How to Build a Low-Cost Orifice Plate Mass Flow Meter for Compressed Air (0.07–0.2 kg/s) – Completely DIY in India
If you need to measure compressed air mass flow in the range 0.07 to 0.2 kg/s (roughly 250–700 m³/hr at 6 bar) in a 1-inch pipeline, and you cannot afford a ready-made thermal/Coriolis meter (₹1.5–3 lakh), here is a proven, fully fabricable orifice-plate-based solution that costs only ₹25,000–₹30,000.
Service Conditions
- Fluid: Dry compressed air
- Mass flow range: 0.07 – 0.2 kg/s
- Pipe size: 1 inch Schedule 40 (ID = 27.3 mm)
- Max gauge pressure: 5 bar → ~6 bar(a)
- Max temperature: 60 °C
Step-by-Step Design Calculations
1. Selection of β ratio
Chosen β = 0.50 (best compromise between accuracy and pressure loss)
Orifice bore d = 0.5 × 27.3 = 13.65 mm → 13.6 mm (easy to machine)
2. Expected Differential Pressure
Density at 6 bar(a), 60 °C ≈ 6.27 kg/m³
Using ISO 5167-2 formula →
- At 0.2 kg/s → ΔP ≈ 11.2 kPa ≈ 1140 mmWC
- At 0.07 kg/s → ΔP ≈ 140 mmWC
This range is perfectly measurable with commonly available Indian DP transmitters.
3. Final Orifice Plate Specification
| Parameter | Value |
|---|---|
| Pipe ID (Sch 40) | 27.3 mm |
| Orifice bore (d) | 13.6 ± 0.02 mm |
| β ratio | 0.50 |
| Plate thickness | 3–4 mm |
| Material | SS304 |
| Taps | Corner taps |
| Bevel | 45° on downstream side |
| Permanent pressure loss | ≈ 55% of ΔP (≈ 600 mmWC max) |
Fabrication Drawing (Give This to Any Local Workshop)
- Plate size: 80 mm OD circular or 50×50 mm square with handle tab
- Centre hole: 13.6 mm reamed
- Downstream face: 45° bevel
- Upstream face: polished (Ra < 0.4 µm)
- Weld two 6 mm SS nipples on pipe for corner taps
Recommended Instruments (Easily Available in India – 2025 prices)
| Parameter | Make & Model | Range | Approx Price |
|---|---|---|---|
| Differential Pressure | WIKA A-10 / Waaree / Galaxy | 0–2000 mmWC | ₹9,000–12,000 |
| Cheap mechanical option | Galaxy Magnehelic type | 0–2000 mmWC | ₹3,500–5,000 |
| Gauge Pressure | WIKA 233.50 / H Guru | 0–10 bar | ₹1,200–2,000 |
| Temperature | Pt-100 RTD or bimetallic | 0–100 °C | ₹800–1,500 |
Field Calculation Formula (Mass Flow)
Mass flow (kg/s) = 0.000178 × √(ΔP in mmWC) × √(P_abs in bar × 288/(T+273))
You can put this formula in Excel or display live value using Arduino + 4-20 mA DP transmitter.
Total Estimated Cost (India – Nov 2025)
- Orifice plate + welding: ₹4,000
- DP transmitter: ₹10,000
- Pressure & temperature gauges: ₹3,000
- Tubing, valves, manifold: ₹5,000
- Optional display/logging: ₹4,000
- Total: ₹25,000 – ₹30,000 only
Is Thermal Mass Flow Meter Easier to DIY?
Absolutely NOT.
- You still need to buy the sensing element (₹15,000–40,000)
- Complex electronics & calibration required
- Not truly DIY
Orifice plate + DP cell + P&T compensation is the cheapest, most robust and 100% locally fabricable solution in India.
