Odd Fluke 179 RMS measurement

[WaveyDipole]

I was looking at a transformer yesterday to assess its suitability for a particular project. the primary is mains AC, but the voltage of the secondary with a single winding with no taps was unknown except that it came from a domestic piece of equipment. I measured the AC voltage with no load using a fluke 179 which has True RMS measurement and got 1.8something volts. Well this did not seem right at all so I had a look at it with my analogue oscilloscope and got a P2P voltage of over 45v. The Rigol measured this at 47.2v. Calculating the RMS voltage form this, I arrived at about 16.69v. I then measured it on an AVO 8 Mk6 and got 16.5v which is quite close. Another DMM without true RMS measured 13.62v and variations like this might be expected, but I then re-checked with the Fluke and got the same 1.8something volts. So why might the Fluke, which is supposed to measure True RMS misbehaving like this? 

[mmagin]

A picture of the transformer might be interesting to see.  Also:
What does the secondary waveform look like on the scope?
Are you sure the Fluke's AC voltage measurement is functioning correctly?  Either check for a plausible reading on another low voltage transformer secondary, or probe the AC line (carefully!)

[Gregg]

Try putting a slight load on the transformer and measuring again; something like a 1K resistor sounds about right for the figures you mention.  You may be getting a small amount of capacitive coupling or inductive spikes in the open circuit condition that will not be significant as soon as the transformer is loaded.
If the known primary voltage and frequency is applied you can try loading the secondary to about half of what the secondary wire size would indicate; which will get you into the ballpark of the lower end of the transformer design parameters.  Most inexpensive transformers perform poorly unless they have a load.