Can I Use The SCC-AIxx Family For Thermocouple Measurements?Hardware: SCC>>Carriers>>SC-2345
Problem: I want to use the SC-2345 for more than eight channels of thermocouple readings, but the SCC-TC modules are limited to only eight per carrier. Can I use the SCC-AIxx modules (which have two channel inputs) to access channels eight through fifteen for thermocouple measurements? Solution: The AIxx family of SCC modules offers isolated analog input signal conditioning for two channels of a data acquisition board. Although most thermocouples do not require isolation, the amplification provided by these modules can be useful for thermocouple signals. For most measurements, the best SCC module (aside from the SCC-TC's) to use with thermocouples is the SCC-AI07 since it provides the highest gain in the family and covers most of the range found in different thermocouples. The SCC-AI07 has an input range of +/-50 mV, provided by a constant gain of 200. (For more information on acceptable voltage ranges for different types of thermocouples, see the application note on thermocouple measurements linked below. There may be instances where +/- 50 mV will not cover the extremes of a thermocouple's range. The SCC-AI06, with a range of +/-100 mV, will most likely cover these circumstances.) With that said, there are major issues with using the SCC-AIxx modules for thermocouple measurements. Most importantly, the AI modules are lacking Cold Junction Compensation (CJC), an important feature for accuracy in the SCC-TC modules. It would be possible to dedicate an AI channel as the CJC for your entire SC-2345 carrier, but due to the thermal gradients inside the carrier, the dedicated CJC channel would be largely ineffective for modules close to electrical "hot spots" (due to power dissipation on chips). This is one of the major differences between the SCC-TC modules and SCC-AIxx modules; the reason only half of the channels are available is because of an onboard CJC for each module. Having an onboard CJC is the only way to effectively eliminate inaccuracies due to the cold junction temperature. Another disadvantage of using the SCC-AI07 or the SCC-AI06 for thermocouple measurements is the lacking of low frequency filtering. These modules provide a fixed lowpass filter at 10 KHz. To accomplish the appropriate filtering for noise reduction on the thermocouple's signal, a filtering module as a second stage SCC module would need to be added. For example, the SCC-LP01 provides lowpass filtering at 25 Hz. This module would eliminate unwanted lower frequency noise, including noise from AC power (but require doubling the number of required modules). For more information on these SCC modules, please follow the links below. Related Links: Product Manuals: SCC-TC Series Thermocouple Input Modules User Guide and Specifications Product Manuals: SCC-AI Series Isolated Analog Input Modules User Guide and Specifications Developer Zone Tutorial: Measuring Temperature with Thermocouples Attachments:
Report Date: 09/24/2002 Last Updated: 08/02/2007 Document ID: 2PN9098O |
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