Fluke 773 Milliamp Application Note

: Fluke Fluke-773-Milliamp-Application-Note-807505 fluke-773-milliamp-application-note-807505 fluke pdf

Open the PDF directly: View PDF .
Page Count: 2

Troubleshooting 4 to 20 mA

process control systems

without breaking the loop

Where pH is important

Anyone who works in any kind

of manufacturing plant knows

that nothing can ruin a day faster

than unscheduled downtime.

The lost production time means

lower output, which can have

a sizable price tag. In a process

plant that lost productivity can

be magnified because an entire

batch of a product could be

ruined if the outage occurs at a

critical point in the process.

Keeping processes up and

running properly requires

efficient and accurate trouble-

shooting tools. An instrument

technician in the oil, gas, and

chemical industry for the last 15

years knows that only too well.

He’s responsible for maintaining

and troubleshooting pressure,

flow, temperature, and pH trans-

mitters in a chemical processing

plant. As you might expect, pow-

ering down the equipment is the

option of last resort.

He has been using Fluke tools

since he first started in the field.

“When you’re working in process

control, you need to have equip-

ment that you trust; I don’t know

an instrument tech who doesn’t

almost exclusively use Fluke for

all of their test equipment.”

When he saw the Fluke 773

Milliamp Process Clamp Meter at

a trade show in early 2011, he

saw huge possibilities for using

it in the plant where he works.

The 773 measures milliamp (mA)

signals without breaking the

loop, which makes it ideal for

troubleshooting the transmitters,

valves, and programmable logic

controllers (PLCs) found in pro-

cess plants. It also sources and

measures dc voltage, so it can be

used to troubleshoot voltage input

and output devices. The clamp

section of the meter is detach-

able and can be used as a remote

jaw, connected to the main body

by an extension cable to make it

easier to take measurements in

tight spaces.

Finding intermittent

problems without break-

ing the loop

“The biggest thing I’ve found

with the 773 is that I can

troubleshoot a live device without

having to power down, and pos-

sibly miss something going on in

the process. When you’re dealing

with chemicals you can have an

ongoing problem that’s very quick

and intermittent and eventually

leads to a single major break-

down. I can take the 773 to the

process instrument, hook it up,

observe the output, and compare

it to an indicator. It can eliminate

the transmitter as a point of error

The clamp section of the Fluke 773 clamp meter is detachable, connected to the main body of

the test tool by an extension cable.

in the loop using mA simulate,

and the clamp meter provides

its own 24-volt loop power for

performing substitution and isola-

tion tests.”

Before using the 773, he had

to go to the power supply, de-

energize the circuit, go back to

the instrument, hook the meter

up in series, and then go back

and re-energize the circuit and

do the calibration check. “Every

time I don’t have to power down

an instrument, it probably saves

me three to four minutes per

instrument, and that adds up

because every hour that one of

our processes is down runs sev-

eral thousand dollars.”

Application Note

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

2 Fluke Corporation Inrush current

Beyond the litmus test

Most often he uses the 773 on pH

transmitters. “Our process is very

tough on pH measuring equip-

ment, so I frequently have to go

out in the field and verify the

calibration on our pH transmit-

ters. With the 773 I never have to

power the instrument down and I

can test my pH probes as well as

the transmitter.”

When an operator first con-

tacts him about a problem with

pH measurements, he asks the

operator to take a sample of the

product in process and check the

pH with litmus paper. “Litmus

paper doesn’t lie, but it’s not

exact enough. So if they tell me

that the litmus test is indicating

that the pH is wrong then I go

out and do some troubleshoot-

ing on the pH probes and the

transmitters.”

The process environment is

harsh and can take its toll on

instruments. He estimates that

the problem is process related

rather than instrument related

probably only 20 percent of

the time. “Using the 773 helps

me quickly eliminate a process

problem so I don’t tear everything

apart and start replacing parts

when I don’t need to.”

He carries a pH scale printout

that shows the correct milliamps

for the product in process. He

hooks up the clamp meter to

the transmitter and then puts

the pH probe into a standard

buffer to verify that it is work-

ing correctly. “If I don’t get the

milliamp reading that I expect,

I know right away the probe is

no good.” If the probe is bad,

he replaces it and runs a quick

calibration using the 773. “Since

it’s a troubleshooting situation I

can use the clamp meter to do a

field calibration. Besides using

the milliamp reading, I sometimes

use the milliamps source function

to send the signal back into my

control system to make sure there

isn’t an issue there.”

Two tests are better

than one

He finds the voltage reading

function in the 773 comes in

especially handy. “When you’re

in instrumentation everything

is pretty much 24 volts power

and you need to make sure your

loop has the correct voltage in

it. If you can read the voltage

and the current with the same

instrument, you can troubleshoot

pretty quickly to see if you have

a power supply issue at the

instrument.”

For example, if a cable gets

nicked as it’s being installed, it

may cause an intermittent power

issue. “That’s where it’s a big

help to be able to read the volt-

age with the same device that

you read the current. You start by

reading voltage at the instrument

to find out whether you’re get-

ting good power or inconsistent

power. If it shows good power,

then you know that the circuit

should be intact.”

Separating process from

instrumentation

The 773 has also come in handy

in verifying reinstallations of

instrumentation after mainte-

nance. A while ago his team

performed maintenance on an

entire process area, removing all

the instrumentation and taking it

to the shop to be calibrated with

the Fluke 744 Documenting Pro-

cess Calibrator. They rebuilt some

valves and put everything back

into service. It wasn’t long before

plant operations noticed that the

flow rates were not the same—

they were a little bit lower.

Also, the control valves were not

opening as they had before at a

certain set point.

“We were able to use the 773

clamp meter to quickly show

the operations manager that

our process instruments were

operating exactly the way they

did before the maintenance. We

had the calibration from the 744

to prove that the equipment was

good when it was in the shop,

and after it was reinstalled we

could show him that the output

in milliamps matched everything

that the displays on their control

system were showing them. We

were able to show them six or

seven things in the loop really

quickly because we didn’t have

to power anything down.” It

turned out there was a process

problem, which they were able to

find and fix quickly.

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

Fluke. The Most Trusted Tools

in the World.

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

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

©2012 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 10/2012 4285933A_EN

Modification of this document is not

permitted without written permission

from Fluke Corporation.