Thermocouple temperature signal error:
1.Have not add cold-junction compensation wire. If the user has not use cold junction compensation wire, the thermocouple sensor terminal temperature and the temperature from other instruments are not matched, thus the data acquisition results are different.
2. Wire length and interference. The thermocouple sensor signal mV or uV signal is very tentative and weak, if the wire used is too long, the signal attenuation and the high voltage interference will cause thermocouple signal distortion, ripple, noise, etc. The thermocouple lead wire resistance value recommended is less than 150Ω.
3. Wrong wiring connection, reversed connection between positive and negative terminals.
RTD PT100 temperature signal error:
1.How to check if the PT100 sensor signal output is standard correctly, normally or not? RTD sensor has good linearity in temperature range 200-650℃. If needs to check it if an RTD PT100 sensor outputs signal normally or not, user can measure the RTD PT100 sensor output signal resistance value by a high precision (at least 0.01%) multimeter. Then check the PT100 resistance value table to confirm it the PT100 sensor outputs correctly or not.
To check if a temperature measuring meter is accurately or not, user needs a high precision RTD PT100 simulator or temperature calibrator or PT100 signal source. The low-cost method is that using a 0.01% grade high accuracy resistance to compare their outputs by referring to the PT100 to Ohm signal reference table.
2.The lead wire is too long, and has lead wire error. The thermal resistance working theory is that the wiring resistance will change along with the temperature changes. If the lead wire is long, the wire resistance value is higher, and the lead wire resistance value causes error in measuring process. The lead wire resistance value recommend is less than 2.5Ω.
Following solutions to reduce lead wire error:
a. Use 3-wire, 4-wire wiring methods to reduce error.
b. Do linear correction based on the applications requirement and data acquisition instruments functions.
c. Use temperature transmitter or RTD signal conditioner to convert thermal resistance signal into 4-20mA signal output, and use data acquisition device to do 4-20ma signal data acquisition after long-distance transmission.
3. Wrong wiring connection cause large error. User needs to recheck and connect wires correctly.
4. Thermal resistance signal output value measured is an infinite value. In this case, usually the wiring screw is not tight or breaking wire.
PT100 thermal resistance commonly used wiring modes and differences.
1. PT100 Two-wire wiring mode: Connecting lead wire at both ends of the thermal resistance to lead out the resistance signal is called the two-wire mode. This wiring method is very simple, but the lead wire has resistance R. The resistance value R depends on the material and length of the wire. This wiring mode is only suitable for applications with low measurement accuracy.
2. PT100 Three-wire wiring mode: Connecting one lead wire at one end of the thermal resistance and connecting two lead wires at the other end is called three-wire mode. This wiring mode is usually used in conjunction with a bridge, which can better eliminate the influence of lead resistance. It is the most commonly used lead resistance in industrial process controlling applications.
3. PT100 Four-wire wiring mode: Connecting two wires at each end of the thermal resistance is called a four-wire mode, in which two lead wires provide constant current I for the thermal resistance to convert R into a voltage signal U, and then transmits signal U to the secondary meter through the other two lead wires. This wiring mode can completely eliminate the influence of lead resistance. It is mainly used for high-precision temperature detection.
In temperature signal data acquisition applications, a technician must have at least 0.05% F.S. precision temperature calibrator or thermocouple PT100 calibrator simulator.