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Since errors from each component are not correlated with each other, the total system error will be the root-sum-square (RSS) of all the errors: 2 ETOTAL 2 = EDAQ 2 + ESIG_COND 2 + ETHERMOCOUPLE Example: Assume the following conditions: • DAQ: Agilent U2355A • Signal Conditioning: Agilent U2802A • Ambient temperature: 23 °C • Thermocouple type: J-type, standard limits of error • Temperature to measure: 600 °C Assume the following error specifications: • U2355A: Gain error = 0.02% of reading • Offset error = 1 mV • U2802A gain = 97.673 • Gain error = 0.06% of reading • Offset error = 15 .V (with respect to input) • Zeroing error = 6 .V (with respect to input) • CJ measurement accuracy = 1 °C • Thermocouple = greater than 2.2 °C or 0.75% error • Noise error has been omitted to simplify the example Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications With zeroing, the offset errors from the DAQ and the U2802A can be removed, and replaced with the zeroing error. Based on the ITS-90 Thermocouple table, a J-type thermocouple will output 33.102 mV at 600 °C, and changes at a rate of approximately 59 .V/°C. This corresponds to (33.102 mV . 97.673) or 3.2332 V at the input of the DAQ. Hence, EDAQ, ESIG_COND, and EZEROING are calculated as follows: • EDAQ = 0.02% . 33.102 mV = 6.62 .V = 6.62 .V . 59 .V/°C = 0.112 °C • ESIG_COND = (0.06% . 33.102 mV) . 59 .V/ °C = 0.337 °C • EZEROING = 6 .V . 59 .V/°C = 0.102 °C Next, the cold junction sensor error is calculated. At 23 °C, a J-type thermocouple output voltage changes at a rate of 52 .V/ °C. Thus, the CJ sensor error of 1 °C at 23 °C corresponds to 52uV/ °C . 1°C = 52 .V. At 600 °C, • ECJC = 52 .V . 59 .V/°C = 0.88 °C Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications Therefore, ETHERMOCOUPLE = 0.75% . 600 °C = 4.5 °C Using the above individual component errors, the total measurement system accuracy is calculated as below. System accuracy without thermocouple sensor error: ETOTAL = SQRT(EDAQ2 + ESIG_COND 2 + EZEROING 2 + ECJC2) = 0.95 °C System accuracy with thermocouple sensor error: ETOTAL 2) = SQRT(EDAQ2 + ESIG_COND 2 + EZEROING 2 + ECJC2 + ETHERMOCOUPLE = 4.6 °C Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications System Typical Performance Thermoelectric Characteristics The thermoelectric characteristics for various thermocouple types is shown in Figure 2-10. Figure 4-10 Thermoelectric characteristics for various thermocouple types Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications Typical Error vs. Measurement Performance The U2802A measurement error with U2355A or U2356A at 23 °C ± 5 °C is shown in Figure 2-11. Figure 4-11 U2802A measurement accuracy plot for various thermocouples type NOTE Assume a ±1 °C error in the CJ measurement due to sensor error and temperature gradient error in the accuracy numbers in Figure 4-2. Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications Making Accurate Temperature Measurements Thermocouple measurement accuracy is very sensitive to cold junction sensor errors and temperature gradients across the terminals. Keep the module away from any heat sources and drafts to minimize any variation between channels. The channels located closest to the center near the reference thermistor will have the best accuracy. It is important to use channels that are physically close together on the screw terminals when taking relative measurements. Channels that are closest together will have the best agreement. Agilent U2802A 31-Channel Thermocouple Input User’s Guide Product Specifications Agilent U2802A 31-Channel Thermocouple Input User’s Guide Agilent U2802A 31-Channel Thermocouple Input User’s Guide 5 Calibration Calibration 54 Calibration Information 54 Zeroing Function 54 Restore Factory Calibration 55 This chapter contains the calibration information and factory restore calibration procedure for the Agilent U2802A 31-channel thermocouple input device. Agilent Technologies Calibration Calibration Calibration Information The Agilent U2802A is factory calibrated and the calibration constants are stored in the EEPROM. During initial setup, the calibration constants are read from the EEPROM before any measurements are taken. Zeroing Function The Agilent U2802A thermocouple input device operating in thermocouple mode can be set to zero mode, where the differential inputs of each channel are shorted together. This zeroing function is used to measure the total system offset errors due to initial offset error, temperature drift error, and long term drift error from the DAQ (U2355A or U2356A) and the U2802A. This measurement can then be subtracted from subsequent measurements in order to remove the system offset error. Agilent U2802A 31-Channel Thermocouple Input User’s Guide Calibration Restore Factory Calibration The Restore Factory Calibration function in the Agilent U2802A is used to restore calibration da...