Application Note

ManualsBrandsFluke ManualsCalibratorsFluke 718Ex

The Fluke 707Ex is an intrinsically safe loop

calibrator

Loop calibration

and maintenance

Introduction

Process instrumentation

requires periodic calibration

and maintenance to ensure that

it is operating correctly. This

application note contains

infor

mation to help g

uide tech-

nicians

through some of the

more common loop calibration

tasks using Fluke tools.

Fluke loop calibrators are

ideal for a wide variety of cali-

bration applications. They

include:

•

Fluke 707 and 707Ex

Loop

Calibrators

•

Fluke 705 Loop Calibrator

•

Fluke 715 Volt/mA Calibrator

•

Fluke 787 and 789

ProcessMeter™ tools

A summary of the features of

Fluke’s loop calibrators and a

glossary of terms are at the end

of this application note.

Field testing loop

powered isolators

The loop-powered isolator and

the two-wire isolating trans-

mitter are two of the more

prevalent dev

es in use in

4-20 mA control loops today

(see Figure 1). The testing and

troubleshooting proc

edures for

each are different and need to

be understood by the techni-

cians performing operational

checks on these units in the

field.

Loop-powered isolators

The main purpose of a loop

isolator is to eliminate ground

loops in control systems while

sending the control signal

current to another part of the

system

. L

oop-powered

isolators, unlike two-wire

transmitters, draw their operat-

ing power from the “input” side

of the isolator (see Figure 1),

which requires a pickup voltage

from 5.5 V to 13.5 V depending

on the manufacturer.

The output of the loop isola-

tor is an electrically isolated

mirror image of the input side

current

. The c

omplianc

e voltage

assoc

iated w

ith the output is

greatly reduced from that of the

input side and ranges around

7.5 V. This produces a total loop

loading capability of 350 Ω.

This limited loop drive capabil

ity is the primar

y limitation of

the loop isolator.

Two-wire transmitters

Isolating two-wire transmitters

provide the same isolating

functionality as loop isolators

with the added advantage that

many provide signal condition-

ing for a variety of inputs such

as thermocouple, frequency, dc

current, RTD, strain gauge, and

other process inputs.

The transmitter uses supply

power to convert those input

signals to standard 4 to 20 mA

signals for the transmitter

output to the loop. Typical

power supplies for two-w

ire

transmitters are 24 V or 48 V.

ower supplies of this size

allow a sig

nificant loop load

capability on the output.

Output

from

2-wire

transmitter

Loop-powered isolator

Two-wire transmitter

Loop

power

supply

Input types; typical

Thermocouple

Frequency

RTD

DC volts

AC volts

DC current

AC current

Strain gauge

Resistance

Loop power

supply

24 V–48 V

Figure 1

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Summary of content (8 pages)

PAGE 1
Loop calibration and maintenance Application Note Introduction Process instrumentation requires periodic calibration and maintenance to ensure that it is operating correctly. This application note contains information to help guide technicians through some of the more common loop calibration tasks using Fluke tools. Fluke loop calibrators are ideal for a wide variety of calibration applications.
PAGE 2
Field checking a loop-powered isolator Fluke loop calibrators have a unique current simulation feature that, when connected to an external power source, allows you to precisely control current between 0 mA and 24 mA. When field checking a loop-powered isolator, the twowire loop transmitter supplying signal current to the isolator for the loop may be removed and the calibrator connected in simulate mode to control loop current (Figure 2).
PAGE 3
Fluke 707 Positioner Flow Figure 3 Step 3: Span (full open) position check Using the 25 % button, step the calibrator output value to 20 mA and allow the valve to stabilize. Step the input to 20.1 mA using the vernier knob in the depressed position turning clockwise while watching or feeling for movement of the valve stem. Minimize this movement using the span adjustment on the valve positioner. Using the vernier knob in the depressed position, adjust current up and down between 20.10 mA and 19.9 mA.
PAGE 4
Calibrating voltage input signal conditioners The precision current sourcing and simulation capabilities of Fluke loop calibrators make them ideal for calibrating many 4-20 mA signal conditioners. However, there are many signal conditioners that require a precision voltage source for proper calibration. Using a simple precision resistor and standard connector, Fluke loop calibrators can field calibrate many standard and non-standard voltage input signal conditioners.
PAGE 5
Calibrating the signal conditioner The following is a procedure for calibrating a 1-5 V input, 420 mA output signal conditioner using the precision current shunt constructed in Figure 5. Step 1: Setting the Fluke 707 to source 4-20 mA 1. With the shunt assembly in the “source” jacks, power the 707 on. 2. Check the display. If the display does not read 4 mA, turn the 707 off and on again while holding the mode button for two seconds. The display should now read 4.000 mA. Step 2: Calibrating 1.
PAGE 6
2. Set the source current of the 707 to 20 mA using the 0-100 % button. Note the “output” meter display reading. Adjust the span control of the signal conditioner one-half the delta from 20 mA. Example: If the output display reads 21 mA, adjust the span control to 20.5 mA (one-half the delta to the required value of 20 mA). 3. Repeat this “one-half step” process until the required output is obtained. There are signal conditioners that have non-interactive controls that do not require this procedure.
PAGE 7
Fluke Loop Calibrator Considerations and Selection Fluke loop calibration tools have a variety of different performance features and capabilities that differentiate the models.
PAGE 8
Glossary of terms 4-20 mA loop A process control circuit in which the devices communicate process variable signals and control signals in a range from a low of 4 mA to a high of 20 mA. This corresponds to the older pneumatic system, which ranged from 3 to 15 PSI. Compliance voltage The voltage a current source develops when attempting to drive a mA signal through a resistive load. DMM Digital multimeter. A meter that measures multiple electrical quantities, such as voltage, current, and resistance.