High resistance measurement by 4071 and 2575

I used NI 4071 to measure a high resistance. The resistance is 78.6MΩ.
When I use 4071 directly to measure it(2-wire), the result is 78.58MΩ.
The measurement is ok. But wnen I connect the resistance to NI Switch 2575,
the result become smaller, about 76.4MΩ。
It is strange, and how to deal with the problem? Thanks

Hi faili,
I have never witnessed this issue before, so I attempted to reproduce your problem. I used the following hardware:
I PXI-4071 DMM
NI PXI-2575 Switch
LFH200 to 50-Pin DSub Switch Cable
NI TBX-50, 50 Pin Dsub Screw Terminal Block
Short 24-gauge wires between resistors and terminal block
Resistor Combination in breadboard (28 2.7MΩ resistors = 75.6MΩ)
I used the following procedure:
Opened Switch Soft Front Panel
Configured PXI-2575 in 2-wire 95x1 Topology
Connected ch94->com in Switch Soft Front Panel
Wired CH94+ (50-pin DSub pin 21) to one side of 75.6Mohm resistance, CH94- (50-pin DSub pin 22) to other side
I made the following measurements:
Opened DMM Soft Front Panel
With DMM, measured one side of resistance to other side (after settling) = 75.6MΩ
Wired COM+ (50-pin DSub pin 23) to DMM HI Lead, COM- (50-pin DSub pin 24) to DMM LO Lead
With DMM, measured COM+ to COM- (after settling) = 75.6MΩ
So, I wasn't able to reproduce your issue. Perhaps the above will help. Can you tell me more about your system and how it is configured? Perhaps there is a hidden resistance to GND somewhere in your setup. Looking at the equation, to receive an error of:
78.6MΩ -76.4MΩ = 2.2MΩ (or 2.8%)
you could have a hidden 2.73GΩ path to GND. How are you making your connections to the PXI-2575? Just a thought...
Let me know if you have further questions or if this does not resolve your issue!Chad Erickson
Switch Product Support Engineer
NI - USA

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Current measurements between 4071 and 2575 sometimes not correct

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When running the scan list, I rarely (randomly) get a very small measurement in the nA to uA (when it should be 4-20mA). I'll run the scan list again and that channel will be correct, so it doesn't seem to correspond to a channel.
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Go to Solution.

Cswieson,
Is the relay board relatively old? If a board is close to end of life, there is a chance that the relay will sporatically not close and/or open. If the relay is stuck open, the DMM might measure very low current because no current is flowing (and only noise is coming through).
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Pxi 2575 high resistance

I am having problems with the resistance across relays on a PXI-2575 increasing over time (faster than expected). I am using a PXI-2575 in combination with a PXI-4070 for manufacturing test. In some cases I am looking for an expected short circuit. The data sheet states that with no voltage applied (which I have none) I should see < 1 ohm for up to 2.5 million cycles. However, I am seeing upwards of 16 ohms across a particular relay after ~2500 cycles in many cases. I have a total 6 of these setups, and I am finding myself replacing PXI-2575 cards after less than 1 year of service due to these high resistances.
To prove to myself that it is the switch matrix (after measuring the UUT with a Fluke DMM) I used the following setup:
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The PXI-4070 is configured for a 2-wire resistance measurement. The PXI-2575 is configured as a 95x1 2-wire mux. The DMM + terminal is connected to pin 46 of Backshell 0 (CH22+) and the - terminal is connected to pin 47 of Backshell 0 (CH22-). A small 24 awg jumper wire is used between pins 23 and 34 of Backshell 3 (COM+ and COM-). On a brand new switch matrix I see ~0.16 ohms using this test. On the most recent one I pulled out of production use, I saw ~16 ohms initially, with it falling to ~5 ohms after leaving it connected for about 5 minutes.
I am up to 4 PXI-2575 cards that suffer from this issue. According to the data sheet I should not be seeing this. Any help would be appreciated.
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Steven
Thanks for the reply. I have done some more testing as you have asked. I realized I actually have 11 PXI-2575's, recently purchased 2 spares, and 3 more for new test equipment. They are all used in an identical manner.
We have made a total of 3 purchases. The first was for 6 PXI-2575's, the next was for 2 (spares), and the final was 3. With all of the swapping around, I am not sure where these failing ones came from at the moment, other than to say they were not part of the most recent purchase of 3. I am more than happy to provide serial numbers, if you can tell me which of the stickers on the board is the serial number. There are two identical looking stickers, one on each of the boards, that have different serial numbers on them.
Below is the data I have collected. Cards 5 & 6 can be considered "good" cards. Note that their readings did not change from their initial readings. They are from the most recent batch. We have no examples of cards with lots of cycles on them that do not exhibit this behavior. Prior to attempt 1 the cards had been powered off and removed from chassis anywhere from days to months. Between attempts 1 and 2 several switch cycles were performed and power was cycled on the PXI and the PC.
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Below is a diagram of the setup I am using to test the switch matrices after they fail in the field.
Installing a new PXI-2575 immediately resolves the issue in the test set, and that test set continues to work for another several months.
Any help would be appreciated.
Thanks
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Why is the PXI-2575 not recommended for 2-wire resistance measurements?

The spec for the NI PXI/PXIe-2575 says "Note The NI PXI/PXIe-25-75 is not recommended for 2-wire resistance measurements". Why is this? If I want to perform 2-wire resistance measurements, what would be a good alternative?

It really depends on your measurement. The problem with 2-wire resistance measurements is that the resistance of the wiring and the switch path will be included in your measurement. If you are measuring in the kOhms to MOhms, this should be neglible. But measuring anything smaller will add a large error. That is why 4-wire resistance measurements are recommended. The sense lines can measure the voltage across the part you actually want to measure and an accurate measurement can be made.
This is true for all switches.
My philosophy is to use 4-wire measurements except when checking for an open.
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What's the difference between 4-wire and 2 wire resistance measurements w/ 9219?

It appears that the NI-9219 provides a small current in either 4-wire or 2-wire resistance measurement mode. What is the difference between the two methods? Just accuracy?
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Resistance thermometer wiring configurations:
Two-wire configuration
The simplest resistance thermometer configuration uses two wires. It is only used when high accuracy is not required as the resistance of the connecting wires is always included with that of the sensor leading to errors in the signal. This applies equally to balanced bridge and fixed bridge system.
Three-wire configuration
In order to minimize the effects of the lead resistances a three wire configuration can be used. Using this method the two leads to the sensor are on adjoining arms, there is a lead resistance in each arm of the bridge and therefore the lead resistance is cancelled out. High quality connection cables should be used for this type of configuration because an assumption is made that the two lead resistances are the same.
Four-wire configuration
The four wire resistance thermometer configuration even further increases the accuracy and reliability of the resistance being measured. In the diagram above a standard two terminal RTD is used with another pair of wires to form an additional loop that cancels out the lead resistance. The above Wheatstone bridge method uses a little more copper wire and is not a perfect solution. Below is a better alternative configuration four-wire Kelvin connection that should be used in all RTDs. It provides full cancellation of spurious effects and cable resistance of up to 15 Ω can be handled. Actually in four wire measurement the resistance error due to lead wire resistance is zero.
Message Edited by JoeLabView on 10-24-2008 01:00 PM

How to acquire data through multiple channels in parallel using PXI 6070 E, PXI 4071 and LabVIEW?

Hi,
I am using NI LabVIEW, NI PXI 4071, and NI PXI 6070E to measure current through a variable resistance. Now, I am using one channel from SCB-68, but I want to add another channel in parallel so that I can have two resistors instead of one that I cam measure current through them.
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Go to Solution.
Attachments:
IV copy for HS.vi ‏55 KB
Layout of NI Cards.pdf ‏248 KB

Your 4071 can only do one measurement at a time. Your DAQ cannot measure resistance either nor has it got any analogue inputs.
However, you could use a multiplexer and multiplex your 4071 DMM. This wont give you simultaneous measurements but can acquire data one after the other, the speed is obviously dependent on the multiplexer you choose!
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I'm developing a Labview user interface for setting up analog input channels. AI Config.vi has a measurement mode input cluster that includes a 4-wire ohms option. I hate to admit it but I don't know what that is, when one would want to use it, or how. Can anyone enlighten me?

4-wire resistance measurements are used when you need either very-high accuracy, or the measurement leads are very long. The way a normal 2-wire resistance measurement works is that a fixed current is passed through the resistance being measured and the meter reads the voltage drop across the unknown resistance, and Ohm's law allows the meter to calculate the resistance. The problem is that this technique read the total resistance (including the test leads and any connection resistance).
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What i'm trying to do is simple, i'm trying to monitor 50 channels or should i say 50 points of resistance measurement and then datalogging a change but i have a restriction and that is the sampling rate. i can't use a realy since there is a DEBOUNCE issue so i'm stuck right now on this. i have to pretty much scan a 50 channels in 20uS or less and then logg everything that changes. please help thanks.

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