Low-Cost Gravimeter for Earth Tide Detection

ISEF Category: Physics and Astronomy

Subcategory: Other  ·  Difficulty: Advanced  ·  Setup: School Lab  ·  Time: Full Year

The Hook

Earth’s gravity changes all day, but the shift is tiny. If your setup is sensitive enough, you can catch the Moon and Sun tugging on your scale. That means your project can measure a real planetary signal from a desk-sized device. Few high school experiments get this close to geophysics.

What Is It?

A gravimeter measures tiny changes in local gravity. In this project, you are trying to build a simple one with a spring and a light-based readout, or with a smartphone sensor averaged over long periods. Think of the spring like a ruler for weight, except the ruler moves so little that a laser spot or electronic signal helps you read it more precisely.

The target signal is the Earth tide. That name can sound like ocean tides, but it means the ground, air, and everything on Earth flex slightly because of the Moon’s and Sun’s gravity. The effect is tiny, so you need careful calibration, long recording times, and a clean way to compare your data with published tide predictions from the Bureau Gravimétrique International.

Why This Is a Good Topic

This topic is great for a science fair because the signal exists in the real world, but you still have room to design your own method. You can test how well a low-cost sensor tracks a known gravity pattern, then compare your measurements with prediction data. That gives you a clear engineering challenge, real physics, and room for careful analysis without needing a university lab.

Research Questions

• How does the choice of readout method, laser and photodiode versus smartphone IMU averaging, affect the ability to detect Earth tides?
• What is the effect of different calibration strategies on the agreement between measured gravity changes and BGI tide predictions?
• Does increasing the recording duration improve the correlation between your gravimeter signal and predicted Earth tides?
• To what extent does temperature drift change the stability of a spring-based gravimeter over several days?
• Which filtering method gives the clearest Earth tide signal from noisy gravimeter data?
• How does the orientation or mounting of the device affect baseline drift and signal quality?

Basic Materials

• Spring with known stiffness or a small hardware-store extension spring.
• Laser pointer with stable beam output.
• Photodiode or light sensor module.
• Microcontroller or data logger for sensor readings.
• Rigid frame or tripod mount.
• Smartphone with accelerometer or IMU logging app.
• Meter stick or ruler for calibration.
• Digital kitchen scale or small weights for calibration checks.
• Notebook for time-stamped observations.
• Computer with spreadsheet software.
• Thermometer for tracking room temperature.

Advanced Materials

• Optical bench or rigid vibration-isolated mounting frame.
• Low-noise photodiode amplifier circuit.
• High-resolution analog-to-digital converter.
• Precision spring or cantilever element.
• Temperature sensor with continuous logging.
• Reference mass set for calibration.
• Vibration isolation pads or table.
• Data acquisition interface.
• Computer for signal processing and time-series analysis.
• Access to local gravity prediction tables from the Bureau Gravimétrique International.

Software & Tools

• Excel or Google Sheets: Organizes time stamps, calibration data, and simple trend plots.
• Python: Helps you filter noise, smooth time-series data, and compare signals with prediction curves.
• ImageJ: Measures laser spot movement from photos or videos if you use an optical readout.
• Audacity: Can inspect audio-rate sensor logging if your setup exports data in that format.
• R: Supports correlation tests, drift analysis, and clear statistical graphs.

Experiment Steps

1. Define the sensing method you will trust most, then decide what signal counts as a gravity change and what counts as noise.
2. Build a calibration plan that turns sensor output into a real displacement or acceleration value.
3. Choose one baseline-stability strategy, then plan how you will check drift across multiple days.
4. Set up a timing system that matches your measurements to public Earth-tide predictions.
5. Plan your comparison metric, such as correlation, phase lag, or amplitude agreement.
6. Decide how you will test alternative filtering or averaging methods without changing the hardware at the same time.

Common Pitfalls

• Letting room temperature change the spring length, which looks like a gravity signal when it is really drift.
• Mounting the device on an unstable table, which adds vibration that hides the tiny tide pattern.
• Skipping calibration at multiple points, which makes the readout hard to convert into useful units.
• Comparing your data to tide predictions with mismatched timestamps, which shifts the signal out of alignment.
• Using a sensor setup with too much noise floor, which erases the Earth tide signal before filtering can help.

What Makes This Competitive

A strong version of this project does more than show a wiggle on a graph. You need solid calibration, careful drift control, and a fair comparison to prediction data. If you test two sensing methods, compare multiple filters, or quantify the lag and error against Bureau Gravimétrique International predictions, your project starts to look like real instrumentation work. That kind of design and analysis usually stands out more than a simple demonstration.

Project Variations

• Test the same gravimeter indoors and outdoors to see how environmental vibration changes signal quality.
• Compare a spring-and-laser readout with a smartphone IMU setup to see which method tracks Earth tides better.
• Use the device to measure tidal patterns at different times of year, then compare the seasonal fit to public prediction data.

Learn More

• Bureau Gravimétrique International: Search for Earth tide prediction tables and gravity reference materials.
• NOAA National Centers for Environmental Information: Look for geophysics and Earth system data pages that explain tidal forces and related measurements.
• USGS: Search for educational material on gravity, geodesy, and Earth deformation.
• NASA Earth Observatory: Find plain-language articles on how the Moon and Sun affect Earth systems.
• MIT OpenCourseWare: Search physics courses on oscillations, measurement, and data analysis for background on sensor calibration.
• PubMed: Search for review articles on gravimetry, geodesy, and low-noise sensing methods if you want a more technical overview.