Temperature Sensing Helpful Hints
(VID-TE-0008)
Learn how to save time and money with your temperature sensing system.
In this video we'll cover a bunch of tips and tricks that you can use to help you save time and money. Compression fittings are single use. Once you have tightened down the compression nut, this ferrule gets pressed into the metal sheath of the probe and it's permanent. So make sure the probe is exactly where you want it BEFORE you tighten the compression nut down. The good news is you don't have to replace the entire sensor head if you do change your mind on the position, or even the type of probe. You can get the probes by themselves and just replace that part. You can also salvage the compression fitting and nut and just get new ferrules, again saving you time and money. And if you decide to change from a Thermal Couple to An RTD or vice versa or maybe the terminal block gets messed up somehow - don't go buy a whole new sensor head you can get the terminal base for a couple bucks and replace that. That saves you time and money. Ground the cable shield at the Sensor end only. Don't ground it at both ends that will create a new possible current path, which can create a ground loop which can actually make things worse than having no shield at all! And while we're talking about noise, don't bundle temperature sensor wires with power lines, high speed digital lines or other electrically noisy wires. Any of those can wreak havoc on your temperature measurements. Some thermocouples have the sensor grounded to the case that gives you better thermal conductivity which gives you quicker response times BUT beware grounded probes require temperature measuring equipment that is specifically designed for grounded probes they won't work with regular temperature measuring devices and likewise, the ungrounded probes won't work with the devices expecting grounded sensors. While you can use copper wire with RTD's, Thermocouples HAVE to have special wire which depends on the type of thermocouple. The wire colors should always match up when you are using Thermocouples to ensure you have the exact same kind of wire going from the sensor all the way back to the controller. If the wires aren't the same type of metal, then you just created a new thermocouple junction that will throw off your measurements right? Which begs the question Doesn't the sensor head change the Thermocouple wire type at the terminals? Doesn't that create a new thermocouple junction? Well, yeah it does, BUT it turns out it doesn't matter. The junction of the wire going to the terminal creates a small voltage, and the terminal to the wire creates another small voltage in the opposite direction. That voltage will change over temperature that's what a thermocouple junction does right? So they only cancel AS LONG AS THEY ARE BOTH AT THE SAME TEMPERATURE. Since both of these junctions are in the same small piece of metal, they are going to be at the same temperature so they will cancel. Could you use this trick with a long piece of copper wire? No, because the two junctions will most likely be at different temperatures and they won't cancel each other out. A normal DIN rail terminal block connects wires via a metal bar. You don't want to do that with thermocouples, because it creates a new thermocouple junction. Instead, use these special DIN rail Thermocouple terminal blocks. They actually clamp the Thermocouple wires together so you maintain metal continuity WITHOUT creating a thermocouple junction. They are inexpensive and an excellent way to maintain the integrity of your temperature measuring system. Some RTDs have 3 wires. The third wire lets the system measure the total resistance of this loop which it then uses to determine the length of the wire. That's how RTD's give you accurate temperature readings over much longer distances than thermocouples. You can extend the distance of RTD's and Thermocouples to miles using temperature transmitters that's covered in the video on remote temperature sensing. So, if you if you are within 300 feet and you need accuracy, use the three wire RTD. If you have a three wire RTD sensor, but only a 2 RTD wire controller, just connect the two red wires together. AutomationDirect offers 3 and 4 wire RTDs. The four wire sensor is for those controllers that require it there really isn't any accuracy or distance improvement over the 3 wire sensors so unless you have a 4-wire controller, the 3-wire sensors are all you really need. And while we're on that subject, make sure you use the correct module in your controller. You can't plug an RTD into a Thermocouple module, for example. They are completely different animals. All of the thermocouple probes are bendable so you can customize them for your application. Just be mindful of the Bend Radius spec for each probe. In general, you can expect the minimum bend radius to be around twice the sheath diameter. Check your probes data sheet to be sure. If you have any questions, please don't hesitate to contact AutomationDirect's free, award winning tech support during regular business hours they will be happy to help. And don't forget the forums there are lots of folks there that love to share their years of experience. Just don't post any support questions there, AutomationDirect doesn't monitor the forums on a regular basis.
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