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

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Process and temperature
switch applications with
the 740 Series DPCs
of normally open and normally
closed contacts. You select which
This note discusses applicontacts to use based on the
cations for process
desired output for a given condition and a given failsafe condition.
switches and calibrating
Control logic. You must think
temperature switches
of
switch
actuation and contact
using the Fluke 740 Series
state separately. Actuating the
Documenting Process
typical process switch means
Calibrators (DPCs). Let’s
opening one set of contacts and
begin by looking at what a
closing another at the same time.
process switch is and
Whether actuation opens or
what it does.
closes a set of contacts depends
on whether you are using the
normally open or normally closed
contacts and whether the switch
Process switches
is in an activated or deactivated
A process switch is a device that state during normal operation.
Failsafe operation is the first
can sense a process variable —
criterion to assess when deciding
such as temperature or pressure
— and change the state of one or which set of contacts to use. For
example, you should use normally
more sets of switch contacts
closed contacts if breaking the
when that variable reaches a
circuit will result in a failsafe
predetermined value. This value
is called a setpoint. A switch can condition. Because loss of power
have multiple setpoints. Let’s look and an open circuit (via a broken
circuit wire, broken connection,
at some important concepts of
or intentional operation) have the
how process switches work.
same effect on circuit operation,
Contacts. Contacts come in
the normally open contacts would
pairs, and a pair is either norbe the correct ones to use. Upon
mally open or normally closed.
loss of power, these contacts
“Normally” means without enerwill open. So, you would want
gization — just the way the
contacts would be on the shelf or them to be closed for normal
operations and to open when
if you disconnected the power
operations go into alarm or
wires from the switch.
control change conditions.
Many process switches have
It is not true that, for example,
four sets of contacts — two
a high level switch will necessarnormally open and two normally
ily close contacts when you reach
closed. But, there are many
a high level condition. Good convariations. A single switch may
trol practices usually dictate the
operate just one set of contacts,
opposite.
or it may operate multiple sets

Application Note

What about actuation? You
might want the switch to failsafe
upon loss of level in a cooling
tank. So, normal level would activate 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 highhigh 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

Closed

50 °C

Setpoint

Deadband
Reset
Open

High Limit
Process
Variable
Open

Reset

Deadband
20 °C

Setpoint
Closed

Low Limit

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

Here’s an example of a complex 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 operation. At 98 %, it may actuate
isolation doors in the room containing the tank. And those
actions are just for high level.
This same switch, or another,
might control low level operations. In some configurations, you
might have separate switches for
each setpoint.

2 Fluke Corporation

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 setpoint 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 percent of the setpoint value.
Direction. Switch actuation
(and, therefore, control) is directional, due to hysteresis.
Sometimes, the hysteresis value
can exceed the setpoint tolerance. For non-critical applications
with wide setpoint tolerances,
you can probably ignore hysteresis. But, standard practice is to
observe direction when calibrating a setpoint. When you
calibrate a low level switch, you
do so with the level dropping.

Process and temperature switch applications with the 740 Series DPCs

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 accurate 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 triggering 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 indicate a normal condition. For such
switches, reset is usually not an
issue.
Hysteresis. This is the tendency of the switch to stay in the
last position it was in. This
means that when you are calibrating 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 increasing pressure input and let the
498 PSI trip serve as the maximum reset value.
Band. This is the area around
the setpoint where the switch is
controlling the process. 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 band of 3 feet.

Deadband. This is closely
related to reset. Deadband prevents a switch from cycling
around a setpoint. Hysteresis provides some deadband,
automatically. But for some
processes, hysteresis is not
enough to prevent undesirable
on/off cycling. So, many switches
have additional deadband intentionally designed into them. That
deadband may be fixed, fixed
selectable, or variable. For example, 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 operation. 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 calibration 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

1.

2.

ENTER

V
RTD

mA

V

mA

RTD

SOURCE

30V MAX

3.
4.

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 thermostat in your home. The Low
Limit example in Figure 1 illus5.
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 temperature moves back up and past
the setpoint in an amount greater
than its deadband. The deadband
is a minimum of 1 °C and maximum 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 instructions. Keystroke entries for the
DPC are surrounded by quotation
marks.

3 Fluke Corporation

Beginning in the power up
state of the calibrator, or
Measure mode, depress the
“ohms/continuity” key twice
to enable continuity mode.
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.
Figure 2. MEASURE/SOURCE split screen, contacts open.
c. Depress “ENTER” again to
select “Linear T.”
To Limit
d. Enter a temperature outSwitch Contacts
put of “25” and depress
“ENTER.”
e. Depress the
“MEAS/SOURCE” key to
To Limit Switch
obtain the split screen
Thermocouple
display. The display of the
Input
74X should be as per
Figure 2.
Connect the DPC, per
Figure 3.
Fluke 741/743
Take As Found
Figure 3. Connecting the DPC.
measurements.
a. Select the “As Found”
6. Enter the setpoint tolerance and deadband
softkey.
settings.
b. Move the cursor to “1 Pt.
a. Move the cursor to tolerance and enter a
c. Switch Test” with the “↓”
setpoint tolerance of “1” °C.
key and depress “ENTER.”
b. Move the cursor to Deadband Min and
You should now see the
enter a minimum deadband of “1” °C.
switch test setup screen.
c. Move the cursor to Deadband Max and
Enter the setpoint.
enter a maximum deadband value of 3 °C.
a. Depress “Enter” and enter
The test setup screen should now be as
a setpoint of “20” °C, then
per Figure 4. Depress the “Done” softkey.
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 temperature to the switch is
below 20 °C.
b. Depress the “Done”
softkey.

Process and temperature switch applications with the 740 Series DPCs

Figure 4. Test setup screen.

MEAS

300V
MAX

TC

8.

9.

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.

Enter Tag information.
a. Depress the “Done” softkey and enter the Tag
information for your test.
b. Depress the “Done” softkey when tag entry is
complete.
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 toggles 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 properly, 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.

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” softkeys. Monitor the DPC as
it performs the As Left
evaluation.
d. Once the post test summary 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” softkey, and “D ne” again to
save the Tag information.
f. If there were inverse
video indications of a failure, 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” softkey, then depress the
“Tag” softkey to review
your Tag information.

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

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

Process and temperature switch applications with the 740 Series DPCs

Our examples in this application note have been for
In the preceding step by step
temperature switches. The 740
description, the switch has been
series DPCs allow you to test
removed from its operational cir- pressure switches, too — in fact,
cuit and the switch contact
in 11 different engineering units.
closure is monitored to determine Pressure switch tests are similar
state change.
to temperature switch tests — you
You can perform this test with vary the process variable (source)
the switch installed in its circuit.
at the input, and monitor for a
In this instance, the switch conchange of state at the output. You
tacts will open and close and you need to use a hand pump to
can use the 740 Series DPC to
source pressure into a pressure
measure the presence or absence module and the switch. You can
of system voltage (e.g. 120VAC) as manually document the results
switch contacts change state. A
by depressing the “Accept Point”
typical example would be measur- soft key when the test has been
ing the voltage applied to a
completed.
heater as controlled by the output
With the 740 series DPCs, you
of the switch. The 740 series
can source and measure many
DPCs can work with dc voltage in key variables. And these tools are
addition to the continuity and ac
useful for calibrating any process
voltage examples previously
switch. Of course, you will need
described.
to supply your own inputs for
many types of process variables —
such as level, flow, and pH. The
principles of switch operation outlined here apply universally.

Other switch tests

For detailed information on
calibrating pressure switches,
reference Fluke Application
Note 2069058.

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up and running.
Fluke Corporation
PO Box 9090, Everett, WA USA 98206
Fluke Europe B.V.
PO Box 1186, 5602 BD
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From other countries +1 (425) 446-5500 or
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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

5 Fluke Corporation

Process and temperature switch applications with the 740 Series DPCs



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