What Is Small Signal Cutoff?
Have you ever encountered these situations?
- The pipeline is completely stopped, but the flowmeter still displays 0.2 m³/h.
- The process has been shut down, yet the totalized flow continues to increase.
- The DCS constantly shows a tiny flow value even though there is no actual flow.
In many cases, the instrument is not faulty. The real cause is that Small Signal Cutoff (Low Cut) has not been configured correctly.
Small Signal Cutoff is a software function that filters false measurements caused by electrical noise, zero drift, or signal fluctuations near zero. It is primarily used for flow measurement, especially differential pressure flowmeters (orifice plates, wedge flowmeters, and V-cone flowmeters). It is generally not recommended for pressure, level, or temperature measurements.
Most control systems only provide Low Cut (Lower Small Signal Cutoff). A few advanced systems also support High Cut (Upper Small Signal Cutoff), although it is rarely used in industrial applications.
1. Low Cut (Lower Small Signal Cutoff)
Definition
If the measured value is below the configured Low Cut threshold, the system forces the process variable (PV) to zero.
Logic
- Measured Value < Low Cut → PV = 0
- Measured Value ≥ Low Cut → Normal measurement and totalization
Why Is Low Cut Necessary?
Low Cut helps eliminate false measurements caused by:
- Zero drift when there is no actual flow
- Electrical interference and signal noise
- Amplified errors after square-root extraction in differential pressure flowmeters
- False totalization during equipment shutdown
Without Low Cut, a flowmeter may continue accumulating non-existent flow over long periods, resulting in significant measurement errors.
Typical Low Cut Settings
| Flowmeter Type | Recommended Setting |
|---|---|
| Orifice Plate / Differential Pressure | 3–8% FS |
| Wedge Flowmeter | 3–8% FS |
| V-Cone Flowmeter | 3–8% FS |
| Electromagnetic Flowmeter | 1–5% FS |
| Vortex Flowmeter | 1–5% FS |
| Ultrasonic Flowmeter | 1–5% FS |
| Custody Transfer Applications | Keep as low as practical |
Note: For custody transfer applications, Low Cut should be configured carefully to avoid under-measurement of legitimate low flow rates.
Example
Flow Range: 0–100 m³/h
Low Cut: 3 m³/h
- Flow = 2.5 m³/h → Display = 0
- Flow = 3.2 m³/h → Display = 3.2 m³/h
2. High Cut (Upper Small Signal Cutoff)
Definition
High Cut is an optional function available in a small number of advanced control systems.
When the measured value exceeds the configured threshold, the system may:
- Clamp the value at the configured upper limit, or
- Ignore measurements above the threshold.
Important Notes
High Cut is not the same as a High-High Alarm.
Unlike alarms, High Cut directly affects the measured process value.
Typical Applications
High Cut is only suitable for a few specialized situations, such as:
- Special protection loops
- Micro-flow monitoring systems
- Laboratory or research applications
For over 95% of industrial flow measurement applications, High Cut is unnecessary.
Do not enable High Cut for normal process flow measurement, as it may suppress valid measurement data.
3. Commonly Confused Concepts
Low Cut ≠ Lower Range Value
Lower Range Value (LRV)
Defines the starting point of the instrument’s measurement range.
Low Cut
Only creates a software threshold for filtering measurements and does not change the instrument’s actual measuring capability.
Low Cut ≠ Alarm Limits
Low Cut
Changes the displayed and calculated process variable (PV).
High/Low Alarm
Only generates an alarm notification while leaving the measured value unchanged.
Differential Pressure Flowmeters: Correct Processing Sequence
The correct processing order is:
Low Cut → Square Root Extraction
Never reverse the sequence.
Square-root extraction amplifies very small differential pressure signals. If square-root extraction is performed before Low Cut filtering, tiny noise signals near zero may appear as measurable flow, leading to inaccurate totalization.
4. Practical Considerations
Custody Transfer Measurement
Do not configure an excessively large Low Cut value.
Any adjustment should be reviewed and approved by process personnel to avoid under-measurement.
Safety Instrumented Systems
Use Low Cut with caution for measurements participating in safety interlocks.
If a low-flow protection function relies on the measured PV, forcing the PV to zero may result in unintended trips.
Transmitter vs. DCS Configuration
Enable Small Signal Cutoff in only one location, preferably in the DCS.
Enabling it simultaneously in both the transmitter and the DCS may result in:
- Double filtering
- Unexpected value jumps
- Difficult troubleshooting
Pressure, Level and Temperature Measurements
Small Signal Cutoff is generally not recommended.
Minor fluctuations in these measurements often represent actual process conditions.
Filtering them may hide slow leaks, gradual level changes, or other important process information.
5. Common Troubleshooting Cases
Problem 1
A small flow exists, but the DCS always displays zero.
Cause
The Low Cut threshold is configured too high.
Problem 2
The equipment is stopped, but the totalized flow continues increasing.
Cause
Low Cut is disabled, allowing zero drift and signal noise to accumulate.
Problem 3
The displayed value repeatedly switches between zero and the actual flow.
Cause
No hysteresis has been configured.
A typical solution is:
- Low Cut = 3%
- Recovery Threshold = 3.5%
This prevents frequent switching when the measured value fluctuates around the cutoff point.
Key Takeaway
Small Signal Cutoff does not improve instrument accuracy.
Its purpose is to prevent false signals near zero from participating in process indication and totalization.
A Low Cut setting that is too small allows false flow accumulation during shutdown.
A setting that is too large may suppress legitimate low flow measurements.
Therefore, the optimal Low Cut value should always be determined according to the specific process conditions and application requirements—not simply set as high as possible.