# Measurement Method

Hi there, I have an existential question about how the EmonPi/Tx measures power. The two ways I can make up in my tiny, not-electronically-trained brain are as follows:

Let’s say I have a 5 second interval of measurement.

1. The EP measures the current and the voltage at a certain point in time, say 5,10,15,… seconds, multiplies them and writes them into our data-sink.

2. The EP somehow forms an average of power over the measurement interval and writes it into the data sink when the interval is over.

Assuming we have significant load changes within these 5 seconds, the first method would ignore this and just calculate as if the load was constant on the level of the moment of measurement for all the interval.
Which method is being used? Am I forgetting/not-knowing anything?

You are right. There are two measurement methods available. The “standard” method, which is the default that is loaded into the emonTx and emonPi when you buy them, does indeed sample the power at discrete intervals, hence it is called the “Discrete Sample” method. It’s prime advantage is when the emonTx is powered by batteries, as it prolongs battery life and over a period of time it can be quite accurate, provided that you don’t have something like an induction hob that switches on and off frequently.
The alternative method we call “Continuous Monitoring”, which pretty much does exactly the same as an electronic electricity meter. The “CM” method can cope with rapidly switched loads, but is not suitable for battery operation. The “CM” method is presently in development.

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Is this potentially suitable to measure brown-outs?

What is the sampling rate in “CM”?
I will certainly order some for home, as well as for our server room in the office (where we had some seconds outage lastly).

The maximum speed that the ADC will run! Depending on the number of channels, it varies, for an emonTx, from one voltage and one current sample every 2 × 104 μs, to one voltage and four current samples every 5 × 104 μs, i.e. a bit less than 2000 sample sets per second to 4800 sample sets per second. It will report the measurement average up to 10 times per second. (i.e. It has not been tested below 0.1 s nor above 5 minutes (300 s). Note that emoncms.org will not accept data faster than once every 10 s, but your local emoncms can.)

Note also, the present ‘CM’ library as published on Github is faulty and returns incorrect values for some measurements.

Wow, that’s great to hear.
I’ll go ahead and do some orders!

Thanks!

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Remember, the CM library is not yet available - it is being tested by a few select users in order to find any remaining bugs. No release date has been set. You will need a programmer and the Arduino IDE to load it into your emonTx. An emonPi version is expected to follow.