Tech Tip: Remote Temperature Sensing Part I from AutomationDirect

Most Temperature sensors come with a couple feet of lead wire, but what if you want to extend that distance? You can get accurate readings from 3-wire RTD sensors out to about 300 feet using copper wire, but you'll probably want to use 3-wire RTD extension cables that are designed expressly for this purpose. Thermocouples can be extended only to around 100 feet, but you have to use special thermocouple wire that matches the sensor type you're using and you also want to make sure you try to avoid using connectors that create extra thermocouple junctions. It's best to use these special thermocouple DIN rail terminal blocks that clamp the wires together instead of connecting them through a bus bar terminal block like this one that would just create more thermocouple junctions which would mess up the temperature you are trying to measure - right? The special Thermocouple terminal blocks ensure you have one continuous flow of the thermocouple wire all the way back to the controller which gives you clean unbiased temperature measurements. Extension cables are great but what if you need for the sensor to be even farther away from the controller, or maybe you have an electrically noisy environment that could interfere with the small temperature signals especially on long wires? Well, then you definitely want to consider using a temperature transmitter. There's all kinds of these transmitters, but at the end of the day, they all do the same thing they convert the small temperature signals from the probes to a 4-20 milliamp signal so you get a solid reliable signal back at your controller. A properly setup 4-20 Milliamp loop can be used reliably for thousands of feet even miles that all depends on your system setup of course - but the point is these temperature transmitters can send reliable temperature data a really long way. There's DIN Rail Mounted Standard and Programmable Universal Transmitters, Sensor Head mounted standard and Programmable Universal Transmitters, Integral Transmitters that have the transmitter and probe in one unit, M12 Transmitters that let you instantly convert an M12 Probe output to 4-20mA and a programmable universal signal conditioner that also happens to do temperature. The standard transmitters are quick and easy you just plug'em in and go. The universal transmitters give you a lot more control, they work with a variety of sensors and have some awesome features that just make your life a whole lot easier. That's a lot to cover in one video, so we'll do the standard transmitters in this video and then learn how to configure the universal transmitters in a separate video. For this video let's focus on the standard temperature transmitters. The ProSense temperature transmitters service ONE type of sensor over a specific temperature range. This one is for a Type J Thermocouple. You plug the thermocouple in here, and connect your 4-20mA loop here. Easy. Nothing to set or calibrate. There's another one for Type K, one for Type T and one for the RTD sensors. You get the idea you order a separate transmitter for each type of sensor. You also need to specify a fixed temperature range for these ProSense transmitters. You have a number of temperature range options to choose from for each sensor type. Why? To give you better resolution. If you choose a 0-2000 deg F transmitter which sends 4mA here and 20mA here to the controller - but your temperatures are only in the 0-200 degree range then you will only ever see this much of the 4-20mA range. But if you chose the 0-200 degree transmitter, now 20mA is at 200 degrees so you get the benefit of the full 4-20 mA range which means you get much more accurate results. So be sure to choose the transmitter that best suits your temperature range if resolution is important to you. The Blue Temperature Transmitters from FACTs Engineering do the exact same thing, but you have a few more options with these. The one Thermocouple unit works for all the same Thermocouple probes as the ProSense units - and several more - but you don't need a separate transmitter for each probe - Select the probe type using these dip switches. It also operates as a millivolt to milliamp convertor so you can use ANY small voltage source you want instead of a probe. The ProSense transmitters work from 12 to 35 Vdc, the Blue ones only work at 24Vdc. If you have a known calibrated input you can adjust the zero and the span of the output of the blue transmitter to optimize your system. For some special applications that can be important. Most of the time you probably won't need that. Both units send a large or upscale current on fault like if the Thermocouple burns out or shorts but only the Blue unit allows you to set that to a low or Down Scale current value. The yellow LED on the front of the ProSense unit indicates power. The LED on the front of the blue thermocouple transmitter is Green for running, but RED during power up, and if there is an internal calibration error or a thermocouple burnout condition. That's handy to be able to see if there is a fault indication right on the front of the box. The blue transmitter has a burnout detection disable that is it doesn't allow the current to go beyond the 4-20mA range that's in case you have a calibration unit that can't handle the over current condition. You also need to disable the burnout function if you are applying millivolts to the input instead of a probe. Both units have internal cold junction temperature compensation but only the blue unit has a disable. You would use this if you want to do your own compensation or if you are not using this as a millivolt to milliamp converter. The blue transmitters don't allow you to choose optimized temperature ranges like the ProSense transmitters which means if you select the J Type thermocouple on the blue unit, the 4-20mA will be spread across the Full J- Thermocouple range whether you need it nor not. Of course for most applications that really isn't an issue especially since thermocouples are spec'd to plus or minus several degrees anyway. So if you need any of these special features then definitely consider the blue thermocouple transmitter from Facts Engineering. Otherwise the economical ProSense transmitter is very capable and will work great in the majority of your applications. As a side note, the programmable ProSense Transmitters that we cover in the next video do just about everything these blue transmitters do and a lot more, but you have to purchase the special programming cable to use them. Check out Part II of this video for more on those. RTD transmitters have a similar comparison with the only real differences being: The ProSense transmitters only handle the Pt100 RTD's. The Blue transmitters work for several types of RTD's again, switch selectable. And the ProSense RTD transmitters offer several temperature ranges just like the thermocouple units. The blue Facts Engineering unit only covers the entire RTD range. Both the Thermocouple and RTD Prosense transmitters are available in a sensor head mountable version. You just swap out the terminal block for the transmitter and you are done! Let's do a live demo using the ProSense DIN Rail thermocouple transmitter. We have a J-Type thermocouple Probe so we select a J Type Transmitter and we want to use it over a 0 to 200 deg temperature range so we select the 0 to 200 deg J-Type transmitter. Attach the thermocouple here, and the 4-20mA loop here. We'll apply power to the loop and normally your controller would go here. For this demo, we are going to bypass the controller and simply put a current meter here so we can see what is going on. Power it up and for the first few seconds we see we get an under current of something well below the bottom 4mA. That's a signal to the controller that the transmitter is powering up and calibrating. Once that is done we're ready to read the temperature. Looks like we are at 10.3 ish milli amps, which if you do the math, comes out to a warm room temperature of about 79 degrees. Perfect. If we drop the sensor in a cup of ice water, it takes thermocouple a couple seconds to respond to the temperature change because it takes time for the temperature change to get through the metal mounting block and reach the actual probe. Looks like it is going to settle out at around 6.7mA which if we do the math is about 34 degrees F - pretty much exactly what we would expect in an ice water bath. Now if we move the probe to a cup of hot water, again it takes a couple of seconds to respond. By the way, this probe really isn't intended for immersion it's just one I happened to have laying around and works fine for this little demo. Looks like it's going to settle out at around 17 mA which is 162 degrees F. The Integral sensors come in several temperature ranges and with quarter inch NPT threads so you can screw it right into the process tank or with a straight shaft that you would insert into a compression fitting. These integral sensors are real handy because it's everything you need in one package, but they don't have as many temperature range offerings and they are only available as RTD sensors. The M12 transmitter is for 3 and 4 wire RTD probes. You just plug it onto the probe and you instantly have a 4-20 mA temperature transmitter that will work with any m12 4 pin cable. You can also use an M12 extension cable between the sensor and the transmitter. Just like the other guys you have to specify the fixed temperature range that you want to use. And of course, you would take the other end of the copper wire that used to go to an RTD module on the controller, and instead run them into a 4-20mA analog input card. The bottom line here is given the probe type, temperature range and the form factor, selecting and using a temperature transmitter is easy. If you need any help, please don't hesitate to contact AutomationDirect's free award winning tech support, they will be happy to help you. And don't for get the forums there are lots of folks there that love to share their years of experience just don't post any support questions there the forums aren't monitored on a regular basis by AutomationDirect's support staff.