An modern NO True RMS DMM? - Page 1
Forum section: electronics / testgear

author: GarySnail
author_rank: Newbie
timestamp: September 06, 2019, 12:59:30 pm
content: I’ve on the search for something unusual.I’m looking for an Benchtop DMM that isn’t TRUE RMS. I’m interested in average rectified value (ARV) and the formfactor of the Signal.I want to test Current Transformers and see the Result in ARV because RMS falsifies it.The CT’s will be in saturation therefore my output signal will be distorted. With the ARV it’s still comparable.I’ve checked most of the bigger manufacturer but no one did provide information, if it’s possible to show the value.Only the Keithley DMM6500 shows in one of its Sample pictures an AVG value. But it is mAmps and I’m not sure if it’s the correct average.I know that lots of old multimeters use exactly this way to calculate the RMS value, but I would like to integrate it in my system so an USB or any other modern Interface would be nice.Any suggestions?

author: 001
author_rank: Super Contributor
timestamp: September 06, 2019, 01:22:21 pm
content: Is it sort of trolling?Use scope for amplitude readings

author: GarySnail
author_rank: Newbie
timestamp: September 06, 2019, 01:31:56 pm
content: I’d like to automate the process, therefore its not an option to look on an scope every time.All I need is a value which I can compare to the min/max that is given.

author: HKJ
author_rank: Super Contributor
timestamp: September 06, 2019, 01:32:25 pm
content: I do not know of a bench meter, but a high end hand held meter:Kyoritsu KEW1062 or YOKOGAWA TY720 it is basically the same meter.It can be switches between mean and RMS reading.

author: 001
author_rank: Super Contributor
timestamp: September 06, 2019, 01:42:53 pm
content: Quote from: GarySnail on September 06, 2019, 01:31:56 pmIt is basic function for DSO

author: exe
author_rank: Unknown
timestamp: September 06, 2019, 01:53:42 pm
content: RC-filter?

author: pigrew
author_rank: Frequent Contributor
timestamp: September 06, 2019, 01:53:50 pm
content: Is this for an automated test setup? If so, you can program a computer to get data from a USB oscilloscope, and then do processing on the computer.You could also build a precision rectifier circuit and measure its output in DCV mode.What voltage and frequency (ranges) are you needing to measure? With what accuracy?I wonder if a low-end DMM from China would be what you need, but getting calibration certs would be difficult.The Keysight U2741A didn't mention True RMS in its datasheet, not sure if it is or not.

author: Fungus
author_rank: Super Contributor
timestamp: September 06, 2019, 01:55:33 pm
content: Quote from: GarySnail on September 06, 2019, 01:31:56 pmNo need. Modern DSOs have USB/Ethernet connections so you can hook them up to a PC and automate everything they do.It's usually an ASCII plaintext interface.

author: jfiresto
author_rank: Frequent Contributor
timestamp: September 06, 2019, 02:51:36 pm
content: If you can accept a handheld meter with a 300–350 hour battery life while logging, the theB&K 390Ais "average sensing, RMS indicating". It has a pretty much standard, USB interface with a well defined protocol. It can also capture the voltage or current min/max-values-so-far that have lasted longer than 1 ms. B&K's support is very good.Should you need it, I can send you a Python utility I have written, that collects its data. i have run it, so far, under OSX, Windows and RPi Linux.

author: Fungus
author_rank: Super Contributor
timestamp: September 06, 2019, 02:55:26 pm
content: Quote from: jfiresto on September 06, 2019, 02:51:36 pmYou could run it off a power supply if it's going to be fixed to a bench and battery life is an issue (via a couple of wires in the back).

author: CDaniel
author_rank: Frequent Contributor
timestamp: September 06, 2019, 04:15:32 pm
content: I doubt that B&K390A , or similar crappy 1KHz bandwidth multimeter is any good , knowing that the current waveform is highly distorted , so even at 50Hz the harmonic content is very high ...You have to use an 100KHz true rms meter that can measure average up to aprox 20KHz , average is lower bandwidth , like recomanded in reply #3 , or there are similar benchmeters .

author: KedasProbe
author_rank: Frequent Contributor
timestamp: September 12, 2019, 06:37:28 pm
content: The average of the DMM6500 is sum/count (what you would expect)But can't you just get the samples and calculate what you need.DMM6500 also has scripts so you would be able to do it standalone.I don't know if the quality of the samples will be good enough for you at max samples speed.but I can do a test though and give you the samples, so you can decide.Script:sum = 0for index = 1, defbuffer1.n dosum =sum + math.abs(defbuffer1.readings)endif defbuffer1.n > 0 thendisplay.settext(display.TEXT1, sum / defbuffer1.n .." V")display.changescreen(display.SCREEN_USER_SWIPE)end

author: macboy
author_rank: Super Contributor
timestamp: September 13, 2019, 02:19:27 pm
content: Keithley 2001 (& 2002) can display AC as True RMS, Average, or Average calibrated to RMS (reads average value but adjusts to give equivalent to RMS value of a sine wave). Bandwidth is several MHz. This last option is only available on certain later firmware, but a workaround is to use the math function to scale the reading. Coupling can be either AC or DC, so you can measure a signal with DC offset. I would think that the DMM6500 is similar. I'd suggest to download and read the Keithley DMM6500 operator manual. Keithley manuals are usually very good at fully describing every feature.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 13, 2019, 03:37:30 pm
content: I agree with other posters that a modern DSO is the way to go, even for automated tests.They all now have some sort of serial comms, and can be set up for automatic readings of many parameters.Otherwise as other poster mentioned, build a precision full wave rectifier with a pair of opamps, feed to any DC meter, and there you have it.

author: ogden
author_rank: Super Contributor
timestamp: September 13, 2019, 04:28:36 pm
content: Quote from: schmitt trigger on September 13, 2019, 03:37:30 pmAC (amplitude) accuracy of most generic/8-bit DSO's are around 3% while 4-digit DMM's have 1% or better. I agree with those who suggest to manufacture DIY-rectifier for DMM in DC mode.