Fluke 753, 754 Service Guide

Process and temperature switch applications with the 740 Series DPCs

Application Note

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.

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 c how proc

pairs, and a pair is either nor mally open or normally closed. “Normally” means without ener-

ization — just the way the

g contacts would be on the shelf or if you disconnected the power

ires from the switch.

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

ess switches work.

Contacts. Contacts come in

Many process switches have

onc

epts of

of normally open and normally closed contacts. You select which contacts to use based on the desired output for a given condition 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

orrect ones to use. Upon

e the c

b loss of power, these contacts will open. So, you would want them to b operations and to open when operations go into alarm or

ontrol change c

a high level sw ily close c a high level condition. Good control practic opposite.

e closed for normal

onditions

It is not true that, for example,

itch w

ontacts when you reach

es usually dictate the

ill nec

essar

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 many level switches come with low-low, low, high, and highhigh sets of contacts — each with its own setpoint.

But, it can get more c than that, depending on the required control scheme and the type of sw 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).

. For example,

omplex

itch used. There are

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

Deadband

Open

High Limit

Process

Variable

Low Limit

Setpoint

Reset

Closed

°C

20 °C

Deadband

Closed

Reset

Setpoint

Open

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

y trigger a high level alarm

ma light. At 90 %, the switch may trigger a high-high level alarm

light plus an audio alarm 93 %, it ma

y trigger a feed valve

. A

closure. At 95 %, it may trigger

7 %,

t 9

dump valve operation

y trigger drain pump opera

it ma tion. At 98 %, it may actuate

. A

isolation doors in the room c taining the tank

. And those actions are just for high level. This same sw might c

itch, or another,

ontrol low level operations. In some configurations, you might ha

ve separate sw

each setpoint

itches for

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 3 point tolerance might allow you consider the switch calibrated if it trips w setpoint expressed in engineering units or

ent

in perc

ent, that normally means

perc percent of the control explain band b

ent of the setpoint value.

Direction. Switch actuation (and, therefore, c tional, due to hysteresis Sometimes, the hysteresis value can exc

e. For non-critical applications

anc with wide setpoint tolerances, you can probably ig

. But, standard practice is to

sis observe direction when calibrating a setpoint calibrate a low level switch, you do so with the level dropping.

rees, your set

3 deg

ithin 5 deg

olerances may be

. T

. When expressed in

rees of the

elow), not in per

band (we

ontrol) is direc

eed the setpoint toler

nore hystere

. When you

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 PS

I when you are

decreasing pressure. If this is a

ontrol

high pressure sw

itch (c function requires a trip on rising pressure), you would calibrate it to trip at 500 PS

I 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

or exam

. F

ontrolling the proc

c ple, if the sw

tank to maintain a level between

ess

itch will control a

6 feet of water and 9 feet of water, it has a band of 3 feet

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

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