Letter combinations as labels in I&C

This post was adapted from the standard ANSI/ISA-5.1 -2009

The first step to understand/read/interpret a I&C diagram is to know the possible combinations of letters in labels.

Once you know these combinations you may start creating your own diagrams!

What is the purpose of labels in I&C diagrams?

The answer to this question is not completely obvious. Labels were in fact the solution to a technical problem: communication or conveying of information.

Labels are condensed information that help to understand complex diagrams based on a code. The standard ANSI/ISA-5.1-2009 is an example of this code. Labels are a key element of I&C diagrams because simple letter-combinations along with other symbols provide vital information on how a control loop works, for example.

One may say that it could be difficult to gather so many details in a simple label of five letters or that some sort of device would be needed to extract the information from a label (as in a bar code). However, engineers have managed across the years to get an easy to use tool in the form of labels. 

How and I&C diagram label is formed?

Labels are formed from 5 different letters each one having a different interpretation according to its position in the label. Here are the details,

Fig. 01A general sketch of the letters forming a label

• First letter. Represents the process variable being measured or on which some action is taken, 
• Second letter. This letter is called a modifier. It is a further detail of the process variable so that a reasonable description of the variable is conveyed.
• Third letter. It is called as a readout/passive function. However, we can also say that this letter represents an action performed based on the measurements of the process variable. It is called passive function because it is a functionality or feature of the device link to the process variable.
• Fourth letter. This letter is used to indicate and output/active function of the device using the process variable data.
• Fifth letter. This last letter can be used to provide another feature of the output function of the device in use.

Labels are made of alphabetical letters only and go from A to Z.

A list of letters for measured process variables

Knowing the meaning of the letters in a label is key. Here is a list common process variables and the letter to represent each of these.

A Analysis

B Burner, combustion

C User's Choice

D User's Choice

E Voltage

F Flow, flow rate

G User's Choice

H Hand

I Current 

J Power

K Time, schedule

L Level

M User's Choice

N User's Choice

O User's Choice

P Pressure

Q Quantity

R Radiation

S Speed, frequency

T Temperature

U Multivariable

V Vibration, mechanical analysis

W Weight, force

X Uncalssified

Y Event, state, presence

Z Position, dimension

Many variables such as pH and viscosity, to say two very common ones, are not in the lis above. However, the letter A is to be used always to represent all the variables related to the stream composition and physical properties.

Other variables not in the list can also be represented with one othe letters identified as: User's choice. These letters are: C, D, G, M, N and O. Examples of process variables are: density, modulus of elasticity, electrical conductivity, humidity, etc.

Examples of letters for process variable modifiers

You will possibly need to add some other detail so that process variable be clearly identified. Examples of these features can be:

D Differential

F Ratio

K Time rate of change

Q Integrate, totalize

S Safety

X X-axis

Y Y-axis

Z Z-axis, safety instrumented system

Examples of usage of these letters are shown in the figure below.

Fig. 02 Examples of labels using the first and second letters.

Examples of readout/passive functions

The third letter usually refers to a feature of the device handling the process variable data. Here is a list of these functionalities,

A Alarm

B User's Choice

E Sensor, primary element

G Glass, gauge, viewing device

I Indicate

J Scan

L Light

N User's Choice

O Orifice, restriction

P Point

Q Integrate, totalize

R Record

U Multifunction

X Accesory devices, unclassified

These letters, as you may imagine, refer to characteristics of a device in the field. Once you read the label you can easily come out with a physical image of how that device looks like. Below are given some examples.

Fig. 03 Examples of devices for measurement of process variables and their corresponding labels

Notice that in the examples in Fig. 03 labels are only made of two letter and three-letters labels were expected. Do not worry: for all cases in Fig. 03 the process variable can be well defined with the first letter only. This will also lead us to another conclusion: the second letter is optional.

Example labels with output/active functions

A fourth letter can be used to depict an extra feature of a device: a so called active functionality. So, what are examples of these output/active functionalities?

Again, a set of letters is used to represent these features,

B User's Choice

C Control

K Control station

N User's Choice

S Switch

T Transmit

U Multifunction

V Valve, damper, louver

X Unclassified

Y Auxiliary devices

Z Driver, actuator, unclassified final control  element

Fig. 04 Examples of labels to indicate features of some devices

Notice that a PID controller may have several features. However, not all of these may be in use at a time. In some cases you could use its alarm functionality and in some other not.

Any question? Write in the comments and I shall try to help.

Other stuff of interest

• LE01 - AC and DC voltage measurement and continuity test
• LE 02 - Start and stop push button installation 24V DC
• LE 03 - Turn on/off an 24V DC pilot light with a push button
• LE 04 - Latch contact with encapsulated relay for turning on/off an AC bulb light
• What is a process variable?
• What are the most important process variables?
• Time dependence of process variables
• A list of process variables

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Ildebrando.