Remote Temperature Sensing Part II
(VID-TE-0007)
Learn how to extend the distance between your temperature sensors and your controller using universal temperature transmitters. Part I discusses how ot extend the distance with fixed dedicated/temperature transmitters.
The Universal temperature transmitter does the exact same job as the fixed temperature transmitters - it converts the small temperature probe signals to 4-20mA loop signals. The difference is you don't need to worry about getting the exact unit you need; you just get this one universal unit either DIN Rail or Sensor Head form factor - and configure it for your probe type and temperature range. You'll need this special USB programming cable and a copy of the FREE software to configure the universal units. Plug the cable in here on the DIN rail transmitter or here on the sensor head transmitter and connect the transmitter to a 4-20mA loop. In this video we're going to bypass the controller and simply put a milliamp meter here so we can see what the current loop is doing. The software looks like this. If you know the com port (or can look it up in the Device Manager) put it here. You can use the auto detect, but it typically takes several minutes to scan all the possible ports and I don't have the patience for that. But it does work if you want to do it that way. Select the type of transmitter you are using XTD for DIN rail Mounted, XTH for sensor Head mounted. I'm using a head mounted unit in this demo so I'll select XTH. This release code is a password that we'll discuss later, but if someone previously set this, you'll need it to get access to the unit. Make sure you have power applied to the4-20mA loop and that the cable is plugged in then hit OK. The connection initializes and then you get a window like this. In the standard settings we select the sensor type this universal transmitter supports a wide range of sensors including both RTD and Thermocouples. We're using a J thermocouple in this demo. You can select the units you want to use in the rest of the menus. And finally select the temperature range your sensor supports. You can find that in the sensors datasheet or right on the AutomationDirect website - looks like the temperature range for this probe is 32 to 900 deg F. If you like all of that you can download the configuration to the transmitter and you are done. Easy. You can read the settings back from a sensor by hitting this button I'll reset my numbers so we can see them come back and sure enough they do. You can print out the configuration using this button, save the configuration to file here and read a configuration from file here. That's nice because you don't have to re-type everything each time you want to configure a transmitter. Just open it and send it to the transmitter. In the expanded settings you can fine tune things a bit. Do you want the transmitter to use its own internal compensation or do you want to take care of it yourself. It's rare that you would do it yourself so you will usually use the built in compensation. If you DO do it yourself then specify the fixed reference temperature you want to sensor to compensate to here. Of course all of this compensation stuff only applies to Thermocouples if we had chosen a 2 wire RTD and wanted external compensation you would see a box here for the cable resistance. You would simply measure that and put it here. We're using a thermocouple and we'll go with the built in compensation. If there is a fault like a short or a sensor break you can tell the transmitter to send a low something below 3.6mA or high something above 21 mA. That's an awesome feature that you will want to make sure you build into your controller program so you can actually detect when there is an issue with your sensor! You can invert the output so cold is high current and hot is low current that's usually just a personal preference or dictated by the system you are working with. You can change the filter time that's how much time do you want to average temperature over it helps smooth out the results and saves you the effort of having to do it in your controller program! You can specify an offset. Suppose you know there is a difference of let's say 3 degrees between your target and the probe you can enter that here so your controller gets the actual temperature of the thing you are trying to measure again so you don't have to do it in your controller program! You can adjust that by up to +/- 18 degrees F (+/-10 deg C). You can type any 8 characters you want here to help identify this configuration in the printed reports. I'll type xmtrdemo. And when I print it out I see that here to remind me what this configuration was for. The last section Service functions is where you set the password. Remember when we first started we had asterisks here? If we had put a 4 digit code here and wrote it to the transmitter, then anytime anyone tries to access this transmitter in the future they will have to enter that same code. PLEASE be careful with this. If you set a password and forget it there is no way to get access to the teansmitters configuration. There isn't even a way to reset the transmitter to factory default. So if you call support for help all they can do is send you to the store to buy another one. So be REAL careful with this. This output current option is an awesome debug feature. If you enable it and enter a Current value, it will set the output current of the transmitter to that value when you write the configuration to the sensor. So let's say we want 10 mA. Write it out. And sure enough, our loop now has 10mA of current running through it. I love this because now I can force any value between 3.8 and 20.5mA on the loop and then check it back at the controller to see if the controller responds accordingly. You can also verify the current by using a current meter like I have here. Very handy. Make sure you click away from the field after entering the value if you don't the software won't see anything new and won't do anything. Also, make sure you turn this OFF when you are done, or the sensor will stay in this mode until you come back and change it. So let's turn this off. Check the other settings everything looks good so we'll do one last write and we are done. The instructions say you have to remove the cable before the transmitter will work. In my experience it works just fine with the cable plugged in, but just to be safe make sure the cable is unplugged before doing any real measurements. A couple final notes on using universal Transmitters. To comply with UL61010-1 the unit must be supplied by a class 2 power supply. You should always disconnect power before making any connections. Pay attention to the shielding recommendations in the insert and documentation notes. In particular: 1. If you mount the temperature transmitter remotely from the sensor in a field housing, the shield on the 4-20mA signal must be at the same potential as the shield on the sensor. 2. If you are using GROUNDED thermocouples, you definitely want to use shielded 4-20 mA cable. 3. If you have a lot of electro-mechanical noise, you are strongly encouraged to properly shield ALL cables. 4. Cables that run outside of buildings should be shielded to help protect from things like lightning strikes. 5. You want to connect the shield to ground at one end only, usually the end closest to the power supply. The reason for that is if you connect the shield at both ends, you just created a new current path which could introduce ground loops which could make things worse than having no shield at all. So again, connect the shield at one end only. If you need any help, please don't hesitate to call AutomationDirect's FREE award winning tech support during regular business hours, they will be happy to help you. And don't forget the forums there are a lot o people there that love to share their years of experience. Just don't post and support questions there AutomationDirect's support staff doesn't monitor the forums on a regular basis.
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