8/2/2019 1989137_b_w_fluke

1/5

Application Note

Tracking down problems withina process loop can be a difficultchallenge. However, properlyapplying the right tools will allowyou to resolve most problemsefficiently. This application notewill demonstrate some practicalapplications of process meters,loop calibrators, and digital multi-meters (DMMs) to troubleshootprocess loop problems.

Loop calibrators

Loop calibrators, such as the Fluke705, 715, 707 and the intrinsi-cally safe 707Ex can temporarilyreplace the power source in acurrent loop. With the calibratorin control of the current, you canaccurately set the currentbetween 4 and 20 mA. Thisallows you to control the loop

Troubleshootingprocess loops

Theres far more you can do, butlets look more closely at the pre-cision resistor technique.

Placing a precision resistoracross the output leads of a loopcalibrator creates a voltage acrossthe resistor. You can control thisvoltage by varying the output ofthe calibrator. For example, plac-ing a 250 resistor across thesource output jacks and driving itwith a 4 to 20 mA current willproduce 1 to 5 V across the resis-tor. Place this voltage on theinput of a signal conditioner, andyou create a test system to setlinearity as well as the zero andspan points. With a DMM orProcessMeter (e.g., the Fluke789), you can measure the outputof the signal conditioner andensure it correctly matches thecorresponding input voltage.

ProcessMeter

The Fluke 789 is a loop cali-brator and true-rms DMM in onepackage. Although not as accu-rate as other Fluke loopcalibrators, the 789 works well asa current source in most trou-bleshooting situations. You canstill control devices on a currentloop to see if they are function-ing, and set them to variouslevels to aid in locating problems.

The DMM measurement capa-

bilities within the 789 make it avaluable tool in troubleshootingprocess loops. You can use thebasic DMM measurement func-tions like volts ac and dc, currentac and dc, resistance, and fre-quency at different stages oftroubleshooting.

F r o m t h e F l u k e D i g i t a l L i b r a r y @ w w w . f l u k e . c o m / l i b r a r y

and the devices connected to it which means you can control andcalibrate actuators and indicatorsin that loop. For example, you cansupply a 20 mA signal to a valvepositioner and observe the valverespond it will move to a fullyopen or fully closed position(depending on the configuration).Or, you can supply a 12 mA sig-nal to a panel-mounted levelindicator and see if it indicates50 % of scale.

But, you can do far more thansingle-point checks. For example,suppose you want to calibrate acontrol valve and positioner.Typically, you would first cali-brate the positioner for properoutput at each step, then use thepositioner to drive and calibratethe valve. You can use the cali-brator to provide positioner inputcurrent corresponding to the typi-cal 0 %, 25 %, 50 %, 75 %, and100 % control valve calibrationpositions. At each of these stepinputs, you can check the posi-tion of the valve and make anyneeded adjustments. You can alsodetermine if the valve is openingproperly, rather than jumping offits seat. And, you can determineif it is seating properly to beginwith.

Suppose you want to simulatean output from a signal condi-

tioner that has a voltage input(from, for example, a thermocou-ple). You can use a calibrator tosimulate that input and calibratethe signal conditioner. You canalso simulate the signal condi-tioner itself, regardless ofwhether it operates with voltageor with current. If it operateswith voltage, you can use a pre-cision resistor with the calibratorto generate accurate voltages.

8/2/2019 1989137_b_w_fluke

2/5

Another feature of the 789that comes in handy is themin/max function, which can beused to determine the range ofvalues a current loop experiences

over a period of time. If the cur-rent loop has a dropping resistoracross which we can make avoltage measurement, place the789 in volts dc and connect theleads across the dropping resistor(Figure 1). This will give you avoltage reading that will vary inproportion to loop current. If adropping resistor is not available,you can open the current loopand use the ProcessMeters cur-rent jacks to close the loop(Figure 2).

With the Fluke 789 in the

proper function and connected tothe circuit, press the mix/maxbutton to activate the mode. TheFluke 789 will look at eachmeasurement and test whether itis higher than the highest read-ing it has measured so far, orlower than the lowest measure-ment. If it detects a new high orlow, the meter will store thatvalue. This process continuesuntil the meter is shut off oranother function selected. Whilein the min/max mode, selectingthe min/max button will cycle the

display through the high, low,and average readings stored inthe ProcessMeter.

The Fluke 789 has an on-boardloop power supply. This allows adevice to be powered independ-ently of the in-house supply. Thatway, you can either eliminate itin troubleshooting or power andtest a device when a loop supplyisnt available. In Figure 2, bymoving the test lead from B to Cand enabling the transmitter 24 Vsupply, the 789 will measure themA output of the transmitter.

Digital multimeters

The Fluke 189, besides being anextremely accurate DMM, has thecapability to store time stampedmeasurements taken periodically,or in a series. These readings canthen be used for future analysis,reference, and process documen-tation. Like the min/max mode ofthe Fluke 789, this can be helpfulin looking for values on a loopthat are unexpected and thatoccur intermittently. However,

this DMM takes the min/maxrecord method to the next level.

PROCESSMETER789

%STEP COARSE FINESpanCheck

mA

mA

mA

mA

mV

V

V

A

OUTPUT

LOOPPOWER

250

HART

OFF

mA

SOURCEOUTPUT 0-24mA

SIMULATE

A COM V

MINMAX100%

0%

HOLD

REL Hz

RANGE

24 VdcLoop Supply

TESTDC PWR++

TemperatureTransmitter

Thermocouple

MeasuredProcess

250

1-5 Vdc

mA Indicator

ProcessIndicator

Figure 1. Meter across dropping resistor.

PROCESSMETER789

%STEP COARSE FINESpanCheck

mA

mA

mA

mA

mV

V

V

A

OUTPUT

LOOPPOWER

250

HART

OFF

mA

SOURCEOUTPUT 0-24mA

SIMULATE

A COM V

MINMAX100%

0%

HOLD

REL Hz

RANGE

TESTDC PWR++

TemperatureTransmitter

24 VdcLoop Supply

B

Thermocouple

MeasuredProcess

mA Indicator

Process Indicator

C A

Figure 2. Meter in series with loop.

2 Fluke Corporation Troubleshooting process loops

8/2/2019 1989137_b_w_fluke

3/5

LoggingThe Fluke 189 can be set torecord a series of intervalsinstead of a single interval ofhigh and low readings. In addi-

tion, these intervals are timestamped in real time so you cantell exactly when a min/maxperiod took place. However, toview all the data from theselogged measurements, you musthave access to a PC and use soft-ware to transfer and view thedata.

The Fluke 189s logging fea-ture gathers and records highand low readings over a period oftime which will be referred tosimply as a period. The begin-ning and ending of this period

can be triggered in two differentways; either by a set time period(interval logging), or by one oftwo events (event logging).

Interval loggingThrough the Fluke 189s setupfeature, you can set the timeinterval, or period that high-lowdata is to be collected, from 1second to 99 minutes and 59seconds. If, for instance, you setthe logging interval to five min-utes, from the instant you startthe logging process, the highestand lowest readings over thenext five minutes are recorded.The average of all readings takenduring that five-minute period iscalculated and recorded as well.At the end of that period, anotherfive-minute period starts record-ing. This continues until the DMMhas no more memory to storedata or the logging process is ter-minated. After the loggingsession, data is downloaded to aPC for storage and analysis.

Event loggingAs mentioned earlier, two events

control the measurement periodfor event logging. One is a sta-ble measurement and the otheris an unstable measurement.

A stable period will continueto be stable if the input signaldoes not vary more than 4 per-cent from the input signalamplitude at the beginning of astable period. If during this timethe input signal jumps or driftsoutside the 4 percent windowand is detected by the meter as

being outside the window, themeter will end that stable periodand log (record) the input sig-nals high, low, and averagevalues for that stable period of

time.The meter will then attemptto start another stable period. Ifthe meter finds the input signalcannot stay within the 4 per-cent window after trying to starta new stable period, it thendefines a period of time asunstable.

Logging a process loopAs before, we can either placethe Fluke 189 across a droppingresistor placed in the currentloop, or break the circuit and

place the DMMs current jacks inseries with the current loop,making sure to select the voltageor current function as appropri-ate. If we want to conductinterval logging, we use logginginterval through the DMMs setupfunction to set the desired timeinterval. If we are only interestedin event logging, then we simplyturn off the interval logging bysetting the interval to 00:00, andactivate the logging session bystarting the logging function.

At this point we can leave theDMM running and come back ata later time to retrieve thelogged data. With interval log-ging, the DMM can store up to

288 periods. Of course, how longthat is will depend on the inter-val. For instance, an interval offive minutes gives 24 hours oflogging, while 15 minute inter-vals allows for 72 hours. Eventlogging capability is determinedby the number of stable andunstable periods detected.

FlukeView FormsWith FlukeView Forms loaded ona PC, you can transfer loggingand measurement data from theDMM to the PC and display thatdata in a graphical as well astabular form. This software pack-age is ideal for documentingmeasurements taken by theFluke 180 Series DMMs.

Shown in Figure 3 is the tab-ular presentation of a five-minutelogging session on a currentloop. For this session, intervallogging was used and the inter-val period was set to tenseconds. You will see that eachrecorded period has a start time,duration, high value, averagevalue, low value, a description,and a stop time. The recordedtimes are in real time or time-of-day, not elapsed time.

Figure 3. Five-minute logging session on a current loop.

Troubleshooting process loops Fluke Corporation 3

8/2/2019 1989137_b_w_fluke

4/5

FlukeView Forms creates agraph of the recorded data aswell. The graph is a series ofrectangles strung together witheach rectangle representing a

period. The top of each rectanglerepresents the maximum valuewhile the bottom is the minimumvalue. A bar across the inside ofa rectangle represents the aver-age value. This gives us a visualapproximation of the data col-lected by the DMM and makes iteasy to spot any anomalies in thecurrent loop measurement.

The graph in Figure 4 is setup to show just the intervalperiods. Youll see that the valuechanges very little with theexception of the time period

around the 2:35:58 mark. Thatperiod had a large variationoccur which is not typical of atemperature measurement loop.By adding the events loggingperiods, we see that the anomalyis very short in duration. Lookingat the event in the tabular form(Figure 5) shows the transitionhappened in a 3.1 second period.

Figure 4. Graphical representation of logging.

Figure 5. Tabular representation of logging.

4 Fluke Corporation Troubleshooting process loops

8/2/2019 1989137_b_w_fluke

5/5

5 Fluke Corporation Troubleshooting process loops

AC and DC readingsThe Fluke 189 has the ability todisplay both the ac and dc com-ponents of a signal simultane-ously. In addition, this DMM will

combine the two components anddisplay the result as a true-rmsvalue. This gives us the capabilityof measuring exactly what adevice on a process loop sees fora signal. This can be handywhen were looking for noise orcurrent loop interference.

As before, you either measureacross a dropping resistor orinsert the meter into the currentloop. Now you switch the DMMto either dc volts or dc amps asappropriate. The DMM nowshows the current flowing in

the loop (if measuring acrossdropping resistor, the voltagerepresents the current in theloop). Next, you press the bluefunction button to switch thefunction to ac + dc. The displayof the DMM now shows twoseparate readings. The large, orprimary, digital display indicatesthe dc measurement while thesmaller, or secondary, display isthe true-rms ac measurementriding on the dc signal.

Pressing the blue function but-ton again swaps the two digitaldisplays. Pressing it a third time,combines the two voltages into atrue-rms value of the total signal.

The ac + dc feature of theFluke 189 can help us track downnoise or EMI signals on thecurrent loop. As long as the acsignal stays relatively small incomparison to the dc signal, allworks well. If the ac signal startsapproaching .5 mA, then youmay have a noise or interferenceproblem to resolve.

Summary

As this application note hasdemonstrated, you can use sev-eral different tools to troubleshootprocess loops. Calibrators arehandy for calibrating processloops, and they can help youtroubleshoot typical problems.An advanced DMM brings yourtroubleshooting abilities to thenext level. The ProcessMetergives you both the calibrator andthe DMM in one instrument.

Fluke CorporationPO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.PO Box 1186, 5602 BDEindhoven, The Netherlands

For more information call:In the U.S.A. (800) 443-5853 orFax (425) 446-5116In Europe/M-East/Africa (31 40) 2 675 200 orFax (31 40) 2 675 222In Canada (800) 36-FLUKE orFax (905) 890-6866From other countries +1 (425) 446-5500 orFax +1 (425) 446-5116

Web access: http://www.fluke.com

2004 Fluke Corporation. All rights reserved.Printed in U.S.A. 7/2004 1989137 A-ENG-N Rev C

Fluke.Keeping your worldup and running.