I am making some noise measurements of tubes (valves) and I need some help
in turning observations into measured values. I do not have a true rms
audio millivoltmeter, just a Ferrograph RTS2 test set which has a
millivoltmeter calibrated for sine waves. From tests I have done it clearly
under reads white noise significantly. So I need help inconverting what I
see on the scope (connected to the Ferrograph's scope output) into rms
noise voltage.

For these measurement the Ferrograph is on its 1mV range. On any range, full
scale gives 1V rms out of the scope output. Initially the Ferrograph reads
at one tenth full scale (100uV) but it is clear from the scope trace that
this is mainly hum. So I engage the LF filter and the hum disappears from
the scope leaving what looks to me like regular white noise. The Ferrograph
meter now reads zero at this point. I increase the scope resolution so I
can see the noise clearly and it looks like it is about 400mV peak to peak.
If this were a sine wave I would say it was about 141mV rms which
translates to 141uV measured and should have been clearly visible on the
Ferrograph meter - that's why I think the Ferrograph under reads noise.

So, I have what appears to be a 400uV peak to peak noise signal on the
scope. How do I convert this to rms. I have read an article about this that
discusses the noise amplitude distribution (Gaussian) and how, as we all
know, a 2 sigma range encompasses over 68% of the signal range, 4 sigma
encompasses 95% and 6 sigma over 99&. So the question is, is what I see on
my scope likely to be 2, 4, 6 sigma or some other value.

Cheers

Ian