Instrumentation Abbreviations Used for Level Measurement in Tanks
Introduction
Level measurement is one of the most common instrumentation applications in process plants, particularly in oil & gas, petrochemical, refinery, chemical, power and water-treatment facilities.
Instrumentation engineers and technicians use standardized abbreviations to identify level instruments, alarms, switches, controllers and control functions on P&IDs, Instrument Indexes, Cause & Effect diagrams, Loop Diagrams and DCS graphics.
The ANSI/ISA-5.1 standard provides a standardized method for identifying and representing instrumentation and control functions. The latest revision is ANSI/ISA-5.1-2024, Instrumentation and Control – Symbols and Identification.
1. Typical Tank Level Arrangement
A typical tank may have the following level instruments:
- LT – Level Transmitter
- LI – Level Indicator
- LIT – Level Indicating Transmitter
- LG – Level Gauge
- LS – Level Switch
- LSH – Level Switch High
- LSHH – Level Switch High High
- LSL – Level Switch Low
- LSLL – Level Switch Low Low
- LAH – Level Alarm High
- LAL – Level Alarm Low
- LAHH – Level Alarm High High
- LALL – Level Alarm Low Low
- LIC – Level Indicating Controller
- LCV – Level Control Valve
These abbreviations are based on the general ISA identification philosophy; individual projects may have additional or modified tag conventions defined in their project specifications.
2. Understanding the Letters
The easiest way to understand level instrument abbreviations is to break the tag into individual letters.
First letter: L
L = Level
For example:
LT
Therefore:
LT = Level Transmitter
Similarly:
LSH
- L = Level
- S = Switch
- H = High
Therefore:
LSH = Level Switch High
ISA's identification system uses leading letters to identify the measured variable or function and succeeding letters to identify functions such as indication, transmission, switching and control.
3. Common Level Instrument Abbreviations
| Abbreviation | Full Form | Function |
|---|
| L | Level | Process variable – liquid level |
| LI | Level Indicator | Indicates level |
| LT | Level Transmitter | Continuously measures and transmits level |
| LIT | Level Indicating Transmitter | Measures, transmits and indicates level |
| LG | Level Gauge | Local visual level indication |
| LS | Level Switch | Detects a predetermined level |
| LSH | Level Switch High | Switch operates at high level |
| LSHH | Level Switch High High | Switch operates at very high level |
| LSL | Level Switch Low | Switch operates at low level |
| LSLL | Level Switch Low Low | Switch operates at very low level |
| LAH | Level Alarm High | High-level alarm |
| LAHH | Level Alarm High High | High-high-level alarm |
| LAL | Level Alarm Low | Low-level alarm |
| LALL | Level Alarm Low Low | Low-low-level alarm |
| LIC | Level Indicating Controller | Indicates and controls level |
| LCV | Level Control Valve | Control valve used for level control |
Important: Exact tag definitions and combinations can vary between companies and projects. Always check the project's Instrument Index, P&ID legend and Engineering Design Criteria before interpreting a tag.
4. What Do H, HH, L and LL Mean?
The letters H, HH, L and LL describe the level condition.
HH – High High
This represents a very high or critical level.
Example:
LSHH-101
When the tank level reaches the high-high set point, the switch may initiate:
- High-high alarm
- Pump shutdown
- Inlet valve closure
- Emergency shutdown
- Other protective action defined in the Cause & Effect
The exact action depends on the process design and Cause & Effect philosophy.
H – High
H = High Level
A high-level condition normally provides an early warning that the tank is approaching its upper operating limit.
Example:
LSH-101
Possible action:
Tank level ↑ → LSH operates → High Level Alarm
The operator can then take corrective action before the level reaches HH.
L – Low
L = Low Level
A low-level condition indicates that the tank level has fallen below the normal operating range.
Example:
LSL-101
Possible action:
Tank level ↓ → LSL operates → Low Level Alarm
LL – Low Low
LL = Low Low Level
This is normally a critical low-level condition.
It may be used to:
- Protect pumps from dry running
- Protect equipment
- Initiate a shutdown
- Generate a low-low alarm
- Initiate an interlock
Example:
Tank level ↓ → LSLL operates → Pump Trip
Again, the actual action is determined by the Cause & Effect and control philosophy.
5. Normal Tank Level
The tank normally operates between predetermined minimum and maximum levels.
A simplified arrangement is:
TANK
┌───────────────────┐
│ HH │ ← High High
│───────────────────│
│ H │ ← High
│───────────────────│
│ │
│ N │ ← Normal Operating Level
│~~~~~~~~~~~~~~~~~~~│
│ │
│ L │ ← Low
│───────────────────│
│ LL │ ← Low Low
└───────────────────┘
The Normal (N) level is the desired operating range/set point. The H/HH and L/LL levels provide progressively higher or lower protection limits.
6. Level Transmitter – LT
LT = Level Transmitter
An LT provides continuous measurement of the liquid level.
For example:
LT-101
The transmitter measures the level and sends a signal to the control system, commonly:
4–20 mA
or a digital communication signal depending on the system and instrument.
Typical technologies include:
- Differential Pressure (DP)
- Radar
- Guided Wave Radar
- Ultrasonic
- Displacer
- Magnetostrictive
- Capacitance
Example:
LT-101
│
│ Level Signal
↓
┌──────────┐
│ DCS │
│ │
│ LIC │
└──────────┘
The DCS can display the actual tank level, for example:
Tank Level = 65%
7. LIT – Level Indicating Transmitter
LIT = Level Indicating Transmitter
LIT combines:
- L – Level
- I – Indication
- T – Transmitter
It indicates the measured level and transmits the signal to the control system.
For example:
LIT-101
The DCS may display:
LIT-101 = 72%
8. LG – Level Gauge
LG = Level Gauge
A level gauge provides local visual indication of the liquid level.
Common types include:
Sight Glass
The operator can visually see the liquid level.
Magnetic Level Gauge
A magnetic float moves an indicator along an external scale.
Typical application:
TANK
┌───────────┐
│ │
│ │ │
│ │ LG │ ← Local Level Gauge
│ │ │
│ │ │
└───────────┘
9. LS – Level Switch
LS = Level Switch
Unlike an LT, which provides continuous level measurement, an LS provides a discrete ON/OFF signal when the liquid reaches a predetermined level.
For example:
LSH-101
may change state when the tank reaches the high-level set point.
The signal can be connected to:
- DCS
- PLC
- SIS/ESD system
- Alarm system
- Motor control system
10. LSH – Level Switch High
LSH = Level Switch High
The switch operates when the liquid reaches a predetermined high-level set point.
Example:
TANK
┌──────────────┐
│ │
│ LSH ●───┼── High Level
│ │
│~~~~~~~~~~~~~~│
│ │
└──────────────┘
Possible function:
LSH → High Level Alarm
11. LSHH – Level Switch High High
LSHH = Level Switch High High
This is generally a more critical level switch than LSH.
Example:
HH ─── LSHH
H ─── LSH
Normal Operating Range
A typical protection philosophy may be:
LSH → Alarm
LSHH → Trip / Shutdown
For example, in an oil storage tank:
LSHH → Close inlet valve / stop transfer pump
The actual protective action must be confirmed from the project's Cause & Effect.
12. LSL – Level Switch Low
LSL = Level Switch Low
LSL operates when the tank level falls below the low-level set point.
Typical function:
LSL → Low Level Alarm
This alerts the operator that the tank level is becoming low.
13. LSLL – Level Switch Low Low
LSLL = Level Switch Low Low
LSLL operates at a critical low level.
A common application is pump protection.
For example:
Tank
┌──────────────┐
│ │
│ │
│ │
│ LSL │ ← Low
│ │
│ LSLL │ ← Low Low
└───────┬──────┘
│
Pump
Possible sequence:
Tank level decreases
↓
LSL operates
↓
Low Level Alarm
↓
If level continues decreasing:
LSLL operates
↓
Pump Trip
This prevents the pump from operating without sufficient liquid.
14. LAH and LAL
LAH = Level Alarm High
Indicates that the level has reached the high alarm set point.
LAL = Level Alarm Low
Indicates that the level has reached the low alarm set point.
Typical DCS indication:
100% ───────── HH
90% ───────── H / LAH
65% ───────── NORMAL
20% ───────── L / LAL
10% ───────── LL
0% ───────── EMPTY
15. LAHH and LALL
LAHH = Level Alarm High High
Indicates a critical high-level condition.
LALL = Level Alarm Low Low
Indicates a critical low-level condition.
These may be associated with interlocks or trips, depending on the plant's safety philosophy.
It is important not to assume that every alarm automatically causes a trip. The actual action should be verified from the Cause & Effect Diagram and approved control philosophy.
16. LIC – Level Indicating Controller
LIC = Level Indicating Controller
An LIC performs three basic functions:
L = Level
I = Indicating
C = Controller
For example:
LT
│
│ Level
↓
┌──────────┐
│ LIC │
│ DCS │
└────┬─────┘
│
│ Control Output
↓
LCV
│
↓
Process Outlet
The LIC compares the measured level with the set point and generates a control output.
For example:
Set Point = 60%
Actual Level = 70%
The LIC may command the outlet control valve to open further to reduce the tank level.
17. LCV – Level Control Valve
LCV = Level Control Valve
An LCV is the final control element used to maintain tank level.
Typical arrangement:
LT
│
↓
LIC
│
↓
LCV
│
↓
Outlet Flow
The LIC controls the LCV to maintain the desired tank level.
18. Difference Between LT, LS and LIC
| Instrument | Measurement | Output/Function |
|---|
| LT | Continuous level | Measurement signal |
| LS | Discrete level | ON/OFF signal |
| LIT | Continuous level | Indication + transmission |
| LI | Level | Local/remote indication |
| LIC | Level | Indication + control |
| LG | Level | Local visual indication |
| LSH | High level | High-level switch |
| LSHH | High-high level | Critical high-level switch |
| LSL | Low level | Low-level switch |
| LSLL | Low-low level | Critical low-level switch |
| LCV | — | Final control element for level |
19. Typical Tank Level Control Philosophy
A typical tank might have:
LT → LIC → LCV
for continuous level control.
And:
LSH → High Level Alarm
LSHH → High-High Trip
LSL → Low Level Alarm
LSLL → Low-Low Trip
Simplified:
TANK
┌────────────────┐
│ LSHH │ → HH Trip
│----------------│
│ LSH │ → H Alarm
│ │
│ LT │
│ │ │
│ │ │
│ ↓ │
│ LIC │
│ │
│ LSL │ → L Alarm
│----------------│
│ LSLL │ → LL Trip
└───────┬────────┘
│
↓
LCV
│
↓
Outlet
This arrangement provides both process control and protective functions.
20. Alarm vs Trip
One of the most important concepts for an instrumentation engineer is the difference between an alarm and a trip.
Alarm
An alarm informs the operator that the process has reached an abnormal condition.
Example:
LAH → High Level Alarm
The operator may take corrective action.
Trip
A trip automatically initiates a predefined action to protect personnel, equipment or the process.
Example:
LSHH → Pump Trip / Valve Closure
The exact action depends on the approved Cause & Effect.
21. High and Low Level Protection
High-Level Protection
The purpose is generally to prevent overfilling.
Typical sequence:
Normal → H Alarm → HH Trip
Low-Level Protection
The purpose may be to prevent:
- Pump dry running
- Loss of suction
- Equipment damage
- Process interruption
Typical sequence:
Normal → L Alarm → LL Trip
22. Quick Reference
| Condition | Abbreviation | Typical Meaning |
|---|
| Normal | N | Normal operating level |
| High | H | High level |
| High High | HH | Critical high level |
| Low | L | Low level |
| Low Low | LL | Critical low level |
| Level Transmitter | LT | Continuous measurement |
| Level Indicator | LI | Level indication |
| Level Indicating Transmitter | LIT | Indication + transmission |
| Level Gauge | LG | Local level indication |
| Level Switch | LS | Discrete level detection |
| Level Switch High | LSH | High-level switching |
| Level Switch High High | LSHH | Critical high-level switching |
| Level Switch Low | LSL | Low-level switching |
| Level Switch Low Low | LSLL | Critical low-level switching |
| Level Alarm High | LAH | High-level alarm |
| Level Alarm High High | LAHH | Critical high-level alarm |
| Level Alarm Low | LAL | Low-level alarm |
| Level Alarm Low Low | LALL | Critical low-level alarm |
| Level Indicating Controller | LIC | Level control |
| Level Control Valve | LCV | Final control element |
Conclusion
Understanding level abbreviations is essential when reading P&IDs, instrument index sheets, Cause & Effect diagrams, DCS graphics, loop diagrams and commissioning documents.
The easiest way to remember the basic terminology is:
L = Level
I = Indication
T = Transmitter
S = Switch
A = Alarm
C = Controller
H = High
HH = High High
L = Low
LL = Low Low
Therefore:
LT → measures level continuously
LSH → detects high level
LSHH → detects critical high-high level
LSL → detects low level
LSLL → detects critical low-low level
LIC → controls level
LCV → controls the process flow to maintain level
For engineering drawings, always use the project-specific P&ID legend and instrumentation specification in addition to ISA-5.1, because individual project/company conventions can differ. ISA states that ISA-5.1 establishes a uniform means of designating instruments and instrumentation systems used for measurement and control.
