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HydraMeter is an unusually ambitious open-source multimeter project—not a finished, safety-certified replacement for a commercial meter. Created by John Duffy, it combines conventional voltage, current, and resistance measurement with a detachable wireless display, a front-mounted replaceable fuse cartridge, and an unconventional toggle-switch interface.
That combination makes HydraMeter notable for makers, electronics engineers, repair technicians, and tool designers. It also makes its limitations important: the published design is still difficult to assemble, requires manual rework, and has unresolved construction and battery-safety issues. It is best approached as an advanced open-hardware design to study, build, and modify—not as a ready-to-use instrument for mains or high-energy electrical work.
What HydraMeter is
HydraMeter is a scratch-built digital multimeter designed by John Duffy. It is not a firmware modification of an existing commercial meter. The project includes its own analog measurement sections, digital electronics, enclosure, controls, display system, firmware, schematics, PCB files, and case CAD.
Its documented measurement architecture separates the voltage, current, and resistance functions into dedicated analog front ends. These sections feed a programmable-gain amplifier and analog-to-digital converter, after which a microcontroller processes the readings and drives the display. The project’s schematic and operation notes provide the clearest overview of that signal path.
#1 Best Overall
- Additional Tips - The following incorrect operations may cause the multimeter not to show results: Firstly, the plugs of test leads are not fully inserted or not inserted into the correct sockets. Secondly, the manual rotary switch is not placed in the correct position. In addition, this meter can not test all AC Current and below 100mV AC Voltage. Please check the user manual carefully before measurement.
- Versatile Digital Multimeter - Accurately measures AC/DC Voltage, DC Current, Resistance, and Diode. This Multimeter is a really useful tool for solving industrial and household electrical issues. Suitable for Household Outlets, Fuses, Batteries (including Vehicles), Automotive Circuit Troubleshooting, Charging Systems, Testing electronics in Cars etc.
- Troubleshooting with Accuracy - This Multimeter has a sampling speed of 2 times per second; Built-in a backlight LCD display with 3 ½ digits (1999 count) 0.6”, and high polarity including negative and positive readings.
- Ensures Safety - Double fuse is anti-burn and protects from overloading. The silicone cover can protect the multimeter from failing damage and prevent electric shocks. And low battery indication will be displayed when battery power is low.
- Ease of Use - Support Data Hold and Continuity Buzzer. Includes Convenient feature like LCD Backlit Screen makes it easy to use in dimly light areas. Batteries/Set of Test Leads/User Manual are Included.
- Probe inputs receive the electrical signal.
- A dedicated voltage, current, or resistance front end scales and conditions it.
- A programmable-gain amplifier adjusts the signal for measurement.
- An ADC converts the conditioned signal into digital data.
- The microcontroller calculates and presents the result locally or on the detachable display.
This is a complete instrument design exercise, not simply an inexpensive microcontroller project.
Why the design is unusual
A detachable wireless display
The display can be separated from the main meter body. That could be useful when the body is behind equipment, inside a crowded test setup, or positioned where the user cannot easily see it while manipulating probes.
The feature also adds complexity. HydraMeter needs a second display and communication subsystem, along with additional power, mechanical, and software considerations. The available project material does not establish a particular wireless range, latency, security level, or battery life, so those should not be assumed.
A replaceable front fuse cartridge
HydraMeter’s front-mounted fuse cartridge is intended to make fuse replacement faster than opening a conventional enclosure. That is a thoughtful serviceability feature, particularly for a tool expected to be used and modified by its owner.
It does not, by itself, prove that the instrument is safe for high-energy circuits. Fuse type, interrupt rating, arc containment, spacing, enclosure strength, and the rest of the input-protection system still matter. A cartridge must never be bypassed with wire or fitted with an incorrect fuse.
Toggle switches instead of a rotary selector
Rather than using the familiar rotary range switch, HydraMeter exposes its configuration through physical toggle switches. The state of the switches is visible and mechanically accessible, which suits the project’s open-hardware character. Switches can also be replaced, rewired, or customized.
Rank #2
- VERSATILE FUNCTIONALITY: Measures AC/DC voltage up to 600V, 10A DC current, 2MΩ resistance; additional features include continuity, diode test and battery test
- LEAD-ALERT PROTECTION: LEDs on the meter illuminate to indicate proper test lead placement, enhancing accuracy and safety during measurements
- BACKLIT DISPLAY: LCD shows clear readings in low-light conditions for enhanced visibility
- BATTERY TEST: Battery test mode can be used for checking if batteries are working
- CONVENIENT FEATURES: Test lead holders on the back of the meter, kickstand and optional magnetic hanger (Cat. Nos. 69445 or 69417) for hands-free operation
The trade-off is a more complicated user interface. Multiple switches may be less intuitive than a single selector, incorrect combinations may be possible, and the controls occupy more panel space. A later design goal identified in the 2025 coverage was adding a rotary range switch for version 2.0.
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What “open source” means here
The HydraMeter repository publishes schematics, PCB material, case CAD, and code directories. The hardware is released under the CERN Open Hardware Licence Version 2, Strongly Reciprocal. That license is designed to keep applicable derivative hardware open under its terms.
For a technically capable builder, this openness offers genuine value:
- The measurement architecture can be inspected rather than treated as a black box.
- Hardware and enclosure designs can be modified.
- Firmware and interface behavior can be studied.
- Faults and design decisions are visible to the community.
- A builder can adapt the instrument for particular experiments or workflows.
However, open files do not automatically make a project reproducible. They do not guarantee a complete bill of materials, part availability, polished firmware, simple assembly, safety certification, or a kit or assembled product.
HydraMeter’s own repository says that detailed assembly documentation is incomplete and that the design is not currently easy to assemble. It documents substantial manual PCB rework, including an incorrect component footprint, trace cuts, hand-wired corrections, and PCB changes that had not been fully verified. The material also notes incomplete switch-wiring information.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsThe difficult engineering behind a multimeter
A multimeter may look simple from the outside, but a reliable instrument has to solve several difficult problems at once. It must measure very different signal levels, protect its inputs, switch ranges without creating dangerous or misleading paths, reject noise, maintain calibration, and present an understandable result.
Rank #3
- Versatile Digital Multimeter - Accurately measures AC/DC Current, AC/DC Voltage, Capacitance, Frequency, Duty Cycle, Resistance, Diode, Continuity and Temperature.
- Thoughtful Design - Support Data Hold, Large LCD Backlit Screen, Auto Shut-off and Kickstand make the process of measurements easier. Professional level is reflected in some features include Auto-Ranging capability, and True RMS for measuring both AC Current and Voltage.
- Suitable For Many Occasions - This Multimeter is a golden partner to help to troubleshoot a variety of automotive and household electrical problems safely and accurately.
- Ensure Safety - Double ceramic fuse is anti-burn and protects from overloading，and it will be more secure and reliable； F400mA/600V and F10A/600V explosion-proof ceramic fuse tubes can protect the multimeter effectively.
- Additional Tips - Please take off the cap before using the test leads. Check the manual for more usage information.
Current measurement is especially demanding. A meter’s current path must carry the expected load while protecting the instrument against misuse. The HydraMeter repository specifies an 18-AWG-or-heavier connection for the 10-A range and describes that range as carrying the full current through a hardwired connection. That is an important construction detail, not a minor wiring preference.
The physical design matters just as much. Probe jacks, fuse mounting, PCB spacing, enclosure materials, barriers, switches, battery wiring, and strain relief all contribute to the behavior of the finished instrument. A schematic that appears sound cannot establish the safety of every physical build made from it.
The uncomfortable part: buildability and safety
The most important qualification is that HydraMeter should not be treated as a validated commercial meter. The repository identifies the published material as HydraMeter 0.4, while also noting that the as-made PCB was version 0.3 and required significant rework. Some newer PCB tweaks were not verified.
The battery section is another serious warning. It requires a protected cell and describes an improvised charging method that can overcharge an unprotected cell. That is not a detail to experiment with casually: lithium-ion cells can vent, ignite, or fail violently when incorrectly charged or protected.
No supplied source verifies independent CAT testing, accredited safety certification, or a complete independently validated accuracy specification for the current HydraMeter revision. The project therefore should not be described as mains-safe, professionally accurate, or equivalent to a certified Fluke, Brymen, Keysight, or similar instrument.
Before using any DIY meter on energized equipment, a technically qualified builder would need to independently evaluate the complete assembly, including:
Rank #4
- Accurately Test Full Features: Accurately measures AC/DC voltage, DC current, resistance, continuity test, diode and batteries.
- High Quality & Safety :Our multimeter is designed to safely and accurately troubleshoot a variety of automotive and household electrical problems. Overload Protection on all ranges to ensure long-term service life,Low Battery Indication, Audible continuity sensor checks that the circuit or wires conduct electricity. Double insulation with a good angled stand for hand free use.
- EASY TO USE: 2.7"Large Backlit LCD Display, Data Hold, Easy to Read Large Back-light Screen forvisibility in dimly light areas. Our mutimeter tester is widely used in Car repair, Homeuse, Schools, laboratories, Families and factories, suitable for Car Drivers, Beginners,Professional Electricians, Housewives, Students Use.
- Nice Protective Orange Shell with Stand Built: Our digital multimeter can be hand held or stood using its fold out stand. Removable thick rubber cover is made of kind of non-slip slightly soft plastic that will help with drop protection & minor bumps protection.
- 2 YEARS Quality Guarantee & Included 9V Battery: The compartment lid at the back is easy to remove. Battery/Set of Test Leads/User Manual are Included.They guarantee against faulty materials and workmanship for 2 years-giving you total peace of mind. If there is any issue within24 months of your purchase, we will replace it for vou with no doubt.
- Input resistor voltage and power ratings.
- Fuse type and interrupt capability.
- PCB creepage, clearance, slots, and insulation barriers.
- Transient protection and fault-current behavior.
- Probe jacks, leads, switches, and enclosure materials.
- Battery protection and charging circuitry.
- Assembly consistency and calibration.
Until that work is done and documented for the exact build, low-voltage electronics experimentation is the appropriate context. Mains, service panels, industrial equipment, and high-energy battery systems require a properly validated instrument.
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Functionally, the design clearly aims to perform ordinary multimeter measurements. It has separate analog front ends for voltage, current, and resistance, along with signal conditioning, conversion, processing, and display.
That is different from proving measurement quality. The available documentation does not provide a complete, independently validated accuracy table for the current revision. It would therefore be wrong to claim that HydraMeter matches a commercial meter, offers professional-grade accuracy, or is more accurate than a Fluke or Brymen.
Its practical value depends on what “practical” means:
- As a learning platform: highly promising for someone comfortable with analog circuits, PCB assembly, firmware, calibration, and troubleshooting.
- As a modifiable tool: potentially valuable because the hardware and software are inspectable.
- As an immediately usable general-purpose meter: a poor fit compared with a ready-made commercial instrument.
- As a mains instrument: not established by the available evidence and inappropriate without independent safety validation.
Who should build it?
HydraMeter makes sense if you:
- Want to understand how a multimeter is designed from the inside.
- Can read analog schematics and assess measurement and protection circuits.
- Are comfortable with PCB rework, hand wiring, calibration, and debugging.
- Want to modify the interface, enclosure, firmware, or measurement system.
- Accept that the current design is not turnkey.
- Plan to begin with appropriately limited, low-voltage experiments.
Buy a commercial meter instead if you:
- Need a working instrument immediately.
- Work on mains, service panels, industrial equipment, or high-energy batteries.
- Need documented accuracy, a warranty, calibration support, or replacement parts.
- Expect a complete assembly guide and reproducible bill of materials.
- Do not want the meter itself to become the project.
HydraMeter versus buying a meter
| Criterion | HydraMeter | Brymen BM235 | Fluke 87V/87V MAX |
|---|---|---|---|
| Open hardware | Yes | No | No |
| Ready-to-use purchase | No verified assembled-product route | Yes, subject to availability | Yes |
| User modification | High | Limited | Limited |
| Documentation maturity | Incomplete project documentation | Commercial product documentation | Mature commercial documentation |
| Safety evidence | Not independently verified in the supplied sources | Manufacturer-published ratings | Manufacturer-published ratings |
| Detachable wireless display | Yes | No | No |
| Best use | Study, experimentation, and customization | Hobby, repair, and general service work | Professional electrical and industrial troubleshooting |
Brymen BM235
The Brymen BM235 is a more practical middle ground for many hobbyists and repair technicians. Its manufacturer-published features include True RMS AC measurement, a 6000-count display, capacitance, frequency, resistance, temperature and current measurement, AutoV/LoZ functions, an input-jack warning, 6-kV transient protection, and EN61010-related CAT ratings.
The official European product page showed €86.13 excluding tax and €105.94 including tax when crawled, but also showed the product out of stock. That is a dated regional price signal, not a universal current price.
Best Value
- VERSATILE FUNCTIONALITY: Measures AC/DC voltage up to 600V, 10A AC/DC current, 50MΩ resistance; additional features include continuity, temperature, capacitance, frequency/duty cycle and diode test
- LEAD-ALERT PROTECTION: LEDs on the meter illuminate to indicate proper test lead placement, enhancing accuracy and safety during measurements
- BACKLIT DISPLAY: LCD shows clear readings in low-light conditions for enhanced visibility
- ACCURATE MEASUREMENTS: Auto-ranging and True Root Mean Squared (TRMS) technology provides precise and accurate measurements
- CONVENIENT FEATURES: Test lead holders on the back of the meter, kickstand and optional magnetic hanger (Cat. Nos. 69445 or 69417) for hands-free operation
Fluke 87V and 87V MAX
The Fluke 87V family is aimed at professional electrical and industrial troubleshooting. Fluke’s product literature describes 1000-V AC/DC measurement, 10-A current measurement with a short-duration 20-A capability, resistance, continuity, diode, frequency, and temperature functions, as well as features intended for noisy variable-speed-drive environments.
HydraMeter should not be presented as a direct competitor on certification, ruggedness, calibration, warranty, support, or field reliability. Its advantage is openness and design freedom, not established commercial assurance.
HydraMeter’s place among open-source meters
HydraMeter is not the first open-source or DIY multimeter. Earlier projects included a 2016 effort aimed at a low-cost open-source CAT-rated meter and a 2019 STM32-based meter offering voltage, current, power, continuity, component testing, and USB connectivity. The latter explicitly positioned its accuracy for prototyping rather than as an equivalent to a high-end commercial meter.
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HydraMeter’s distinction is its breadth of ambition. It treats the multimeter as a complete product-design problem: analog electronics, digital processing, enclosure, controls, serviceability, wireless display, and user experience. That is why it stands out even though DIY multimeters already existed.
The verdict
HydraMeter raises the bar for open-source multimeter projects because it attempts to deliver the whole instrument, not merely a low-cost measurement PCB. Its detachable display, fuse-cartridge concept, toggle controls, and published design files show an unusual level of mechanical and electronic ambition.
But it does not currently raise the bar above certified commercial meters for safety, reproducibility, documented accuracy, or field readiness. The project’s incomplete assembly guidance, manual rework, battery warnings, and unverified revisions are central facts—not footnotes.
For an experienced maker, HydraMeter is an excellent subject for study and a potentially rewarding platform for modification. For a beginner’s first meter or any work where fault energy and personal safety are serious concerns, a properly rated commercial instrument remains the responsible choice.
View the HydraMeter design files on GitHub or read the later design walkthrough for additional project context.
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