Multifunction DAQ
Wiring NI 9219 to collect 4-20mA current
Hello Dave,
Just to clarify your setup, could you please post a diagram of how you have the sensor, the power supply for the sensor, and NI 9219 wired together? Also, what software are you using to read the measurements from the NI 9219, your own LabVIEW code, example code, or NI MAX (task or test panel?)?
Hi Matthew,
Below is the diagram. Attached is a document for the Dwyer Magnehelic sensor which is providing the current we're attempting to measure.
For the DC power supply we tried both a 24 DC transformer plugged into house power ( 110V AC ) and also a simple 9V battery.
The cable lengths are all fairly short, just 3 or 4 feet with maybe the longest being the + side of the connection between power and the Dwyer
sensor. That line is maybe 10 ft.
For the test using the Fluke hand held current meter we simply swapped it in place of the 9219 using the same connections as shown in the diagram.
The fluke had no problems and showed a fairly stable 14mA coming out of the Magnehelic which is the value we expected.
For the test case we used the NIMax test panel. Later we'll be using custom software but for now it fails using the NIMax software ( test panels )
The 9219 is installed in a cDAQ-9171 and connected to the PC using USB.
It never worked when using channel 0 on the 9219.
When using channel 1 it initially didn't work. But after starting and stopping collection in NIMax a few times, it did eventually start working.
It never worked using channel 0 even though we attempted to reset by starting and stopping collection in NIMax. We also tried unplugging the
USB cable to perform a full reset of the 9219. When I say it didn't work, I mean that the NIMax showed a current of 25mA and appeared to
latch which I interpreted to be an error condition since it continued to show 25 mA even when the signal lines were removed.
On my test bench, I just used a resistor in place of the magnehelic and a 9V battery in place of the DC power supply.
Channel 0 works great in that configuration. It's just somehow not happy with the Magnehelic even though the Fluke is fine with it.
Thanks!
Dave
Dave,
It does look like you have the wiring configured correctly.
Just to be clear, channel 1 reads the current (the same as the Fluke hand held), but channel 0 does not (rails out to the full 25mA)?
Are you able to measure resistance with your Fluke hand held, or do you have a DMM lying around, which you can use? I’d like you to check the resistance of channel 0 verses channel 1, essentially this is the effective shut resistance is which the device is using to measure the current, they should be the same. First, disconnect the wires from terminals 3 and 5, so that the circuit is open. Then run the test panel in NI MAX, as you have before. What does it measure the current as for an open circuit? Then connect you DMM to pins 3 and 5, and measure what the resistance is. Do this for both channels 0 and 1. This will tell us if channel 0 is faulty.
Hello Dave,
- I agree with your suspicion about this issue being related to the interaction between the Magnehelic sensor and the 9219 device.
- I wanted to know what the NI MAX test panel read to you, if you had nothing connected to it. It should be 0, as you have observed. Thank you for that information.
- The reason that it depends on history, is because the module is engaging the shut resistor for measuring the current, but does not do this unless you have configured the device to do so. It does not have the shut resistor engaged all of the time, as the device does more than just measuring current from those pins. The device configures its circuitry for the type of measurement you set it to take. The fact that the resistances of both channel 0 and channel 1 are the same, leads me to believe that channel 0 is not faulty. However, that doesn’t meant the device is still not at fault, it may be that the internal circuitry for measuring current, up to the shunts is fine, but the circuitry afterwards, maybe up to the actual ADC even, may be the problem.
What I would like you to do is, instead of measuring current with these channels, let’s try doing a voltage measurement, and convert it into current afterwards using Ohm’s law. This will allow us to troubleshoot the cause of this issue and see if it is related to the current measuring circuitry internal to the device.
Connect the NI 9219 in parallel with a small resistance resistor; see Figure B in the document for the Dwyer Magnehelic sensor you linked me to in your earlier post (make sure to change the input pins of the NI 9219 to be pins 4+ and 5- for voltage). First try a resistor of 30 Ohms; to emulate the NI 9219’s shunt resistance. Then try 700 Ohms, which is what the document suggests in Figure C for a supplied power of 24V. Does reading the voltage/resistance yield the correct current measurement you are getting from the Fluke hand held (14mA)?
Hello Dave,
The test should also determine if the Magnehelic is working correctly yes, but the purpose of this test is more to figure out if the current measuring portion of NI 9219’s circuitry is faulty. If it is faulty, then you should consider sending it to National Instruments as a Return Material Authorization (RMA) request to get it fixed.
I expect that the Magnehelic will not operate as it should with the 30 ohms, but would with 700 ohms, since the documentation for the Magnegelic says to use 700 ohms for voltage measurements, not 30. However, the actual reading is not what we care about; we just want to know if the NI 9219 is reading the same voltage, for either resistance, as the Fluke device.
If you are looking for another test to run while you have the equipment available, I suggest you also try an internal connection test. Open up the Voltage – Continuous Input.vi example (Help>>Find Examples> Hardware Input and Output> DAQmx>Analog input) shipping example. Then right click the Physical Channels control, select I/O Name Filtering, and check off Internal Channels. You should now be able to click the drop down for Physical Channels and select “9205/_aignd_vs_aignd”. Run the VI and see what you get for voltage reading.
