Consistent Inconsistent 1 MegOhm and Higher Ohm Readings

Thread Starter

Laerie

Joined Aug 28, 2020
2
When sourcing 2 Wire or 4 Wire Ohms from my Flukes 5520A, 5522A, 5730A, to any manufactured DMM, handheld to 8.5 Digit Bench the indications vary. Example; 10 MOhm to Fluke 87V indicates 9.800 to 10.182. Example; 10 MOhm to Keithley 2000 indications vary. The varying Starts with all three Calibrators above 0.5 MOhm. Some thoughts?
 

Papabravo

Joined Feb 24, 2006
16,118
High resistance or low conductance readings are difficult to make because of the tiny currents involved and the sensitivity to small changes in the environment of the measurement.
 

Delta Prime

Joined Nov 15, 2019
998
Hello there :) Every resistor that I know has a temperature coefficient. The mere fact that current is forced through the resistance changes the resistance measurement.
Measurement fluctuations or readings from the DMM occur because DMMs are notorious for averaging the resulted measurement.
The four-wire technique is not used when measuring high-resistance values in the general case.
unless shielded test cables are used, in which case the shield is guarded.
High-resistance readings need to use isolating and guarding techniques to reduce sensitivity to environmental noise.
Any decent high-range ohmmeter will have a Guard terminal in addition to the input terminals. .
Also, accurate measurement requires a reasonably high voltage.
The HP 3456A 6½-digit DVM compliance voltage is 5.5v on the 100MΩ and 1GΩ ranges.
Others use a compliance voltage of 500 volts or more. It's all about technique.;)
 

Thread Starter

Laerie

Joined Aug 28, 2020
2
Thank you for all who have taken interest in my Ohm's question. To up date, the varying of meter readings it happens in both 2-Wire and 4-Wire above 0.5 MOhm sourcing from my references; Fluke line of calibrators, 5520A, 5522A, 5730A. Variation of measured DC value Ohm is observed using Fluke 1550B at all sourcing voltages 250, 500, 1k. When using passive source Ohm Keithley 5155 MegOhm resistors the same variation is observed. Environmental conditions are always monitored in our laboratory. No large fluctuations +/- 2 C are noted during the measurements, RH always between 40% to 50%, with no movement around measurements. Leads used are the Fluke 5440 with guard connected, or open, same results.
 

Delta Prime

Joined Nov 15, 2019
998
Hello there :)
No large fluctuations +/- 2 C are noted during the measurements, RH always between 40% to 50%, with no movement around measurements
NIST climate-controlled environmental laboratory relative humidity is set at 35 plus or minus 1%
When calibrating standard resistors, NIST uses four-wire techniques for values up to 10,000 ohms.
They use two-wire for 100,000 and 1 Megohm resistors.
For 10 Megohms up to 1 Teraohm they use a three-wire connection, which is two-wire plus guard.
Since they define accuracy (here in the US), I think we can accept their protocols as definitive.
Here is a great resource
Ref: http://www.nist.gov/calibrations/resistance.cfm.
 
Last edited:

kaindub

Joined Oct 28, 2019
69
Are you multimeters calibrated? If not then they will show differences. Off the shelf meters can be considered to indicate "approximate" values.
 

KeithWalker

Joined Jul 10, 2017
1,780
Another important factor that will affect the accuracy of high resistance component measurements is surface contamination that can be caused during handling and storage. The contamination can occur on the surface of the test leads and measuring devices as well as the components being tested.
 

andrewmm

Joined Feb 25, 2011
1,467
can I suggest that you try your resistors "on" the meter, eliminate the leads as a source of error ?

Also check what there tolerance is , if they are 10 % , the they could well be the values you say

Also remember, the human body has a resistance in this area,
I have seen many measurements taken with the users fingers as part of the circuit
Ensure you are only touching the insulated parts of the system,
 
Keep all cell phones/tablets/WiFI/Bluetooth away from the bench, readings will move around with TX packets.
It could also be AC hum, is the bench metal grounded, no LED desk lights nearby etc.
Put the target DMM on ACV and see what you read.
 
Top