LOX/RP-1 Propellant Level Sensor

Project Overview

Propellant level sensor comparison and prototype

Prototype concentric-rod capacitance sensor assembly

Mission

Design and build a robust liquid level sensor to measure LOX and RP-1 levels in SRL’s liquid engine test stand tanks. The instrument needed to survive the high-pressure environment and cryogenic temperatures while providing a simple, reliable output for stand operations.

Challenge

Create a capacitance-based measurement that works across fluids with very different permittivities, functions in cryogenic and ambient water test conditions, and fits within SRL’s existing DAQ/control loop without adding system complexity.

Solution

Implemented a capacitance-based concentric two-rod stainless-steel sensor with PTFE end caps for electrical isolation and high-pressure sealing. Used an Arduino Uno’s known internal trace capacitance to form a capacitor divider with the sensor, enabling consistent capacitance-to-height conversion and a filtered 0–5 V analog output proportional to tank fill level with optional I2C communication.

Technologies & Tools

C++ (Arduino) Embedded Electronics Signal Filtering SolidWorks Stainless/PTFE Fabrication Cryogenic Testing Test & Integration Design for Manufacturing

Sensor demonstration: capacitance-to-height readout behavior

Design & Development

Capacitive Sensing Principle

Two concentric electrically isolated stainless-steel rods form a capacitor. As liquid fills the annulus between the rods, the effective permittivity increases and the capacitance rises. This shift in capacitance is mapped to liquid height using the fluid’s properties and the sensor’s geometry.

Stainless concentric electrodes
PTFE end caps provide isolation and pressure passthrough
Capacitance range: ~36–58 pF in normal operation
Water testing observed ~2142 pF (high ε_r)

Capacitive sensor principle diagram showing concentric rods and fluid fill

Capacitive sensing principle: concentric-rod geometry and fluid fill region

Electronics & Readout

The Arduino’s internal trace capacitance forms a divider with the sensor. Charging the sensor rods and measuring the voltage over time provides the capacitance estimate, which is then converted to fluid height. Height is output as a 0–5 V analog level or via I2C to the test stand.

Internal capacitance ≈ 30 pF (characterized for accuracy)
Resolution: ~1% for Propellant (3.5–225 pF); ~5% for DI Water (Up to 2 nF)
Minimum height resolution ≈ ±11.5 mm (total propagated error)
Optional parallel cap (~1250 pF) allows for easy conversion to water tests

Capacitor charge/discharge waveform illustrating capacitance-dependent timing

Charge/discharge behavior used to infer capacitance (and thus liquid height)

Sensor head with attached PCB shown outside the tank

Electronics PCB mounted at the sensor head for short, low-noise routing

Mechanical & Integration

The stainless/PTFE assembly was designed for high-pressure plumbing and cryogenic exposure. The analog 0–5 V and I2C outputs integrate directly with SRL’s DAQ for real-time level visualization and control logic.

Stainless-steel rods & tube for LOX compatibility and durability
PTFE dielectric and seals for isolation and electrical tank pass-through
Bench and water-flow calibration prior to stand integration
Compressed PTFE ensured liquid leak tight

Mechanical cross-section highlighting concentric electrodes and sealed pass-through

Mechanical details: concentric electrodes and sealed pass-through for tank integration

Technical Specifications

Measurement

Operating Capacitance: ~36–58 pF (LOX/RP-1)

Extended Range (water): ≈ 2.1 nF with parallel cap

Resolution: ~1% (3.5–225 pF), ~5% to 2 nF

Min Fluid Height Resolution: ±11.5 mm

Environment

Fluids: LOX, RP-1, water (for flow testing)

Conditions: Cryogenic-tolerant, high-pressure tank

Isolation: PTFE end-cap dielectric and electrical passthrough

Mechanical

Sensor Body: Concentric stainless rods

Dielectric/Seals: PTFE

Mounting: NPT threaded vertically into tank

Pressure: 1.6 structural FoS with 700 psi MEOP

Leakage: Verified acceptable leakage at 700 psi MEOP

Interface

Controller: Arduino Uno

Output: 0–5 V analog or I2C

Calibration: Characterizes fluid permittivity & sensor geometry

Results & Impact

±11.5 mm
Minimum height resolution

1% / 5%
Capacitance measurement accuracy

0–5 V
Simple analog level output

Key Achievements

Robust Cryo-Capacitive Design

Concentric stainless rods with PTFE isolation survived handling and plumbing for test-stand integration.

Direct DAQ Integration

Generated a filtered 0–5 V analog signal mapping 0–100% fill with optional I2C output simplifying stand operations.

Sensor Range Easily Extensible for Water Tests

Optional ~1250 pF parallel capacitor extended measurable range for high-ε_r water flow testing.

Lessons Learned

Characterizing the Arduino’s internal capacitance is critical for accuracy and is not consistent across different chips. This should be calibrated independently for each sensing device. Fluid permittivity changes and temperature effects drive the need for fluid-specific calibration. Cleanliness of the system must be maintained for LOX compatibility and to ensure DI-water testing remains non-conductive.

Future Work

Add temperature compensation, implement a dedicated capacitance-to-digital converter for higher resolution, and perform full cryogenic calibration with LOX and RP-1 to refine height mapping across operating conditions instead of solely using DI Water.

Project Gallery

Capacitive sensor PCB layout and board render

Capacitive sensor PCB design

Capacitive sensor circuit schematic

Sensor head assembly with readout PCB

Mechanical cross section of sensing head

Mechanical cross section: concentric tubes & pass-through

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