA output still reads 4.16 and 20.16mA, even though the actual output is4 and 20 mA. The calibrationproblem in the input section hasbeen hidden by introducing acompensating error in the outputsection, so that neither of the digitalreadings agrees with the calibrationstandards.

As published in Intech, May 1996 and also inHART Book 8, July 1998. Reprinted with thepermission of the author.


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Ordering InformationFLUKE-744 Documenting Process Calibrator-HARTIncludes: TL24 Industrial Test Leads (two sets), AC20Test Clips (2 sets), TP20 Test Probes (1 set), BP7235NiMH Battery Pack, BC7217 Battery Charger, serial portcable, HART communications cable, DPC/TRACKSample with free PC communication utility software,Instruction Manual, HART User’s Manual, NIST-traceable calibration certificate and data, three-yearwarranty.

Optional Accessories

Fluke-700 Pxx Pressure ModulesIncluded with each Fluke PressureModule: BSP-ISO to NPT Adapter(s)(except with P29 - P31), InstructionSheet, NIST traceable calibrationreport and data, one-year warranty.

Fluke-700BCW Bar Code WandFluke-700PTP Pneumatic Test Pump, 360 psi/25 barFluke-700TC1 TC Mini-Plug Kit, 9 typesFluke-700TC2 TC Mini-Plug Kit, JKTERSBE9005 Battery EliminatorBC7217 Battery ChargerBP7217 NiCd Battery PackBP7235 NiMH Battery PackC700 Hard Carrying CaseC781 Soft Carrying CaseC789 Soft Carrying CaseFluke-744V20 HART Upgrade Revision 2.3

Documentation of resultsThe scheduling of calibrations, creation of proceduresand documentation of your calibrationresults are facilitated by a number of instrumentationmanagement software packages:

Fluke DPC/TRACK ™

PRM (Plant ResourceManager) from YokogawaElectric Corporation.

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HART Transmitter Calibration Application Note · 3 Fluke Corporation Hart Transmitter Calibration It’s easy to calibrate and maintain HART instrumentation with one powerful tool