RACA AND RMS VS. MEAN
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© ESAB AB 2018
RACA AND RMS VS. MEAN
The ESAB power sources are internally using a circuit that will measure the rectified
arithmetic mean value of any AC signal (referred to as mean value in this document). This
has been the case for many years and has its roots in the early days when RMS
measurement was very complex and was often not cost effective. Old RMS indicating
instruments where basically measuring the arithmetic mean value and multiplying it by a
constant to get to a RMS value. This was only valid for pure sinusoidal waveforms as the
correction factor is different for different waveforms. The welding current and voltage cannot
be considered to be sinusoidal waveforms in most cases. It has been shown that the mean
value is better corresponding to the actual heat input to the base material (Ref 3, M.R.
Bosworth). To have control of the heat input in the base material is one of the most important
factors to consider when welding an object. For the above reasons, ESAB is using mean
value to represent the set and actual values in the welding system. To use the right
measuring equipment when measuring the output voltage and current of the power source is
extremely important. Most RMS multimeters have a high pass filter that filters out the DC
component when they are set to measure an AC signal. This creates an issue since the AC
weld process by nature will contain a DC component. This is due to the melt off rate of the
wire being different on the positive and negative side. It gets even worse when such an
instrument is used to measure the AC output of the Aristo1000 when the AC balance is
used. Measuring on an asymmetrical AC signal (signal containing both DC and AC
components), like the AC weld process, is very difficult with today’s multimeters since most of
them are designed to work with pure AC wave forms and some are even limited to sinusoidal
waveforms.
Since it is very hard to find multimeters that measures the mean value of an asymmetrical
signal in a correct way (mean value of the DC and AC component at the same time) ESAB
has created the RACA. The RACA is a box that converts an asymmetrical signal, both
voltage and current, into a DC voltage that is directly proportional to the mean value of the
asymmetrical signal. This DC value can be measured correctly by any DC multimeter. The
DC voltage measured at the output of the RACA is multiplied by a factor 10 to get the actual
value. Like any measuring equipment, the RACA can and needs to be calibrated on a regular
basis for correct function.
RACA calibration kit is calibrated as a system and components cannot be interchanged
between different kits.