Ratio transformer errors

[bdunham7]

I'm trying to sort out the AC performance on several meters and I have two ratio transformers, both Gertsch--an RT-6 and a PT-5R (essentially an RT-5 rearranged).  I thought I noticed some discrepancies between the two and while I'm trying to quantify and demonstrate that, I had a practical question and a theoretical question about these ratio transformer.

First, while the design of these and their specs seem to indicate that they will be quite accurate as long as you observe the operating parameters, what actual sources of error have you found with them in real-life situations?  I'm driving them with either a 510A @ 1kHz or a 5100B-series calibrator with a calibrated reference meter on the input or output as the situation merits.  The output load is 1-3 multimeters selected from a group of Fluke 8505A, 8506A, 8846A and 289.

Second, although it is fairly easy to recite the various theoretical causes of error, what are the magnitude of these errors and can any of them cause the output to be higher than the input x selected ratio by any appreciable amount?  It seems to me that all of the various causes that aren't totally negligible would only cause the output to be lower.  That's important because it would mean that like mercury barometers, if there is a discrepancy between instruments the highest one is always the most correct.

I've attached the specs for the RT-5, the RT-6 is the same except it doesn't have the potentiometer thus the resolution is 10ppm not continuous.

[Conrad Hoffman]

Don't know if they address your concerns directly but there are two Henry Hall papers that might be of interest. One is on a different electrical model for ratio transformers and the other is on ppb measurements with them. They're probably buried somewhere on the IET Labs site and I have copies here. I'll try to find them in the next couple days if you can't.

[Conrad Hoffman]

The Henry Hall ppb article is here- https://www.ietlabs.com/pdf/GR_Experimenters/1968/GenRad_Experimenter_May_1968.pdf
Henry's model is here- https://www.electronicdesign.com/technologies/analog/article/21808689/a-better-transformer-equivalent-circuit
The GR1493 manual is also a must-read- https://www.ietlabs.com/pdf/Manuals/GR/1493%20Precision%20Deacde%20Transformer.pdf