OK, Sergymax, let us try this in four steps, if you are still confused.
First step: get 24 volts worth of batteries, maybe two 12 volt lantern batteries in series and have a red wire connected to the positive end and a black wire connected to the negative end, so you have 24 volts DC on a red and black wire. Find a 250 ohm resistor. Get a yellow piece of wire. Your temperature measurement device has two terminals on it , + and -, for the 4 to 20 mA current loop. Connect the red wire from the batteries to the + terminal of the temperature measurement device. Connect the yellow wire from the - terminal of the temperature measurement device to one end of the 250 ohm resistor. Connect the black wire to the other end of the resistor. You now have an operating current loop and the temperature sensor is powered up and operating. There is current flowing around this loop, somewhere between 4 and 20 mA. But, you don't know what the current is yet.
Second step: Get some white wire and some blue wire. Connect one end of the white wire to the resistor lead that already has a yellow wire, and connect the other end of the white wire to your USB-6009, on the + or high terminal for a differential analog input channel. Connect one end of the blue wire to the resistor lead that already has a black wire, and connect the other end of the blue wire to your USB-6009, on the - or low terminal for that same differential analog input channel. Now you have a complete electrical system that will be able to measure the current.
Third step: do some data acquisition with LabVIEW and the USB-6009. Look at examples in the Example Finder and read what everybody has already described in this thread about acquiring data. Forget all the things about RTDs and the ConvertRTD vi. You want to measure voltage, on the differential analog input channel you picked in the Second Step. Work on all these things until you are getting a voltage. If something is still confusing, go connect your USB-6009 to a flashlight battery instead and experiment around until you are reading about 1.5 volts. Whatever you do, keep at it until you can measure voltages on the electrical system we set up, so you are measuring somewhere between 1 and 5 volts on the white and blue wires. Now study the documentation and try putting the temperature device into places that have different temperatures, and see the voltage changing, and either by experimenting or by reading the documention or both, figure out the linear, straight-line relationship between the voltage you measure and the temperature. Remember Ohm's law, so you understand that 20 mA current in your loop will give you 5 V voltage on your analog channel, and 4 mA will give you 1 volt. When you get all this right, you will be getting temperature measurements that agree with the documentation and the experiments.
Now you are getting temperatures in LabVIEW.
Fourth step: you still are not finished. The batteries will run down, and all our colored wires probably do not fit neatly into whatever system you are working on. You should be using a DC power supply, or maybe DC power you already have in the system. Maybe my choice of 24 volts is not convenient in your steup. So, you must replace the temporary but working setup we created above with whatever permanent power and wiring the system has or wants or is compatible with. You will have to watch out for whether your USB-6009 or your computer or something else has a ground connection that interferes with what we are dong. Maybe you do not have to use a differential channel, depending on whether you can have a common channel connected to your system, and how the other measurements you maybe making are referenced to ground, and whether your DC power supply has either side tied to ground, and if things are floating do the voltages get too far from whatever your USB-6009 considers ground for it to handle. All of this stuff can be a chore. You have to understand other parts of your system. You are trying to use a 4-20 mA current loop measurement on your system - either you or other people in your shop or a vendor that sold you the system, or somebody someplace, should be able to help you with these issues (they are not temperature measurement issues, they are current loop issues).
The only way I can make this any clearer is to use my own USB-6009 and buy one of those temperature sensor / transmitters, wire them all together, and try to push them through the screen of my computer monitor. And since I am now helping you on a Saturday, I am at home, and do not want to bust a hole through my own monitor. So, good luck with it all!
