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The RMS value is a mean value, for a good estimation it is sufficient to average over enough full periods of the signal.
To compute the RMS over single periods, one can use the FPScript function LevelCrossings() to determine the indices of upward zero crossings as boundaries in the signal to compute one RMS value per period.

// indices of upward zero crossings (with 5 % hysteresis)
Dim idx = LevelCrossings(IA, 0, 0.05 * Range(IA), EVENT_POSITIVE, EVENT_INDEX)

// prepare output datasets in the desired length
Dim rms_y = IA.Y[0L] # Shape(idx[1L,-1L])
Dim rms_x = IA.X[0L] # Shape(idx[1L,-1L])

Dim i_begin, i_end

For Each Row i In idx[1L,-1L] Do
	// extract a full period
	i_begin = idx
	i_end   = idx - 1L
	// use x (time) of the interval center
	rms_x = IA.X[(i_begin + i_end) / 2L]
	rms_y = Mean(IA[i_begin, i_end], MEAN_SQUARE)
End

Signal(rms_y, rms_x)

The code seems above seems to be mangled, maybe in the transition to the new web site. The FPScript formula below should do the job.

//Recherche des index où le signal d'entrée passe à zéro (orientation positive) afin d'isoler les périodes du signal
Dim PMax = Maximum(Acceleration)
Dim Idx = LevelCrossings(Acceleration, 0.0, 0.05 * PMax, EVENT_POSITIVE, EVENT_INDEX)

//Calcul des valeurs RMS
//Code optimisé pour un calcul plus rapide
//Ici on n'étend pas les variables time et rms dans la boucle for avec l'opérateur :=
//On définit au préalable la taille de time et rms et on les remplies dans la boucle For
Dim N = NumberOfRows(idx)
Dim rms  = AdjustUnit(? # N-1, Acceleration)
For k = 0 To N - 2 Do
    rms[k]  = Mean(Acceleration[Idx[k], Idx[k+1]], MEAN_SQUARE)
End

Return Signal(rms, Acceleration.X[Idx])