Configuring an Electrical Power Sensor in BMON
Here is an example of how to configure a Continental Control Systems WattNode Pulse wired to a 250 amp circuit and connected to a Monnit pulse counter. The objective is to configure this setup in BMON to read out electrical Power in kilowatts (kW). As explained in the prior section, the electrical power value in BMON can be used to determine total energy use in kWh for any particular time interval of interest.
We’re assuming your WattNode device is properly connected to the device you are monitoring and that the Monnit pulse counter is connected to the pulse output of the WattNode. In this example the Monnit sensor is posting data every 10 minutes. It is critical that the Monnit Pulse Counter is set to “Accumulate” mode, so that it accumulates a total, increasing, pulse count over time.
Reviewing the WattNode manual and the setup in this example we find that for a WattNode Model 3Y-208/3D-240, wired into a 250 amp circuit, our Watt-hours per pulse (WHpP) is 6.250.
The BMON Transform feature has a special variable named
rate. If a Transform expression uses this variable, BMON knows that a pulse count is being measured, and BMON automatically calculates the pulse rate per second indicated by the last pulse count reading received relative to the prior pulse count reading. This pulse rate is stored in the variable
rate. As an example, assume that the last pulse count received was a total count of 10,435. The prior pulse count reading from the sensor was 9,623 and it was received 605 seconds prior to the current reading. The pulse rate that occurred between the two readings was (10435 - 9623) / 605 = 1.342 pulses/second. This value of 1.342 is automatically stored in the
To properly fill out the
Transform entry in BMON, we need to use that
rate variable in a math expression to calculate the kilowatts of electrical power being used. Here is the formula, with some more detailed explanation in a subsequent paragraph:
Power = rate in pulses/second * 6.25 Watt-hours / pulse * 3600 seconds / hour * 0.001 kW / W
Power = rate * 22.5
So, you would enter
rate * 22.5 in the Transform box for the Sensor in BMON. That sensor will now display the average electrical kW for each 10 minute interval posted by the Monnit Pulse Counter.
Here is a more detailed explanation of the formula:
rate variable contains a number indicating the number of pulses per second put out by the Power sensor. In our example, each pulse is worth 6.25 Watt-hours, so we first multiply rate by 6.25 to then give us the number of Watt-hours of energy used per second. We need to first convert this power use to Watts. Watts of power is the number of Watt-hours used in one hour. So, the per second value needs to be scaled up to a per hour value. That is why we multiply by 3600 seconds / hour. Finally we need to convert Watts to kilowatts, which is done by multiplying by 1 / 1000, or 0.001.