Fluke 753 Ing Application Note Process And Temperature Switch Applications With The 740 Series DPC

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Application Note

Process switches

A process switch is a device that

can sense a process variable —

such as temperature or pressure

— and change the state of one or

more sets of switch contacts

when that variable reaches a

predetermined value. This value

is called a setpoint. A switch can

have multiple setpoints. Let’s look

at some important concepts of

how process switches work.

Contacts. Contacts come in

pairs, and a pair is either nor-

mally open or normally closed.

“Normally” means without ener-

gization — just the way the

contacts would be on the shelf or

if you disconnected the power

wires from the switch.

Many process switches have

four sets of contacts — two

normally open and two normally

closed. But, there are many

variations. A single switch may

operate just one set of contacts,

or it may operate multiple sets

Process and temperature

switch applications with

the 740 Series DPCs

What about actuation? You

might want the switch to failsafe

upon loss of level in a cooling

tank. So, normal level would acti-

vate the switch (compared to its

shelf position). Upon loss of level,

the switch deactivate — that is, it

will assume the same state that

it would be in if it were on the

shelf. For an example of this

control logic, look at a typical

toggle-style light switch. You will

notice the word “ON” under the

toggle handle and the word OFF

above it. To reveal the word “ON,”

you must flip the switch up. If the

toggle mechanism were to fail

mechanically — which could

happen if, for example, it were to

melt due to arcing — the toggle

handle would drop into the “OFF”

position due to gravity. That is the

failsafe position of these switches.

It’s common to implement process

switches the same way.

Setpoint. A switch may have

multiple setpoints. For example,

many level switches come with

low-low, low, high, and high-

high sets of contacts — each with

its own setpoint.

But, it can get more complex

than that, depending on the

required control scheme and the

type of switch used. There are

many ways to accommodate

complex switching schemes —

including the use of an analog

transmitter serves as the input to

a virtual switch (implemented in

software).

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

of normally open and normally

closed contacts. You select which

contacts to use based on the

desired output for a given condi-

tion and a given failsafe condition.

Control logic. You must think

of switch actuation and contact

state separately. Actuating the

typical process switch means

opening one set of contacts and

closing another at the same time.

Whether actuation opens or

closes a set of contacts depends

on whether you are using the

normally open or normally closed

contacts and whether the switch

is in an activated or deactivated

state during normal operation.

Failsafe operation is the first

criterion to assess when deciding

which set of contacts to use. For

example, you should use normally

closed contacts if breaking the

circuit will result in a failsafe

condition. Because loss of power

and an open circuit (via a broken

circuit wire, broken connection,

or intentional operation) have the

same effect on circuit operation,

the normally open contacts would

be the correct ones to use. Upon

loss of power, these contacts

will open. So, you would want

them to be closed for normal

operations and to open when

operations go into alarm or

control change conditions.

It is not true that, for example,

a high level switch will necessar-

ily close contacts when you reach

a high level condition. Good con-

trol practices usually dictate the

opposite.

This note discusses appli-

cations for process

switches and calibrating

temperature switches

using the Fluke 740 Series

Documenting Process

Calibrators (DPCs). Let’s

begin by looking at what a

process switch is and

what it does.

When you calibrate a high level

switch, you do so with the level

rising. This is standard practice

with all process variables, not

just level — you get a more accu-

rate calibration by accounting for

hysteresis.

Trip. This is the value at

which the switch will change the

state of a given set of contacts.

Where a switch trips is a function

of its setpoint and direction. For a

pressure switch with a setpoint

of 500 PSI, the switch should trip

at 500 PSI as pressure rises. Trip

is also called “set.” The opposite

of that is reset.

Reset. Some switches reset

automatically, while others

require a manual reset. In either

case, the reset will not occur until

the switch actuator has moved in

the direction opposite its trigger-

ing direction enough to overcome

hysteresis (and/or deadband —

see below) and allow the switch

to change contact states back to

normal. An exception to this is

when the switch is used to indi-

cate a normal condition. For such

switches, reset is usually not an

issue.

Hysteresis. This is the ten-

dency of the switch to stay in the

last position it was in. This

means that when you are cali-

brating a switch to trip at 500

PSI, the hysteresis of the switch

may cause it to trip at 501 PSI

when you are increasing pressure

and 499 PSI when you are

decreasing pressure. If this is a

high pressure switch (control

function requires a trip on rising

pressure), you would calibrate it

to trip at 500 PSI on an increas-

ing pressure input and let the

498 PSI trip serve as the maxi-

mum reset value.

Band.This is the area around

the setpoint where the switch is

controlling the process. For exam-

ple, if the switch will control a

tank to maintain a level between

6 feet of water and 9 feet of

water, it has a band of 3 feet.

Deadband

Open

High Limit

Process

Variable

Low Limit

Setpoint

Reset

Closed

50 °C

20 °C

Deadband

Closed

Reset

Setpoint

Open

Figure 1. 2-point switch with settings for low and high setpoints.

2 Fluke Corporation Process and temperature switch applications with the 740 Series DPCs

Here’s an example of a com-

plex application. A level switch

may allow a “normal” indication

(such as a light) to display at any

level up to 82 %. At 82 %, the

switch causes normal indication

to go off — placing the indication

between a normal state and an

alarm state. At 85 %, the switch

may trigger a high level alarm

light. At 90 %, the switch may

trigger a high-high level alarm

light plus an audio alarm. At

93 %, it may trigger a feed valve

closure. At 95 %, it may trigger

dump valve operation. At 97 %,

it may trigger drain pump opera-

tion. At 98 %, it may actuate

isolation doors in the room con-

taining the tank. And those

actions are just for high level.

This same switch, or another,

might control low level opera-

tions. In some configurations, you

might have separate switches for

each setpoint.

Setpoint tolerance. This is

the amount of error you can have

between the desired setpoint and

the one you actually set. It’s not

always easy to calibrate a switch

directly on the desired setpoint —

for a variety of reasons. For

example, if you must open a

valve when the temperature

reaches 313 degrees, your set-

point tolerance might allow you

consider the switch calibrated if

it trips within 5 degrees of the

setpoint. Tolerances may be

expressed in engineering units or

in percent. When expressed in

percent, that normally means

percent of the control band (we

explain band below), not in per-

cent of the setpoint value.

Direction. Switch actuation

(and, therefore, control) is direc-

tional, due to hysteresis.

Sometimes, the hysteresis value

can exceed the setpoint toler-

ance. For non-critical applications

with wide setpoint tolerances,

you can probably ignore hystere-

sis. But, standard practice is to

observe direction when calibrat-

ing a setpoint. When you

calibrate a low level switch, you

do so with the level dropping.

Deadband. This is closely

related to reset. Deadband pre-

vents a switch from cycling

around a setpoint. Hysteresis pro-

vides some deadband,

automatically. But for some

processes, hysteresis is not

enough to prevent undesirable

on/off cycling. So, many switches

have additional deadband inten-

tionally designed into them. That

deadband may be fixed, fixed

selectable, or variable. For exam-

ple, an electronic thermostat used

for a heat pump may have a fixed

selectable deadband of 1.5

degrees or 3 degrees.

Range. This is specified with

the low and high points of opera-

tion. For example, if the switch

will control a tank to maintain a

level between 6 feet of water

and 9 feet of water, it has a cali-

bration range of 6 to 9 feet. The

switch itself might have an actual

range of 0 to 50 feet — this range

would appear on the nameplate

of the switch.

Testing a temperature

switch

The switch in the following

example is a temperature switch

with a type K thermocouple input

and a low temperature setpoint of

20 °C. This switch functions in

much the same way as the ther-

mostat in your home. The Low

Limit example in Figure 1 illus-

trates the terminology.

We will be using the normally

open contacts of this switch.

These contacts will close upon

switch actuation, which will

occur with a drop in temperature.

This switch does not have

adjustable reset. The contacts

will re-open upon automatic

reset, which occurs as the tem-

perature moves back up and past

the setpoint in an amount greater

than its deadband. The deadband

is a minimum of 1 °C and maxi-

mum of 3 °C across the range of

the switch.

To set up the Fluke 740 Series

DPC to calibrate the switch,

follow these step-by-step instruc-

tions. Keystroke entries for the

DPC are surrounded by quotation

marks.

1. Beginning in the power up

state of the calibrator, or

Measure mode, depress the

“ohms/continuity” key twice

to enable continuity mode.

2. Simulate the temperature

input.

a. Depress the

“MEAS/SOURCE” key

once to obtain the

Source mode.

b. Depress the “TC/RTD”

key, move the cursor with

the “↓” key to “K” and

depress “ENTER” to select

a type K thermocouple.

c. Depress “ENTER” again to

select “Linear T.”

d. Enter a temperature out-

put of “25” and depress

“ENTER.”

e. Depress the

“MEAS/SOURCE” key to

obtain the split screen

display. The display of the

74X should be as per

Figure 2.

3. Connect the DPC, per

Figure 3.

4. Take As Found

measurements.

a. Select the “As Found”

softkey.

b. Move the cursor to “1 Pt.

c. Switch Test” with the “↓”

key and depress “ENTER.”

You should now see the

switch test setup screen.

5. Enter the setpoint.

a. Depress “Enter” and enter

a setpoint of “20” °C, then

depress “ENTER” again.

The Setpoint Type is set

for low and the Set State

is a short by default —

perfect for this particular

test. (If these conditions

were different, we would

change them here.).

These setup conditions

describe a switch that has

a setpoint of 20 °C and

closes a set of contacts as

long as the input temper-

ature to the switch is

below 20 °C.

b. Depress the “Done”

softkey.

6. Enter the setpoint tolerance and deadband

settings.

a. Move the cursor to tolerance and enter a

setpoint tolerance of “1” °C.

b. Move the cursor to Deadband Min and

enter a minimum deadband of “1” °C.

c.Move the cursor to Deadband Max and

enter a maximum deadband value of 3 °C.

The test setup screen should now be as

per Figure 4. Depress the “Done” softkey.

Figure 2. MEASURE/SOURCE split screen, contacts open.

ENTER

V

RTD

MEAS

SOURCE

mA mA

RTD

V

300V

MAX

30V MAX

TC

Fluke 741/743

To Limit

Switch Contacts

To Limit Switch

Thermocouple

Input

Figure 3. Connecting the DPC.

Figure 4. Test setup screen.

3 Fluke Corporation Process and temperature switch applications with the 740 Series DPCs

8. Enter Tag information.

a. Depress the “Done” soft-

key and enter the Tag

information for your test.

b. Depress the “Done” soft-

key when tag entry is

complete.

9. Adjust setpoints or reset

points.

a. If the switch failed any of

the test parameters, it is

necessary to adjust the

set and/or reset points. To

do that, first select the

“adjust” softkey.

b. Depress the “Step Size”

softkey, then enter a step

size of “.1” °C.

c. Depress the “Done”

softkey.

d. Depress the “↓” key until

the DPC source value is

20 °C (the setpoint).

e. Slowly adjust the setpoint

on the limit switch until

the measure screen tog-

gles from reset to set.

Depress the “↑” key until

the DPC measure screen

toggles to Reset. If the

DPC toggles from set to

reset between 21 °C and

23 °C, the deadband

should be correctly set.

If it does not toggle prop-

erly, adjust the reset point

until it toggles within that

band.

f. Verify the set and reset

points toggle correctly, by

depressing the “↓” and

“↑” keys to slew the DPC

source temperature across

the set and reset values.

g. Once that is complete,

depress the

“Done”

softkey.

Figure 5. MEASURE/SOURCE split screen, contacts reset.

7. You should now see the split screen

(Figure 5). Select the “Auto Test” softkey

and the “Continue” softkey. The DPC will

now ramp the simulated thermocouple

potential into the limit switch back and

forth past the nominal setpoint and record

the sourced temperature values for the

actual setpoint, and then show that value

in the upper left-hand corner of the DPC

display. Once that is done, the DPC will

then test the reset point of the switch by

ramping the simulated thermocouple

potential into the switch back and forth

past the nominal (21 °C - 23 °C) expected

reset value. Once that value is recorded,

you should be presented with a post test

summary similar to that in Figure 6. Errors

exceeding test tolerance are recorded in

inverse video.

Figure 6. Post-test summary, with reverse video.

Figure 7. Post-test summary, with all results normal.

10. Confirm the As Left settings.

a. Depress the “As Left”

softkey.

b. Confirm the test settings.

c. Depress the “Done”, “Auto

Test” and “Continue” soft-

keys. Monitor the DPC as

it performs the As Left

evaluation.

d. Once the post test sum-

mary is displayed, review

the results. If all results

are in normal video (as in

Figure 7), the As Left test

passes.

e. Depress the “Done” soft-

key, and “D ne” again to

save the Tag information.

f. If there were inverse

video indications of a fail-

ure, repeat the

adjustments performed in

Step 9 until a passing

result is obtained.

11. Review results in memory,

a. Depress the “Done”

and “Review Memory”

softkeys.

b. Move the cursor to the tag

entry associated with this

test and depress “ENTER.”

c. Move the cursor to the As

Found entry and depress

“Enter” to review your As

Found result.

d. Depress the “Done”

softkey.

e. Move the cursor to the As

Left entry and depress

“Enter” to review that

result.

f. Depress the “Done” soft-

key, then depress the

“Tag” softkey to review

your Tag information.

4 Fluke Corporation Process and temperature switch applications with the 740 Series DPCs

Other switch tests

In the preceding step by step

description, the switch has been

removed from its operational cir-

cuit and the switch contact

closure is monitored to determine

state change.

You can perform this test with

the switch installed in its circuit.

In this instance, the switch con-

tacts will open and close and you

can use the 740 Series DPC to

measure the presence or absence

of system voltage (e.g. 120VAC) as

switch contacts change state. A

typical example would be measur-

ing the voltage applied to a

heater as controlled by the output

of the switch. The 740 series

DPCs can work with dc voltage in

addition to the continuity and ac

voltage examples previously

described.

Our examples in this applica-

tion note have been for

temperature switches. The 740

series DPCs allow you to test

pressure switches, too — in fact,

in 11 different engineering units.

Pressure switch tests are similar

to temperature switch tests — you

vary the process variable (source)

at the input, and monitor for a

change of state at the output. You

need to use a hand pump to

source pressure into a pressure

module and the switch. You can

manually document the results

by depressing the “Accept Point”

soft key when the test has been

completed.

With the 740 series DPCs, you

can source and measure many

key variables. And these tools are

useful for calibrating any process

switch. Of course, you will need

to supply your own inputs for

many types of process variables —

such as level, flow, and pH. The

principles of switch operation out-

lined here apply universally.

Fluke Corporation

PO Box 9090, Everett, WA USA 98206

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

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

Fax (425) 446-5116

In Europe/M-East/Africa (31 40) 2 675 200 or

Fax (31 40) 2 675 222

In Canada (800) 36-FLUKE or

Fax (905) 890-6866

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

Fax +1 (425) 446-51

16

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

©2004 Fluke Corporation. All rights reserved.

Printed in U.S.A. 7/2004 1263879 A-US-N Rev D

Fluke. Keeping your world

up and running.

For detailed information on

calibrating pressure switches,

reference Fluke Application

Note 2069058.

5 Fluke Corporation Process and temperature switch applications with the 740 Series DPCs