Overview

Sometimes the questions arise:

  • “How fast does the WattNode respond to a spike in power?”
  • “How quickly can current transformers react to a change in current?”

Generally, this occurs for cases where sudden spikes in power or current are anticipated and someone wants to use the WattNode meter to measure this spike or react to it. Unfortunately, WattNode meters are not optimized for instantaneous response and may not respond as quickly as desired. The actual response times vary for different models.

Current Transformers

Current transformers have a high bandwidth, generally 5 kHz or higher, so they will respond in less than a millisecond to a spike in current or power.

WattNode Pulse

The WattNode Pulse has the fastest response time of all models. It measures the power in 200 millisecond time blocks and updates the pulse output frequency approximately 100 milliseconds after completing a measurement block (note: no power or energy is lost between blocks, because as soon as one measurement block completes, the next one starts). This means the pulse output frequency should update between 100 ms and 300 ms after a change (or spike) in power.

However, depending on the current pulse frequency, it might take a bit longer to see the effect of the higher (or lower) pulse frequency. For example, if the full-scale pulse frequency is 4.0 Hz and the power level is running at 50% of full-scale (2.0 Hz), and then a spike occurs to 100% power, it could take between 250 milliseconds (1 / 4.0 Hz) and 500 milliseconds (1 / 2.0 Hz) to see a new pulse at the higher frequency. This delay can be reduced by selecting a higher full-scale pulse frequency with Option Hz=xxx, where “xxx” can be as high as 600 Hz for the standard bidirectional outputs (150 Hz for other outputs). For example, 1 / 600 Hz = 1.67 milliseconds.

Also, since the WattNode measures 200 millisecond blocks, if the spike is much shorter than 200 milliseconds, the WattNode will report the average power during the 200 millisecond block, not the peak. So if you go from 50% to 100% of full-scale power for 80 milliseconds, the reported power for the interval would be 70% of full-scale power.

Effective Response Time: 100 – 300 milliseconds, plus the new pulse period (can range from 1.67 milliseconds to hours, depending on the current power level and the full-scale pulse frequency).

WattNode BACnet and Modbus

The WattNode BACnet and Modbus meters updates their raw measurements every 1.0 seconds, and processes the raw measurements into final data in approximately another 300 milliseconds. It then will typically take 50-100 milliseconds to request new data from the WattNode and get the response. So, a spike in power could take 0.4 to 1.4 seconds to appear in the output data.

Since the WattNode BACnet and Modbus meters uses a one second basic measurement interval, even if you have configured a meter for no averaging (see the Averaging register in the manual), any spike in power will effectively get averaged over the one second reading. So if you go from 50% to 100% of full-scale power for 80 milliseconds, the reported power for the interval would be 54% of full-scale power, which is relatively small. Only longer spikes in power will be easily measured.

Effective Response Time: 0.4 – 1.4 seconds

Note: If you configure the WattNode BACnet or Modbus meters to use averaging (the default is five seconds of averaging and the maximum is 60 seconds), then it will appear to respond much more slowly to sudden changes in power and other values.

WattNode LonWorks

The WattNode LonWorks updates its raw measurements every 5.0 seconds, and processes the raw measurements into final data in approximately another one second. It then will typically take 200-500 milliseconds to request new data from the WattNode and get the response. So, a spike in power could take 1.2 to 5.5 seconds to appear in the output data.

Since the WattNode LonWorks uses a five second basic measurement interval, any spike in power will effectively get averaged over the five seconds. So if you go from 50% to 100% of full-scale power for 80 milliseconds, the reported power for the interval would be 50.8% of full-scale power, which is essentially unnoticeable. Only longer spikes in power will be easily measured.

Effective Response Time: 1.2 – 5.5 seconds

Other Measurements

The Pulse WattNode only generates pulses, but the Modbus and LonWorks models also measure voltage, reactive power, current, power factor, frequency, etc. In general, the response times for all of these will be the same as the response time for power.

Voltage and Frequency

The voltage and frequency values are sampled for one cycle just before the end of each measurement interval, so they may or may not register brief changes, like a single cycle spike or dip in voltage.

Demand

Demand is the average power over a long time period, commonly 15 or 30 minutes. The demand reading only updates at the end of a full demand period or at the end of a subinterval if you have configured demand subintervals (see the manual for details). In any case, demand will not normally respond to changes for several minutes, possibly several hours if you have configured a long demand period.

Energy

Reported energy (kilowatt-hours) is an accumulation of consumed energy. So a brief spike in power will cause the energy value to start to increase more quickly, but will have an almost unnoticeable change on the total energy. For example, if the WattNode is reporting an energy value of 4200.00 kWh, and your power goes from zero to 5000 watts for one minute, the energy will increase to 4200.08 kWh (an increase of 0.08 kWh). Therefore, it isn’t very meaningful to talk about the response time of energy.