Measuring Large resistances
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From: Shaun on 12 Nov 2009 15:48 "Kreyen" <Ihatespam(a)nospam.com> wrote in message news:8esof5l2tboin13asa185vfbeuqtdrmpbl(a)4ax.com... > On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" <pimpom(a)invalid.com> > wrote: > >>Kreyen wrote: >>> I am having problems trying to measure resistance values above >>> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >>> >>> Is ther anyway apart from the usual dc bridges of getting >>> accurate >>> resistance measurements. >>> >> >>Are you trying to measure a resistor or something else? What >>range variations are you getting? Did you consider surface >>leakage and other interfering factors? What level of accuracy do >>you need? >> > > I'm trying to measure 3 resistances to be used for calibration > purposes.. I'm getting variations of several percent while I'm > looking into a t least a 0.1 percent accuracy level. > > Kreyen > On resistors in those high range, even finger prints on the resistor can affect accuracy. Solution: Clean them with isopropyl alcohol and let them dry, then try measuring them again. Some meters have a nanoSiemens range which is 1/Resistance, they usually give accurate readings. Other that that a megohmeter that uses high voltage to measure resistance is usually used. Shaun
From: pimpom on 12 Nov 2009 16:02 Kreyen wrote: > On Fri, 13 Nov 2009 01:59:20 +0530, "pimpom" > <pimpom(a)invalid.com> > wrote: > >> Kreyen wrote: >>> I am having problems trying to measure resistance values >>> above >>> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >>> >>> Is ther anyway apart from the usual dc bridges of getting >>> accurate >>> resistance measurements. >>> >> >> Are you trying to measure a resistor or something else? What >> range variations are you getting? Did you consider surface >> leakage and other interfering factors? What level of accuracy >> do >> you need? >> > > I'm trying to measure 3 resistances to be used for calibration > purposes.. I'm getting variations of several percent while > I'm > looking into a t least a 0.1 percent accuracy level. > I was about to suggest something similar to Jan's but decided to ask for more details. For that level of accuracy, a bridge would be your best bet. With the jury-rigged method, it goes without saying that the supply would have to be stable and accurate to better than 0.1%, with a similar requirement for the measuring instrument. Perhaps you could try your lab ohmmeter again, making sure that everything is squeaky clean and perfectly dry. In still air. Varying thermal gradients might also have affected the readings.
From: Spehro Pefhany on 12 Nov 2009 16:10 On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihatespam(a)nospam.com> wrote: > >I am having problems trying to measure resistance values above >100MegaOhms. My Laboratory Ohmeter gives unstable values. > >Is ther anyway apart from the usual dc bridges of getting accurate >resistance measurements. > >Thanks guys. > >Kreyen 100M isn't that high, well, depending on what kind of accuracy you're looking for. How much instability are you seeing? How much above 100M? LMC6042 has 2fA (typical) input bias current and costs a couple dollars one-off. A 100M 1% resistor with 50ppm/K tempco runs around five dollars. A 1G 1% resistor with similar tempco is maybe double that. Most inexpensive DMMs on the lower ranges have very high input impedance so the LMC buffer might not be necessary.
From: Fred Bartoli on 12 Nov 2009 16:28 Spehro Pefhany a �crit : > On Thu, 12 Nov 2009 21:12:29 +0100, Kreyen <Ihatespam(a)nospam.com> > wrote: > >> I am having problems trying to measure resistance values above >> 100MegaOhms. My Laboratory Ohmeter gives unstable values. >> >> Is ther anyway apart from the usual dc bridges of getting accurate >> resistance measurements. >> >> Thanks guys. >> >> Kreyen > > 100M isn't that high, well, depending on what kind of accuracy you're > looking for. > > How much instability are you seeing? How much above 100M? > > LMC6042 has 2fA (typical) input bias current and costs a couple > dollars one-off. > > A 100M 1% resistor with 50ppm/K tempco runs around five dollars. > A 1G 1% resistor with similar tempco is maybe double that. > > Most inexpensive DMMs on the lower ranges have very high input > impedance so the LMC buffer might not be necessary. > > > High value resistors are (depends on the model) somewhat unstable. I currently have a batch of Dale 1G/1%. They measure fine with 100V bias which is the datasheet measuring conditions. When measuring them at low voltage, they're all over the place, from +3ish% to +7ish%. Yep, not even grouped... On the contrary I've some Caddok rated <0.02ppm/V! A friend of mine worked at, IIRC, Vishay/sfernice on that specific high value resistors 'feature' and went auditing some of the 'production' lines (the quotes are his). He said the voltage dependency was mostly a prod issue. High value Rs is a very small niche market and as such, production inherited some highest tech tools, like hand lapping and the likes. Being labor intensive it was relocated in low labor cost countries, with poor buildings (he said, almost backyard :-) and less than ideal handling cleanliness. That could explain a lot some of the strange behaviors... -- Thanks, Fred.
From: George Herold on 12 Nov 2009 16:57
On Nov 12, 3:12 pm, Kreyen <Ihates...(a)nospam.com> wrote: > I am having problems trying to measure resistance values above > 100MegaOhms. My Laboratory Ohmeter gives unstable values. > > Is ther anyway apart from the usual dc bridges of getting accurate > resistance measurements. > > Thanks guys. > > Kreyen I've got a fluke 189 multimeter that reads 100M ohms just fine. (Ohmite brand.) Since the meter has 10 Meg input impedance they must do some tricks in software. Which suggests that you might measure a good 10M ohm resistor and then try putting the 100M in parallel. See if you get better numbers... of course you will have reduced accuracy, but it might work. At least you might learn if it's the meter or the resistors that are flakey. George H. |