Below are several things to try if your thermocouple is not reading correctly:
Most Common Thermocouple Problems
CHECK HARDWARE & SOFTWARE SETUP
Make sure your thermocouple is set up properly, as described here.
GROUND TC AT ONE PLACE
Attach an additional wire from the thermocouple Vin- screw terminal to the GND screw terminal.
In general the TC should be grounded at one place. If left floating, it could radiate like an
antenna with respect to instruNet ground. If grounded at more than 1 place, it could be in a ground loop,
and have current flowing through it from one ground to the other (which might have a 5mVpp 10MHz sine between them,
as noted here).
The i100 is most susceptible to RFI, followed by the i420/i430/i60x, followed by the i423; due to internal RFI filters of varying strength.
ARE YOUR THERMOCOUPLE WIRES BACKWARDS?
If your thermocouple leads are backwards, then the measured temperature will be shown as varying
in the opposite direction from ambient (e.g. if the instruNet terminals are at 25°C and the thermocouple
is at 35°C, and the leads are backwards, then instruNet will report 15°C).
The RED wire within thermocouple cables often contains the negative (-) signal, which is the opposite of what one might expect.
This is driven by the ANSI standard for thermocouples; however there are other standards with other color codings,
as noted here.
THERMOCOUPLE SETUP
Make sure the channel is set up for Thermocouple Sensor Type, with 0.001 to 0.016666
Seconds integration (typically), and a Range ≤ ±80mV.
If you are on the ±80mV range and want more accuracy, try the ±20mV range.
INSTRUNET CALIBRATION & TEMPERATURE DRIFT ERRORS
Try pressing the instruNet CALIBRATE button in the NETWORK page. The reading on the
computer screen might change slightly. This change represents the correction of the
thermal drift within the instruNet voltage measuring system (i.e. an instruNet measurement
error that is proportional to the temperature change of the instruNet PCB since the last calibration).
instruNet can be set up to automatically calibrate every X minutes, or when it starts to digitize. This
error can be several degrees C if the instruNet PCB temp has changed >10°C since the last calibrate.
In summary, it is strongly recommend that you work with version ≥ 1.4.1 of
the instruNet software, and
that AUTO CAL RATE be set to approximately
5 minutes and CJC CAL RATE be set to approximately 3 minutes
(in the Record page, press the Setup button, and then the Calibrate button to access these fields).
MUST INSERT THERMOCOUPLE WIRES DIRECTLY INTO SCREW TERMINALS
Both the positive (+) and the negative (-) thermocouple wires must touch the instruNet screw terminal metal in order
to measure temperature at the thermocouple tip (i.e. CJC compensation);
therefore, it is important that the two TC wires, after being stripped bare, are inserted directly into the
instruNet screw terminals without an added extension between the bare
wire and screw terminal (e.g. solder lug, additional wire, component lead, pin crimped to wire, etc).
If a solder lug of the same material as the TC wire is crimped to the TC wired and then
inserted into the instruNet screw terminal, one should expect an additional ±0.5°C error
due to that large object being cooled by air near the screw terminal.
WORKING WITH A THERMOCOUPLE SIMULATOR INSTRUMENT
If instruNet is measuring the signal from a Thermocouple Simulator instrument and
experiences inaccurate or sporadic readings, try adding a small wire between
the instruNet GND screw terminal and the instruNet Vin Minus (-) screw (i.e. the thermocouple - lead). If that does
not fix the problem, try adding a wire between the instruNet GND screw
terminal and the Simulator instrument metal enclosure, to reduce high frequencies signals between the two systems.
Less Common Thermocouple Problems
AVOID BNC CABLES BNC
cables and
Coaxial
cables often have a gold plated center conductor and a copper or nickel outer conductor. These different materials create their own thermocouple and induce an error
within a thermocouple measuring circuit.
TEST INSTRUNET AMPLIFIER
Select "Installed Hardware Report" under the Hardware menu. This will give you the temperature of the instruNet device. Then short the Vin- to Vin+ screw terminals. The shorted channel temperature should be the same as the instruNet box temperature. If this is correct, then something is happening with the thermocouple being attached; otherwise, the channel is having difficulty in some way. Also, try turning the analog low pass filter OFF, and see if that fixes it. Also, try attaching the thermocouple to another measurement channel, to see if your channel amplifier is damaged in some way (e.g. due to static electricity).
DEBRIS & TC MATERIALS
Clean the screw terminal housing and wire with alcohol and visually inspect them for debris or discoloration. And make sure that your TC wiring and connectors are of the correct material. Any non-correct metals in the system will cause your TC to be inaccurate.
TEMPERATURE GRADIANT
Are your thermocouple wires at the measurement device hot or cold with respect to room temperature? If so, you might have a thermal gradient from the wires to the internal instruNet temperature sensor.
INSTRUNET NETWORK
Try disconnecting outbound instruNet devices, and see if this effects the measured temperature. Try measuring the instruNet 5/12/-12V power supply voltages at the instruNet screw terminals. They should be within 1V of their nominal values. If they are not, you might have a power supply problem. Make sure the instruNet cable thumbscrews are snug and the end of the network is terminated. Make sure you are not working with outdated instruNet software. Free updates are available at instrunet.com.
OSCILLATING TC
If touching your TC wires with a moist finger makes the computer reading change, then your added capacitance to ground might be stopping or changing an oscillation w.r.t. earth ground. To view the voltage vs. time waveform at the instruNet screw terminals, set integration to 0.0 and digitize 1 channel for 1000 pts at 100,000 samples per sec, 1 scan; and then look for a sine wave on the screen. If you see this (you might need to change the display min/max to better see the waveform shape), then your TC is radiating w.r.t. earth. To fix this, install an additional wire between your Vin- screw terminal and GND, as noted here.
RFI THROUGH TC WIRE
RFI going through a loop of wire (e.g. your TC wires) can induce a voltage such as 5mVpp sine wave at 10MHz (Reference: Maxwell's equations). To test for this, please move your thermocouple wires around with a plastic pen. If the reading on the computer screen changes, this means that RFI is interacting with your wires since changing the EMF flux through the wire loop changes the voltage at the screw terminals. To fix this, install an additional wire between your Vin- screw terminal and GND, as noted here.
GROUND LOOPS
If your computer is at ground G1, and the instruNet device is at ground G2, and the sensor is at ground G3; and these 3 grounds vary by several milliVolts (AC or DC), then this can effect your 5uV accurate measurement. For details, click here.
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