Wanted to let those of you with original HCCT Weston and Jewell meter problems know Bob Cascisa is working on them. So far, to the extent they do not required irreplaceable parts and only need iron van meter movement repair and cosmetics Bobs your guy.
Here is a photo of a Weston meter Bob is working on for me. He is picky, which is what this task requires. He is adjusting the size of the winding and with a little more work the movement reads exactly 1.3 amps mid-scale.
Additionally John Regan at FunProjects has faithfully duplicated original style meter scales for both the Weston and Jewell meters and Bob will be using them. The Weston meter shown above will ultimately have a new FunProjects scale.
We are lucky to have fellows like these to help us save these old tools.
Ron the Coilman
Good to know. Thanks Ron.
Thanks Ron. I had no idea. My gauge has a red smudge on it. Doesn't affect anything, but it's nice to know there is something out there.
Three Questions, really for Ron:
What power source does Bob use when calibrating the meter? How accurate is his digital meter, when measuring a HCCT magneto's frequency and odd waveform? Should he be using an RMS meter for this measurement, or is a conventional meter sufficient?
I am not completely familiar with Bob's test set up and defer to him.
Ron the Coilman
Having done a few meters let me take a whack at the question. Since all of the meters here are iron vane type movements they will be true RMS responding type meters thus they will be accurate enough with the magneto type waveform even if calibrated using a more typical sine wave type of AC source. Most calibration like this is done with a "variac" AC controller of power line waveform. The problems come in when using a movement that is not a true RMS responding meter such as a D'arsonval movement then used with a rectifier. The latter type being commonly used for AC measurements in typical power circuitry where the meters will usually state "Average responding" meaning that the meter has been calibrated using an AC sine wave source and such meters will indeed have an error when used with another source such as a T magneto waveform at lower RPM's. There are IC modules that can convert AC sources into true RMS values and then fed to a linear meter. Most all AC meters are accurate enough if you use a sine wave source since even the cheap ones are calibrated using sine wave source. A further consideration however is that the only real consideration for the HCCT meter is the 1.2 - 1.4 Amp "window" and even a linear type meter movement used with rectifier can be calibrated to a pretty high degree of accuracy in that small window. It really doesn't matter what the meter on an HCCT reads or its accuracy outside of that 1.2-1.4 amp window so long as it isn't totally insane so as to damage the movement. On some early test equipment the center "zero adjust" was used to mechanically move the needle to give a precise reading at one place on the dial. The St. Louis Magneto tester (MT-1) that my company reproduces was originally calibrated in that exact way although with modern inductor precision that is pretty easy to do now, I can hit a high degree of calibration accuracy across most of the range that the tester covers.
I use a Variac (Variable AC Transformer) as a variable AC voltage source. There is a 25 Ohm, 50 Watt resistor for a load. My DMM (Fluke 8024) is a True RMS type with a 1.5% accuracy rating. I will be validating it to a NIST traceable meter before I do the final calibration on my current projects for Ron.
Calibration is done at 60 Hz (equivalent to 450 RPM). The HCCT meter actually measures the average current due to the dampening effect of the meter movement. The indicating scale is skewed to indicate the RMS value of the current.
Excellent answers, all. I did not expect the HCCT to be an RMS meter, never considered the movement type. This is all good news, to those of us who are trying to calibrate these things!
Bob, I'd also use an isolation transformer on the input to the Variac, just to make sure there is no ground shock hazard. Learned that one the hard way...