AutoCraft Posted October 27, 2011 Report Posted October 27, 2011 Has anyone connected a thermocouple directly to a PLC analog channel? If so, were you satisfied with the readings? Quote
Mickey Posted October 27, 2011 Report Posted October 27, 2011 There are thermocouple cards for this purpose. If that is not what you have then the analog card would have to be able to read millivolts. What PLC and analog card do you have? Quote
AutoCraft Posted October 27, 2011 Author Report Posted October 27, 2011 Thanks for replying, I'm using an Allen Bradley CompactLogix L23 with it's embedded anlog I/O. Note attached screen shot. ScreenShot.doc Quote
Mickey Posted October 27, 2011 Report Posted October 27, 2011 (edited) The embedded analog I/O's have voltage ranges of -10 to 10vdc, 0-5vdc, 1-5vdc and 0-10vdc. Because thermocouples have millivolt outputs you will not have the resolution to read millivolts accurately with the embedded I/O. You need a thermocouple card or a transmitter to convert the thermocouple to a analog value the embedded I/O can read. e.g 1-5vdc or 4-20ma) IMHO Edited October 27, 2011 by Mickey Quote
DanW Posted October 28, 2011 Report Posted October 28, 2011 Thermocouples are not only a low level millivolt output but require 'cold junction compensation' because the look-up tables or polynomial equations used to calculate temperature from millivolts assume that the connection to the PLC input terminals is always at the freezing point of water (0°C, 32°F), yet the input terminal block is rarely, if ever, at that exact temperature. Without cold junction compensation the error can be in tens of degrees, the difference between the temperature of the terminal block at the PLC and the freezing point of water. There are two conventional ways of dealing with a thermocouple. a) Thermocouple input cards have 'cold junction compensation' built into them and the appropriate calculations to covert from mV to temperature for various types of thermocouples. These are supplied by the PLC vendor. Non-isolated models cost a lot less than isolated models. It pays to buy isolated. b) Thermocouple transmitter. These devices take a therocouple input and output a "high level" current or voltage proportional to the programmed temperature range. The device requires a DC power supply. Two wire transmitters have an external power supply in the same 2 wire circuit the provides the current signal to the PLC analog input. Non-isolated models cost a lot less than isolated models. It pays to buy isolated. A Google search for thermocouple transmitter will produce hundreds of vendors. If this is a one-off deal, be sure to limit your consideration to one that is either vendor pre-configured (free or some nominal fee) for the range you need or the software/cable to do so is really inexpensive because there are very inexpensive devices in this category where the cost to configure the output range is multiple times the cost of the transmitter. Quote
AutoCraft Posted October 29, 2011 Author Report Posted October 29, 2011 My Thanks to everyone that replied Quote
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