5.5 digit meters

I just looked at 5.5 digit meters on ebay, mostly HP, and almost all show 0.5 to 1.5 volts with no probes plugged in. "Tested and working." Sure.

Reply to
Tom Del Rosso
Loading thread data ...

Must be all those magical MRI fields circulating around >:-} ...Jim Thompson

--
| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
 Click to see the full signature
Reply to
Jim Thompson

If you set a DVM to hi-z input impedance, it might read anything.

Given a choice, I prefer Fluke. The Keithley benchtops have high EMI sensitivity, and the HPs with the VF display tend to kick out ugly spikes.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

Just about every LCD based scope has nasty Backlight Inverter hash. Our new Tek MSO3xxx seems to be worse than my Keysight. But you need to be fairly close to the scope to see it.

Cheers

Reply to
Martin Riddle

they haven't switched to led back light yet?

Reply to
Lasse Langwadt Christensen

I bought one just like that a couple of weeks ago, and it behaves just like that. I believe that's an expected behavior. The manual says that on the lower voltage ranges, the meter input impedance is above 10 GOhms. It takes very little ambient or leakage current to develop an appreciable voltage across that much impedance!

When connected to an actual voltage or current source or resistance, the meter I bought reads correctly to within the accuracy of the references I have available. Short the inputs with a jumper and the reading drops to microvolts.

I haven't yet tried to see if it's a good enough electrometer to sense the static charge in a comb a few inches away from a test probe, but I suspect it will work!

Reply to
Dave Platt

I have a couple of well-aged Keithley 171s that were cheap on eBay. LEDs, 5.5 digits, no worries.

Cheers

Phil Hobbs

Reply to
pcdhobbs

3.5 digit meters are usually 10 Mohm (as is the HP 3478A) and they show 0 with no connection.

The HP's I see have either LCD or LED. The Fluke 8842A has VF and I was wondering about that.

The HP 3478A has two flaws Dave EEVBLOG Jones pointed out. Sometimes there's a picture of one that says "SELF TEST OK" while the ERROR segment is turned on. (He doesn't say if maybe that can be a HPIB error when nothing is connected to it.) And their calibration is in battery-backed RAM so if you change the battery it loses cal (the Fluke

8842A uses EEPROM). They don't have a second connector for a temporary battery so you have to hack in one.
Reply to
Tom Del Rosso

That's good. The manual says impedance is 10^7 in the spec summary, but the manual appendix says 10^10 in low voltage ranges.

Reply to
Tom Del Rosso

Somebody on the Net figured out the GPIB commands to read and write the cal RAM data nybbles. Practically the first thing I did with mine was to read the data and make multiple backups... cheap insurance.

Reply to
Dave Platt

any links to the method?

Reply to
Chris Jones

I saved the bookmarks on my computer, don't have access from my tablet while traveling. I'll post them next week.

Reply to
Dave Platt

Thanks!

Reply to
Chris Jones

formatting link

Reply to
Dave Platt

Excellent.

Reply to
Chris Jones

Here's a bit of simple Python code I wrote to retrieve the cal data and store it. This uses a Python VXI11 library, which is speaking to an ICS8065 VXI11-to-GPIB gateway which then accesses the 3278A,

import vxi11 import signal import sys print("list_devices: ", vxi11.list_devices()) print("list_resources: ",vxi11.list_resources()) print "Opening instrument" instr = vxi11.Instrument("ics8065", "gpib0,23") print "Got instrument ", instr interrupted = 0 def signal_handler(signal, frame): print('You pressed Ctrl+C!') sys.exit(0) signal.signal(signal.SIGINT, signal_handler) print("Reset instrument to H0 home state") instr.write("H0"); calibration = bytearray() print("Fetching calibration array") for offset in range(256): command = bytearray(b'W') command.append(offset) response = instr.ask_raw(command) calibration.append(response) with open("calibration.dat", "wb") as outfile: outfile.write(calibration)

and here's the corresponding restore-the-calibration routine, which I have not actually executed with the CAL switch enabled.

import vxi11 import signal import sys print("list_devices: ", vxi11.list_devices()) print("list_resources: ",vxi11.list_resources()) print "Opening instrument" instr = vxi11.Instrument("ics8065", "gpib0,23") print "Got instrument ", instr interrupted = 0 def signal_handler(signal, frame): print('You pressed Ctrl+C!') sys.exit(0) signal.signal(signal.SIGINT, signal_handler) print("Reset instrument to H0 home state") instr.write("H0"); calibration = bytearray() print("Loading calibration array") with open("calibration.dat", "rb") as infile: calibration = infile.read() if len(calibration) != 256: print("Calibration data isn't exactly 256 bytes!") abort # Warning - the following will overwrite the instrument's # calibration array if the CAL switch is enabled. To allow # this, uncomment the write_raw line. for offset in range(256): command = bytearray(b'X') command.append(offset) command.append(calibration[offset]) # instr.write_raw(command)

Reply to
Dave Platt

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.