The Dwyer HFL-4-25 is an aluminum-body in-line flow monitor that reads 2 to 25 GPM (7.5 to 95 LPM) of oil-based fluids through a 1 in. female NPT connection, rated to 3500 psig (241 bar) for liquids with ±2% full-scale accuracy.
Flow pushes a sharp-edged floating orifice disk and ring-shaped transfer magnet along a tapered center shaft against a return spring. A magnet follower outside the flow tube tracks the internal magnet, and its red reference line reads flow rate directly on the scale.
Specifications per Dwyer Series HF catalog page 259 (2022-11-29) and Bulletin F-55, FR# 440843-00 Rev. 4 (7/15). Verified October 12, 2026.
Best For
- Detecting low-flow rates for lubricating liquids
- Fluid power systems
- Setting pressure relief valves
- Fluid handling equipment
- Industrial maintenance
Warranty
Warranty coverage excludes monitor damage caused by excessive contamination. Protect the HFL-4-25 with filtration of at least 74 microns (200 mesh) and clean and inspect it every six months. Contact Team Zot for warranty terms and claims.
Key Features
Floating Orifice Design
A sharp-edged floating orifice disk and transfer magnet ride a tapered center shaft, giving stable variable-area readings at fluid viscosities up to 500 SSU.
Direct-Reading Dual Scale
A magnet follower outside the flow tube carries a red reference line that reads directly against the GPM and LPM scale (2 to 25 GPM / 7.5 to 95 LPM).
Sealed Metal Casing
All internal wetted parts are sealed inside the tubular body casing. The magnet follower is sealed inside the polycarbonate window tube, so the unit can be washed down with soap and water.
Mount in Any Orientation
Installs horizontally, vertically or inverted with no straight pipe required before or after the monitor. 90° elbows can be fitted on both ends without significant flow variation.
3500 psig Pressure Rating
Rated to 3500 psig (241 bar) for liquid service and 240°F (116°C). Accuracy is ±2% of full scale with ±1% full-scale repeatability.
Aluminum Body for Oil
Aluminum casing with Buna-N seals and 304 SS internal parts, scaled in GPM and LPM of oil for oil-based fluids.
ZOT Supply is an authorized distributor of Dwyer products.
Technical Specifications
| Model | HFL-4-25 |
|---|---|
| Series | Dwyer Series HF In-Line Flow Monitor |
| Service | Oil-based fluids |
| Flow Range (GPM) | 2 to 25 GPM oil |
| Flow Range (LPM) | 7.5 to 95 LPM oil |
| Scale | Dual scale, GPM and LPM, direct reading |
| Connection | 1 in. female NPT |
| Body Material | Aluminum |
| Wetted Materials | Aluminum body, Buna-N seals, PTFE-coated Alnico magnet, 304 SS internal parts |
| Maximum Pressure (Liquids) | 3500 psig (241 bar) |
| Temperature Limit | 240°F (116°C) |
| Maximum Pressure Drop | 8.41 PSID |
| Accuracy | ±2% FS over entire range |
| Repeatability | ±1% of full scale |
| Maximum Viscosity | 500 SSU |
| Specific Gravity Correction | Actual = Observed Reading x √(0.873 / S.G.) |
| Before Use | Units are tested with light hydraulic oil and some oil residue may remain. Check compatibility with your fluid; cleaning may be required before use. |
| Filtration | 74 micron filter or 200 mesh screen minimum |
| Flow Direction | One direction only, matching the arrow on the scale |
| Mounting | Horizontal, vertical or inverted; no straight pipe required |
| Window Tube | Polycarbonate (Lexan) |
| Dimensions | A (reference): 2.375 in.; B (wrench flats): 1.750 in.; C (reference length): 7.125 in. |
| Cartridge Retainer Clip | Waldes No. 5105-18H |
| Shipping Weight | 3.5 lb (1.59 kg) |
| California Prop 65 | WARNING: This product can expose you to chemicals including Lead, which is known to the State of California to cause cancer and birth defects or other reproductive harm. For more information go to www.P65Warnings.ca.gov. |
Frequently Asked Questions
What flow range does the Dwyer HFL-4-25 measure?
The HFL-4-25 reads 2 to 25 GPM (7.5 to 95 LPM) of oil on a dual GPM/LPM scale, with ±2% full-scale accuracy and ±1% full-scale repeatability.
What is the maximum pressure rating of the HFL-4-25?
The HFL-4-25 is rated to 3500 psig (241 bar) for liquid service, with a temperature limit of 240°F (116°C).
Does the HFL-4-25 need straight pipe before or after it?
No. The monitor installs directly in the line horizontally, vertically or inverted with no straight pipe required, and 90° elbows can be fitted on both ends without significant flow variation.
How do I correct the HFL-4-25 reading for an oil with a different specific gravity?
Multiply the observed reading by the square root of (0.873 / S.G.), where S.G. is the specific gravity of your oil. Indication reads high for heavier fluids and low for lighter fluids.
Can the HFL-4-25 be used on water?
No. Aluminum monitors should not be used for water. The HFL-4-25 is built for oil-based fluids.
What filtration does the HFL-4-25 need?
Install system filtration of at least a 74 micron filter or a 200 mesh screen ahead of the monitor. Monitor damage caused by excessive contamination is not covered by warranty.
Can the HFL-4-25 handle reverse flow?
No. Flow must match the arrow on the scale. In the reverse direction the unit acts like a leaky check valve; for occasional reverse flow, install a check valve in parallel with the monitor.
