What Is Low Flow Cutoff
Low Flow Cutoff is a function that automatically forces the flowmeter output to zero under low-flow or empty-pipe conditions. When the fluid in the pipe is stationary, contains gas, or falls below the set threshold, the measurement signal is prone to noise, causing false accumulation.
Properly setting the cutoff value can suppress false pulses and ensure the stability of totalized flow and current output. This article uses Promag 50, Promass 80, and Prosonic 92 as examples to explain the key setting points.
Preparation and Principles Before Setting
Basic Setting Principles
- The cutoff value should be slightly higher than the minimum actual flow on site to avoid cutting off normal measurement by mistake.
- The cutoff value is usually taken as 1% to 5% of full scale, depending on process fluctuations.
- If the cutoff value is too low, noise will be introduced; if it is too high, low-flow measurement will be lost.
Pre-operation Checks
- Confirm that the pipe is full and free of accumulated bubbles.
- Record the current flowmeter parameters and output configuration.
- Prepare to enter the field menu or commissioning software (such as FieldCare).
Promag 50 Low Flow Cutoff Setting
The Promag 50 is an electromagnetic flowmeter, and the cutoff point is set through the Current Output and Low Flow Cutoff menus.
Operation Steps
- Enter the main menu and select Setup → Current Output.
- Find the Low Flow Cutoff parameter and enter the cutoff value (the unit must match the measuring range).
- After saving, return to the measurement screen and observe whether the output returns to zero at low flow.
If the process medium easily contains gas, the cutoff value can be appropriately increased and used together with the empty pipe detection function.
Promass 80 Low Flow Cutoff Setting
The Promass 80 is a Coriolis mass flowmeter, and the cutoff setting affects the mass flow and density outputs.
Operation Steps
- Enter Setup → System Units to confirm the units.
- Enter Setup → Low Flow Cutoff and enter the cutoff value.
- Optionally enable Zero Point Adjustment; perform zero calibration before setting the cutoff.
Coriolis flowmeters are sensitive to zero drift. It is recommended to complete zero calibration first, then set the cutoff value to avoid false accumulation at low flow.
Prosonic 92 Low Flow Cutoff Setting
The Prosonic 92 is an ultrasonic flowmeter, and the cutoff function is mostly used to suppress empty pipe noise and low-flow velocity fluctuations.
Operation Steps
- Enter Setup → Output → Low Flow Cutoff.
- Enter the cutoff value; it is recommended to take 2% to 5% of full scale.
- Save and observe whether the totalized value is stable.
Ultrasonic flowmeters are greatly affected by bubbles and suspended solids, so the cutoff value can be adjusted in combination with the signal strength diagnostic value.
Parameter Comparison of the Three Instruments
| Instrument Model | Measuring Principle | Cutoff Menu Path | Recommended Cutoff Value |
|---|---|---|---|
| Promag 50 | Electromagnetic | Setup → Current Output | 1% to 3% of full scale |
| Promass 80 | Coriolis | Setup → Low Flow Cutoff | 1% to 5% of full scale |
| Prosonic 92 | Ultrasonic | Setup → Output | 2% to 5% of full scale |
Common Issues and Precautions
- Still accumulating after cutoff: Check zero drift or empty pipe detection settings.
- Normal flow is cut off: Lower the cutoff value or recalibrate the measuring range.
- Output fluctuation: Adjust together with the damping time (Damping).
It is recommended to save the cutoff value in the commissioning record for subsequent maintenance and audit traceability.