conductance function -  nanosiemens - DMM limits .  - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 18, 2011, 12:23:02 am
content: Ok before I do my question an introduction story will be the below text .TextThe conductance function in the meter reads out as a value of nanosiemens.This range allows to measure high resistance values from 20 Megohms to 100,000 megohms.The formula for conversion between nS and megohms is Mohm=1000/nSso for example 10.00 nS=100 megohms.Conductance is another way to measure high resistances, such as leakages in diodes, capacitors, pcbs, or insulators or even motor winding insulation or leakage.The main difference between a megohmeter and the conductance test is the conductance test, uses only a few volts to perform the test where the megohm meter can be set to much higher voltages for "stress" tests.When available stress testing is a better method especially if the insulation being tested is marginal but stress testing electronic components or circuit boards at higher voltage may cause damage so the conductance tests may be more appropriate in these cases.If the insulation was completely deteriorated the conductance method should show bad results as well.INSULATION TESTINGInsulations above 50 megohms is generally considered good.Insulations between 1 megohm and 50 megohms are questionable and should be check frequently as a failure may be about to occur. Insulations below 1 megohm are considered bad.Multimeters in either the ohms mode or the conductance mode (nano Siemens mode), only use about a volt which does not stress the insulation.CONDUCTANCE MODEA Siemen is the recipical of an ohm. If you are going to make resistance measurements over 500 megohms, then the 50.00nS conductance range will allow you to read up to 100,000 megohms. To convert the readings into ohms, you need to recipicate the Siemen reading. Note that nano is 10 to the minus 9. Here are a few values:0.01nS = 100,000 megohms0.1nS = 10,000 megohms1nS = 1000 megohms10nS = 100 megohms50nS = 20 megohms60nS=16.6 megohms (Fluke 28II limit down)90nS = 11.11 megohms300nS= 3.33 megohms (Agilent U1272A limit down)And now my question :I am trying to measure the high impedance resistor of my DMM Fluke 28II and Agilent U1272A.And looks impossible.The Fluke 28II at the Ns mode gets to 0.01Ns with out probes connected,The U1272A shows 00.04 with no probes connected,and the Fluke 8050A shows about 00.08 with no probes connected.I am trying to use relative, but I do not get any reading at all or equal to zero 00.00 NsWhy Is so hard to find out the High impedance resistor in those DMM ?

author: amspire
author_rank: Super Contributor
timestamp: October 18, 2011, 12:49:32 am
content: Well if the testing voltage is, say, 1V, then the current at 0.01nS is 10 picoAmps.I would say you are down at the leakage current of the meter IC's and components. Also the current still has to be read by an A/D converter, and you will probably find the least significant bit of the A/D corresponds to maybe 0.1nA. What is the resolution of your most sensitive current ranges?If I look under my 8050A, it says it reads down to 0.5 nS which means the lower current limit it can read is around 0.5nA.Richard

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: October 18, 2011, 01:19:56 am
content: Much text but I am very dense tonight! What is the "high impedance resistor"? Do you mean the input impedance of the meters? If so, that can be hard to measure because of the sampling when the meter is turned on, and the measurement is not valid if the meter is turned off. The best way is to measure a known voltage through a very high value resistor, then calculate it based on the lower meter reading.BTW, Siemens used to be mhos, which is ohm spelled backwards. It just made so much sense to me that I refuse to update my terminology.

author: Unknown
author_rank: Unknown
timestamp: October 18, 2011, 01:41:18 am
content: Leakage current of the inputs of the meters is likely to be at least in the 10 pA range, this will probably screw up your measurements. I measure about 30pA from a bench meter with 10Gohm input impedance in the 10V range. This swamps any current used by resistance measurements unless you use high voltages. The 'infinite' input impedance only applies to the lower ranges, so high voltages are out. They may not be safe for the meter (overload is often specified for a limited amount of time, assuming auto-range will switch range), and the impedance may be lower due to the activation of protection circuits.Of course these low level measurements are also very sensitive to induced currents and all kinds of neat low-level effects.Quote from: Conrad Hoffman on October 18, 2011, 01:19:56 amFinding a 1Gohm resistor, and making sure you don't have lower resistance paths, may not be trivial, though.

author: IanB
author_rank: Super Contributor
timestamp: October 18, 2011, 02:33:00 am
content: Quote from: Kiriakos-GR on October 18, 2011, 12:23:02 amI don't get it. Why did you stop there? Why don't you go ahead and measure the conductance with the probes connected?Many multimeters have an input impedance of about 10 meg, which would be about 100 nS (I've checked this on my own meters using the resistance range of a second meter). So you would have 0.01 nS with no probes connected, and this would rise to about 100 nS when you connect the probes to the input of a second meter on a voltage range.So I don't quite follow what your question is?Also, before using the conductance range, why not check with a resistance range?

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: October 18, 2011, 03:17:08 am
content: Quote from: IanB on October 18, 2011, 02:33:00 amI did so ... The Fluke 28II is unable to measure lower than 16 MOhms in the Ns mode.And so it does reads the 10Mohm at the ohms range only.The Agilent U1272A can go down to 3.3 Mohm in the Ns mode and so it reads nicely in Ns and Ohms range the 10 Mohm standard impedance.When I boot the Fluke 28II at the high impedance mode, and set it to the mV DC, the Agilent reads OL = more than 300 Mohm,and I am trying with the Ns to measure the impedance , because its over the 300M Ohm range.Simple as that.