Fluke 8502A expectations?

johngalt47 · 12th September 2010, 05:53 PM

I just bought a used 8502A and when measuring resistance, the least 2 significant digits won't settle down. Is this something that calibration will fix or is this just inherent in a 6 1/2 digit device?

dave_gerecke · 13th September 2010, 07:19 PM

This is really just a guess, but with a meter of that accuracy, the last digits will fluctuate due to the fact that you are measuring a very small magnitude with those digits. This is one of the reasons that the meter has a 4 wire resistance measurement. By having separate force and sense lines for resistance measurements, you get a more accurate and stable measurement.

Peace,

Dave

stratus46 · 13th September 2010, 09:39 PM

Quote:

Originally Posted by johngalt47

I just bought a used 8502A and when measuring resistance, the least 2 significant digits won't settle down. Is this something that calibration will fix or is this just inherent in a 6 1/2 digit device?

You're complaining you can ONLY get 4 1/2 digits of accuracy? You can always average out the reading but if you took physics, you'll know how hard it is to get beyond 3 digits reliably.

G²

richiem · 13th September 2010, 10:07 PM

I'll dip my oar in here. The 8502 will have erratic ohms readings, especially at high resistance values of 1M or more, if the current source is noisy. It's a fairly common problem. You should expect some last digit bobble, say +/- 2 digits in these instruments -- and more at very high resistance measurements where noise and stray capacitance are issues.. See if the 4-wire measurement, say at 10k ohm, is any better. I had an 8505 that I never could get to work right on resistance, even though everything else cal'ed well. I sold it before I got to the real problem, then read a note somewhere about the current source issue.

Hope this helps.

tomchr · 15th September 2010, 12:17 AM

How are you connecting the resistor to the meter? Hopefully by some solid form of connection... If you use regular test leads and hold the leads to the resistor with your fingers, I'd expect a few digits of wobble as well.

~Tom

richiem · 15th September 2010, 02:39 AM

RE tomchr's suggestion -- try using gold-plated 4-wire clips -- these are available at fairly low cost from China on eBay. I've found them to work well.

johngalt47 · 15th September 2010, 11:48 AM

For now I have some test leads with alligator clips on the ends. I am planning to purchase the 4-wire clip leads on ebay soon. Also, Transcat said they would calibrate the meter for about $126. Fluke wanted over $500!

richiem · 15th September 2010, 07:06 PM

I recommend verifying that there is or isn't a problem with the ohms function before spending money on calibration -- unless your primary need is for accurate DC and AC volts measurements.

macboy · 5th October 2010, 01:55 PM

Think about the precision (not accuracy) of a 6.5 digit readout. That's +/- 2,000,000 count. Or a range of 4,000,000 count. So the least significant digit is 1/4000000th of the full scale, or 0.000025% (twenty-five millionths of a percent). To put it another way, it's -132 dBFS (132 dB below full scale). To put it yet another way, it's 22 bits of precision. This is around the noise limit of modern electronics. You will never likely see the LSD of a 6.5 digit meter settle down, simply due to noise. It's also why you don't see a 7.5 digit meter out there. It might be possible to measure with more precision if the electronics were cryogenically frozen, but otherwise you just can't get away from the noise.

Keep in mind too that resistance changes with temperature. You can very easily see this on even a 4.5 digit meter with a carbon resistor. Just clip it on, and note the reading, then grab it tightly in your fingers to heat it up (don't touch both leads) and you will see the reading creep up. Let go and the value slowly creeps back down. With metal resistors, the effect is much less (you can test for fakes this way). Even the small amount of current used by the meter when measuring resistance will cause some self-heating of the resistor and cause the value to creep, especially at 6.5 digits.

Don't forget too, that you can't measure anything with 6.5 digits of accuracy, even if the precision is there. The accuracy, even freshly calibrated, is orders of magnitude worse than the 0.000025% precision. The last few digits are only for comparing any two readings taken around the same time. Taking the same measurement at a different time can and will give different readings at 6.5 digits due to temperature of the meter and of the component under test, humidity, drift, etc.

richiem · 5th October 2010, 08:42 PM

@ macboy, I have an 8-1/2 digit HP 3458A DMM sitting on my bench, which works as advertised. And there are a plethora of 7-1/2 digit meters out there for sale from every major manufacturer. I think you have overlooked the impact of digital signal processing on the resolution (and accuracy) of these instruments.

12th September 2010, 05:53 PM	#1
johngalt47 diyAudio Member Join Date: Mar 2004 Location: Texas	Fluke 8502A expectations? I just bought a used 8502A and when measuring resistance, the least 2 significant digits won't settle down. Is this something that calibration will fix or is this just inherent in a 6 1/2 digit device? __________________ Any technology sufficiently advanced is indistinguishable from magic

15th September 2010, 11:48 AM	#7
johngalt47 diyAudio Member Join Date: Mar 2004 Location: Texas	For now I have some test leads with alligator clips on the ends. I am planning to purchase the 4-wire clip leads on ebay soon. Also, Transcat said they would calibrate the meter for about $126. Fluke wanted over $500! __________________ Any technology sufficiently advanced is indistinguishable from magic

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Fluke 8502A acting weird	meconlen	Equipment & Tools	5	16th June 2010 09:15 PM
Fluke 8502A?	nania	Parts	5	22nd May 2010 03:29 AM
Expectations from an inexpensive 3-way	peter_m	Multi-Way	4	27th May 2007 09:14 AM
Isobaric 8" expectations?!	Rarkov	Multi-Way	18	12th September 2002 02:41 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

13th September 2010, 07:19 PM	#2
dave_gerecke diyAudio Member Join Date: Dec 2007 Location: Vermont	This is really just a guess, but with a meter of that accuracy, the last digits will fluctuate due to the fact that you are measuring a very small magnitude with those digits. This is one of the reasons that the meter has a 4 wire resistance measurement. By having separate force and sense lines for resistance measurements, you get a more accurate and stable measurement. Peace, Dave

13th September 2010, 10:07 PM	#4
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	I'll dip my oar in here. The 8502 will have erratic ohms readings, especially at high resistance values of 1M or more, if the current source is noisy. It's a fairly common problem. You should expect some last digit bobble, say +/- 2 digits in these instruments -- and more at very high resistance measurements where noise and stray capacitance are issues.. See if the 4-wire measurement, say at 10k ohm, is any better. I had an 8505 that I never could get to work right on resistance, even though everything else cal'ed well. I sold it before I got to the real problem, then read a note somewhere about the current source issue. Hope this helps.

15th September 2010, 12:17 AM	#5
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	How are you connecting the resistor to the meter? Hopefully by some solid form of connection... If you use regular test leads and hold the leads to the resistor with your fingers, I'd expect a few digits of wobble as well. ~Tom

15th September 2010, 02:39 AM	#6
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	RE tomchr's suggestion -- try using gold-plated 4-wire clips -- these are available at fairly low cost from China on eBay. I've found them to work well.

15th September 2010, 07:06 PM	#8
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	I recommend verifying that there is or isn't a problem with the ohms function before spending money on calibration -- unless your primary need is for accurate DC and AC volts measurements.

5th October 2010, 01:55 PM	#9
macboy diyAudio Member Join Date: Oct 2003 Location: Ottawa, Canada	Think about the precision (not accuracy) of a 6.5 digit readout. That's +/- 2,000,000 count. Or a range of 4,000,000 count. So the least significant digit is 1/4000000th of the full scale, or 0.000025% (twenty-five millionths of a percent). To put it another way, it's -132 dBFS (132 dB below full scale). To put it yet another way, it's 22 bits of precision. This is around the noise limit of modern electronics. You will never likely see the LSD of a 6.5 digit meter settle down, simply due to noise. It's also why you don't see a 7.5 digit meter out there. It might be possible to measure with more precision if the electronics were cryogenically frozen, but otherwise you just can't get away from the noise. Keep in mind too that resistance changes with temperature. You can very easily see this on even a 4.5 digit meter with a carbon resistor. Just clip it on, and note the reading, then grab it tightly in your fingers to heat it up (don't touch both leads) and you will see the reading creep up. Let go and the value slowly creeps back down. With metal resistors, the effect is much less (you can test for fakes this way). Even the small amount of current used by the meter when measuring resistance will cause some self-heating of the resistor and cause the value to creep, especially at 6.5 digits. Don't forget too, that you can't measure anything with 6.5 digits of accuracy, even if the precision is there. The accuracy, even freshly calibrated, is orders of magnitude worse than the 0.000025% precision. The last few digits are only for comparing any two readings taken around the same time. Taking the same measurement at a different time can and will give different readings at 6.5 digits due to temperature of the meter and of the component under test, humidity, drift, etc.

5th October 2010, 08:42 PM	#10
richiem diyAudio Member Join Date: Sep 2010 Location: Grapeview, WA	@ macboy, I have an 8-1/2 digit HP 3458A DMM sitting on my bench, which works as advertised. And there are a plethora of 7-1/2 digit meters out there for sale from every major manufacturer. I think you have overlooked the impact of digital signal processing on the resolution (and accuracy) of these instruments.