Gravimetric fuel consumption measurement is conceptually simple — weigh the fuel before and after, divide by time. The engineering challenge is doing this accurately at the sub-percent level in a vibrating test cell environment, with temperature compensation applied in real time.
Here's the technical architecture.
Hardware Layer
Fuel Tank (150L stainless steel)
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+-- 4x Load Cells (shear beam, rated capacity 200kg each)
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+-- Signal Conditioner / Load Cell Amplifier
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+-- PLC Analog Input (4-20mA or 0-10V)
RTD Sensors (PT100, 3-wire)
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+-- RTD Transmitter (4-20mA output)
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+-- PLC Analog Input
Mass Measurement Accuracy
Load cell selection is critical. For a 150L tank of diesel (~130kg full):
- Load cell rated capacity: 4x 50kg = 200kg total
- Required accuracy: ±0.1% of reading
- Choose load cells with linearity <= 0.02% FS, hysteresis <= 0.02% FS
Install on isolation mounts to reduce vibration coupling from the test cell engine. Sample at 10Hz minimum; average over 30-second windows to eliminate vibration noise.
Temperature Compensation Algorithm
ISO 3046 and SAE J1321 reference correction:
# Fuel density correction to 15°C reference
# For diesel fuel (approximate)
def correct_to_reference(mass_kg, temp_c, ref_temp=15.0):
# Diesel volumetric expansion coefficient ~0.00085 per °C
alpha = 0.00085
correction_factor = 1 + alpha * (temp_c - ref_temp)
corrected_mass = mass_kg / correction_factor
return corrected_mass
# In the PLC, implement as:
# corrected_mass = raw_mass / (1 + 0.00085 * (fuel_temp - 15.0))
For precise work, use the actual density-temperature relationship from ASTM D1250 petroleum measurement tables rather than a linear approximation.
PLC Scan Cycle and Data Logging
PLC Main Loop (100ms scan):
1. Read load cell ADC value -> convert to kg using calibration polynomial
2. Read RTD value -> convert to °C
3. Apply temperature compensation -> corrected_mass_kg
4. Calculate delta_mass = prev_corrected_mass - corrected_mass
5. Accumulate total_fuel_consumed
6. Calculate instantaneous_rate = delta_mass / scan_time (kg/s)
7. Log to historian: timestamp, raw_mass, temp, corrected_mass, rate
Every 1 second: write to HMI display tags
Every 10 seconds: write to data log file
At test end: generate structured report
Calibration Procedure
Dead-weight calibration using OIML Class M1 weights:
- Zero the system with empty tank + known tare weight
- Apply 25%, 50%, 75%, 100% of rated load in 5kg increments
- Record ADC output vs applied mass
- Fit calibration polynomial (linear is sufficient for quality load cells)
- Record calibration date and certificate number in system config
Calibration must be traceable to national standards: NIST (USA), NPL (UK), PTB (Germany), NABL (India).
Structured Test Report Format
Generate reports with these mandatory fields for ISO/SAE compliance:
- Test ID, date/time (UTC), operator
- DUT engine serial number and model
- Load cell calibration certificate references
- RTD calibration certificate references
- Raw data table: timestamp, raw_mass, fuel_temp, corrected_mass, cumulative_consumed
- Calculated fuel economy: g/kWh, L/100km, or BTU/hp-hr as required
- Temperature correction factors applied
- Pass/fail vs specification
The Neometrix FCMS implements this architecture for locomotive and engine test bench applications.
→ https://neometrixgroup.com/products/fuel-consumption-measurement-system
