Configuring an Electrical Power Sensor in BMON

Continuing the discussion from Electrical Power versus Energy in BMON:

Electric meters, gas meters, water meters, and fuel meters often produce pulses as a means of electronically communicating flow through the meter. Each pulse represents a fixed amount of energy or volume of material flowing through the meter. The pulse output of the meter can be connected to a pulse counter that reports total pulse counts to the BMON system. It is generally desirable to convert these pulse count totals into a rate of flow of energy, fluid, or gas. By converting to a rate (units/second), variations in the length of the interval measured by the pulse counter are factored out of the measured value.

Special pulse counter Transform features were developed for BMON to address this situation. The recommended setting for a pulse counter is to have it continually accumulate pulse counts, only resetting to zero when a maximum rollover pulse count is reached. ~ Excerpt from Alan Mitchell’s BMON Documentation

Below are examples of how you can transform the total pulse count values into a usable rate value.

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 rate variable.

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

Final Formula:
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:
The 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.