how would an AC power meter from the 1940s or so have worked? - Page 1
Forum section: electronics / beginners

author: dkter
author_rank: Newbie
timestamp: September 05, 2022, 03:56:06 pm
content: not sure if I'm posting this in the right place. I fell into a bit of a rabbit hole recently and am now in way over my head trying to research things that I have no idea how to begin looking for.I'm wondering how a device that measures RMS power would have worked in the days before semiconductors and bridge rectifiers. Any pointers would be helpful

author: capt bullshot
author_rank: Super Contributor
timestamp: September 05, 2022, 04:00:30 pm
content: They just work by computing i(t)*u(t) the analog way:Some of them really are mechanical wonders ...

author: TimFox
author_rank: Super Contributor
timestamp: September 05, 2022, 05:00:21 pm
content: One of my pet peeves: "rms power".If you measure the rms voltageVacross a known resistanceR, then themeanpower isV2/R.This is true for any waveform, including DC and an AC sine wave.As an exercise for the reader, you can compute the "rms power" for a sine wave, which is 22.5% larger than the mean power, but is a useless parameter.With a device that measures the instantaneous value of the voltageV(t) and the currentI(t), then their productW(t) =V(t) xI(t) is the "instantaneous power", and the time average of that power as a function of time over a long time is the mean power, corresponding to the meaning above for a voltage and resistor.

author: Gyro
author_rank: Super Contributor
timestamp: September 05, 2022, 06:01:48 pm
content: AC power meters were actually quite easy... Back in the 1920s when DC mains was more or less universal, there were all sorts of ingenious integrating electricity meters (attached). These included unipolar motors (Mercury bath), Mercury electrolytic Coulomb meters (a hugely upscaled version of the much more recent fuse sized Mercury column elapsed time indicators), and even DC brushed motors. Each worked on the voltage drop across a large shunt resistor. I think the voltage was assumed to be constant or, at least, ignored (the customers wouldn't have known back then). These devices have been virtually forgotten! I've never seen any of them in the flesh - they would have been scrapped by the electricity companies when they changed to AC mains.AC services (usually industrial 3-phase back then) used the more recognisable dual coil eddy current discs, as featured in the videos above. In the case of 3-phase, all of the discs appear to have been mounted on a common shaft. Presumably the loads were mostly motors, with balanced load across the phases.

author: james_s
author_rank: Super Contributor
timestamp: September 05, 2022, 06:12:55 pm
content: Many modern meters still work exactly the same way as 1940s meters. Many of the "smart" meters are just a standard meter with some additional electronics tacked on to enable real time monitoring of consumption. The meter I have now appears to be solid state but up until about 3 years ago it had a spinning disc just like the antique meters, with a little sensor to count the revolutions.

author: armandine2
author_rank: Frequent Contributor
timestamp: September 05, 2022, 06:26:01 pm
content: https://www.academia.edu/8140873/A_K_Sawhney_A_course_in_Electrical_and_Electronic_Measurements_and_Instrumentationchapter 10 of Sawhney I believe has the electrodynamic wattmeterI got to use my old wattmeter following a YT tutorial in Hindi(?)Ed . added photos

author: MrAl
author_rank: Super Contributor
timestamp: September 05, 2022, 08:12:15 pm
content: Not sure what date this is from but one of them used resistors to measure the power due to the heat created when a current runs through a resistance. They used that to measure RMS current.

author: TimFox
author_rank: Super Contributor
timestamp: September 05, 2022, 08:15:27 pm
content: “Thermocouple ammeters” were common ca. WW II for measuring RF current at transmitter outputs or antennas.

author: Terry Bites
author_rank: Super Contributor
timestamp: September 05, 2022, 10:05:47 pm
content: These electromechanical meters into use about 1900.Ancient reliable tech.For a not too deep dive seehttps://www.electricaldeck.com/2021/05/induction-type-energy-meter-construction-working.html

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 06, 2022, 12:19:37 am
content: From about 1970 onwards, the brides' mothers were very demanding for true RMS.Others didn't' care.Free libations tomorrow for the first correct answer... why?

author: dkter
author_rank: Newbie
timestamp: September 06, 2022, 03:57:17 am
content: Thanks for all the resources! Super helpful.I'm trying to figure out the rationale for the marketing of this specific meter from 1950:(this is an article describing the eponymous "diotron" circuit, for anyone interested)The ad seems to insinuate that these standard analog power meters can't handle arbitrary waveforms. Why is this?From what I can tell, these analog meters smooth out the AC waveform mechanically by storing energy in the coiled spring, which can later release it and average out the power over a small period. I don't see why this action would be dependent on a sinusoidal waveform. Do I have the wrong idea here?

author: jonpaul
author_rank: Super Contributor
timestamp: September 06, 2022, 04:32:14 am
content: Thomas Edison...1880s, plating weight in electrolyte cell, meter shunt1900s..1970s special motor with current and voltage coils, number of turns proportion to KWH.Mechanical counter1970s..1980s as above but optocoupler counts turns and electronic countersee the history of electric power metersJ