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... About Instrumentation

Temperature Measurement
What precision can I achieve using a standard sensor?
The specifications for tolerances have typically +/-2.5ºC for most common thermocouples and +/-0.5ºC for RTD's. Best accuracies can be reached in the order of +/-0.5ºC for thermocouples and +/-0.2ºC for RTD's. Please note that these values are always dependent on temperature. If RTD is used, for better accuracy you should use a 4-wire wiring configuration.
How do I choose between a thermocouple and an RTD?
Basically the required accuracy, process temperature, response time and dimensions of the probe. Thermocouples provide a response time better than the RTD's; RTD's have better accuracy and stability than thermocouples. In relation to the process temperature thermocouples allow measurement of temperature at temperatures higher than the RTD's.
I have my thermocouple installed away from the controller. This is a problem?
Could be. You should get a loop resistance until 20Ohm, maximum 100Ohm for thermocouples and Pt100. Exceed the 100Ohm may result in coarse errors of measurement. Note that installing a 4-20mA transmitter near the sensor, the length of the cable and its impedance ceases to be a problem and can achieve a thermocouple assembly economically advantageous, by using cables with copper wire only.
Should I choose thermocouple type K or type N?
Thermocouple N type is more stable and immune to junction medium contamination than the type K.
Why should I use a protective sheath?
A thermowell allows the removal of temperature probe without being forcibly stop the process. Being appropriately sized, heat transfer will be far-right, without additional measurement errors, shielding the probe from the medium.
What wiring configurations exist for Pt100 and what should I use?
For a better understanding is necessary to present the basic configuration of the sensing element.
At the end of the sensor lead wires are welded and insulated, and the assembly is inserted in a metallic tube.
Since the resistance value that allows the determination of temperature is only the sensor resistance and not the resistance of the connecting wires, there are three basic schemes for sensor connection that circumvent the inevitable fact of having to use lead wires to measure the sensor resistance.
2-Wire Connection:
The Wheatstone bridge has a resistance thermometer with two wires connection. In the equilibrium condition of the bridge, which occurs when the galvanometer "G" indicates zero, one gets the following relationship between the resistance:
Rsensor + RL1 + RL2 = R1.R3 / R2
If R3 = R2 then Rsensor + RL1 + RL2 = R1. Since R1 is known, it is determined the value of Rsensor + RL1 + RL2.
Clearly, the value of sensor resistance values are increased resistance of the connecting wires, which decreases the quality of the measurement with the resulting error in temperature measurement.
3-Wire Connection:
In this case the bridge is connected to a three wires resistance thermometer. Considering again that R2 = R3, the balance of the bridge will result from the condition:
Rsensor + RL2 = R1 + RL1
How is it possible, with good approximation, consider approximately equal to RL1 RL2 RL3 approximately equal to, the effect of the wire is much attenuated.
This is the type of connection used in resistance thermometers for industrial applications.
4-Wire Connection:
The four wire system connection completely eliminates the resistance of the connecting wires, being used in situations that require very precise measurements. Many "Reference" elements are manufactured with four wires connection, for example:
This configuration is intended for use in laboratories, reading instruments of reference sensors allowing the circulation of a constant current (usually 1mA) through the sensor element and the potential difference measuring are made on the sensor terminals thus eliminating the effect of the resistance of the leads in the final result.
Why do you provide RTD assemblies with 2, 3 and 4 wire configurations?
Because in fact the three configuration are used. The most common is the 3-wire, and 4-wire configuration that will achieve greater accuracy. The 2-wire configuration should be avoided if the sensor is for example Pt100. The 2-wire configuration is common for Pt1000, since the impedance of the cables is in most cases negligible compared with the resistance of the sensor.
I need to measure fast variations of temperature. What sensor I should use?
You should use a thermocouple with a lower thermal mass as possible. If the medium and the acquisition unit allow, the hot junction must be exposed or grounded.
A probe with a calibration certificate is more accurate than one uncalibrated?
No. But with the information of the errors and uncertainties compared to a reference sensor, you can get a measurement chain with better accuracy.

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