💡 Core Formula: Flow Rate (L/min) = Pulse Frequency Per Second (Hz) ÷ Coefficient

• USN-HS21TI: Coefficient = 10, so Q(L/min) = f / 10
• USN-HS06PA-1: Coefficient = 76, so Q(L/min) = f / 76

Product Overview

This article explains the calculation methods for instantaneous flow rate and total flow, as well as the formula derivation process. These contents are based on the working principle of water flow sensors such as USN-HS21TI or USN-HS06PA-1.

• ● Refers to the total volume of liquid flowing through the sensor over a period of time.

• ● Unit: Liters (L), Milliliters (mL), etc.

• ● Calculation: Total pulse count × Volume per pulse.

• ● Refers to the volume of liquid flowing per unit time, i.e., the "current" flow speed.

• ● Unit: Liters/second (L/s), Liters/minute (L/min), etc.

• ● Calculation: Obtained by converting pulse frequency per unit time.

This means: Each pulse represents 1/600 liters of water

So:

V_total (L) = N × (1 / 600)

✅ Explanation: If N pulses are detected in total, the total water volume is N multiplied by the volume per pulse.

We know:

V_total (L) = Q (L/s) × t (s)

Combining the two expressions:

N / 600 = Q × t

Divide both sides by t:

N / (600 × t) = Q

→ Q = (N / t) / 600

Since N / t is the pulse frequency f (Unit: Hz), so:

Q (L/s) = f / 600

It can also be written as:

f = 600 × Q (L/s)

✅ Unit: L/s (Liters/second)

Q(L/s) = f / 600

✅ Unit: L/min (Liters/minute)

● 1 minute = 60 seconds

● So flow per minute = flow per second × 60

Q(L/min) = (f / 600) × 60 = f × 60 / 600 = f / 10

✅ Unit: L/hour (Liters/hour)

● 1 hour = 3600 seconds

Q(L/hour) = (f / 600) × 3600 = f × 3600 / 600 = f × 6

f / 10 is L/min, multiplied by 60 gives L/hour → (f / 10) × 60 = f × 6

✅ So:

Q(L/hour) = f × 6

For USN-HS21TI sensor, the Hall sensor outputs 600 pulses per liter of water.

Product Features:

• ● Coefficient: 10

• ● Suitable for household 1/4" pipes

• ● Flow range: 1-30 L/min

• ● Volume per pulse: 1 / 600 liters

• ● Total volume formula: V_total (L) = N / 600

Instantaneous flow rate derivation:

Q(L/s) = f / 600

Q(L/min) = f / 10

Q(L/hour) = f × 6

For USN-HS06PA-1 sensor, the Hall sensor outputs 4560 pulses per liter of water.

Product Features:

• ● Coefficient: 76

• ● Suitable for low flow applications

• ● Flow range: 0.15-1.5 L/min

• ● Volume per pulse: 1 / 4560 liters

• ● Total volume formula: V_total (L) = N / 4560

Instantaneous flow rate derivation:

Q(L/s) = f / 4560

Q(L/min) = f / 76

Q(L/hour) = f × 60 / 76 ≈ f × 0.789

Assume at a certain moment:

● Pulse frequency f = 30 Hz (30 pulses per second)

Then:

• ● Q(L/s) = 30 / 600 = 0.05 L/s

• ● Q(L/min) = 30 / 10 = 3 L/min

• ● Q(L/hour) = 30 × 6 = 180 L/hour

✅ Conclusion: Current water flow rate is 3 liters per minute.

Assume f = 5 Hz

Then:

• ● Q(L/s) = 5 / 4560 ≈ 0.001096 L/s

• ● Q(L/min) = 5 / 76 ≈ 0.0658 L/min

• ● Q(L/hour) = 5 × 60 / 76 ≈ 3.95 L/hour

• 1. The pulse coefficient is determined by the sensor model! Make sure to check your sensor specifications.

• 2. Use interrupt counting to accurately obtain N and f, to avoid missing pulses.

• 3. For high real-time requirements, it is recommended to update f every second to calculate the instantaneous flow rate.

• 4. Total flow can be obtained by accumulating all pulses and dividing by the coefficient.

Sample code snippet (Arduino):

volatile unsigned long pulseCount = 0;
unsigned long lastMillis = 0;

void pulseHandler() {
  pulseCount++;
}

void loop() {
  unsigned long currentMillis = millis();
  if (currentMillis - lastMillis >= 1000) { // Update every second
    float f = pulseCount; // Frequency (Hz)
    float Q_L_per_min = f / 10; // Assume USN-HS21TI
    Serial.print("Flow rate: ");
    Serial.println(Q_L_per_min);
    
    // Update total volume
    totalVolume += f / 600; // Volume added per second
    
    pulseCount = 0; // Reset counter
    lastMillis = currentMillis;
  }
}

Instantaneous Flow Rate = Pulse Frequency ÷ Pulses per Liter

Total Volume = Total Pulse Count ÷ Pulses per Liter

As long as you know the "pulses per liter" of your sensor, you can easily achieve accurate flow measurement!