Measurement Computing DataShuttle - Manual (Page 121)

DataShuttle and DynaRes 1 1-13-01 Appendix A-1
Appendix A--Thermocouple Application Tips A
When implementing thermocouple applications, the following points will prove helpful if any problems are
encountered.
• Ensure that the negative terminal is shorted to the ground (GND) terminal. Do not do this if the
thermocouple is also grounded.
• Make sure that the type of thermocouple chosen in the software is the same as the one in use.
Thermocouples can be identified by the colors of their wires.
• Check that the thermocouple is made from matched wires and not just any two bits of wire of the right
composition. Non-matched wires will show errors due to resistance differences in the two wires.
• No wires should be connecting the ends of the thermocouple wires to the terminal panel. This leads to a
change in the voltage drop due to the extra wire’s different temperature/resistance characteristics.
Extension wire that is specifically designed for a given thermocouple type can be used however. In this
case only use extension wire at low temperatures and in regions of small gradients.
• Use the isothermal block terminal panels. Without it errors of up to 5°C can be seen in certain
environments.
• Use sheathing to decrease the temperature gradient along certain sections of wire. If 90% of the
temperature gradient is along a short section of wire, and that section has some small errors, then those
errors will be significantly magnified.
• Ensure that there are no intermittent or solid grounds at the tip. This can occur in conducting solutions,
or in high vibration or dirty environments. If there is an intermittent ground (seen as occasional spikes),
use a high resistance resistor at R5 as shown in the manuals.
• If the cable is old, has been exposed to extreme temperatures or has been bent frequently, then the
temperature characteristics may have been changed and will likely result in bad readings.
• Don’t use very long, very thin wire as this can increase the resistance in the wires and lead to errors,
especially at high temperatures.
• Check the connection at the end of the thermocouple, where the two wires have been joined. If this was
not done properly then the wires may have been damaged or readings may be in error.
• Use the low noise mode, or at least the highest number of bits possible. This will help filter out
unwanted noise.
• Ensure that calibration setup is engaged in DASYLab. This will prevent errors due to changing ambient
temperatures.
• Make sure that QuickLog PC version 2.0.5 or QuickLog For Windows 1.14 or later is in use. Previous
versions do not check the cold junction compensation.
• To check that the board, software, and terminal panel are functioning correctly, connect the positive
and negative terminals of the channel in use to each other with copper wire. The software should read
room temperature. If it does then the problem is with the thermocouple and the way it is placed in it’s
environment.
• To check that the thermocouple and cold junction compensation are working do the following: Heat the
joined end of the thermocouple and check that the temperature reading increases. Heat the terminal
ends and check that the temperature decreases. If the temperature movement is in the wrong direction
then reverse the thermocouple leads. Heat the cold junction reference (only possible on non isothermal
block boards) and check for temperature increase. Each can be cooled instead of heated, with the
opposite temperature reaction occurring.
• When using Thermocouples connected to multiple units and a common metal surface remove −
common connectors to eliminate ground loops.