Hi, I’m Bill Griffith, an Application Engineer with Agilent Technologies. Today we’re going
to talk about making accurate thermocouple measurements. In a previous video we talked
about how thermocouple works, and today I thought it’d be interesting to start at
the tip of the thermocouple, and look at the different error mechanisms in a thermocouple
measurement all the way back to our data logger. So let’s start at the tip with the three
thermal errors. The first is adhesion. You want to make sure that your thermocouple is
well adhered to the device that you’re measuring, because if it pops up or lifts up, you’re
going to be measuring the air temperature above the device. The next one is you want
to consider the mass of the sensor compared to the mass of the device you’re measuring.
I have an insulated thermocouple, and it has a large metal tube on it, so it has a lot
of mass. So this would be very inappropriate for measuring a small semiconductor device.
These insulated thermocouples, you also need to take into account that it’s going to
take a while for the thermal energy to go through the insulation and actually warm up
the thermocouple. So leaving some extra time in your measurement to make sure that it has
warmed up is important. They do make them where the actual metal tube is connected to
the thermocouple, and that will increase the speed at which the thermal energy gets to
the thermocouple. Let’s move on to some electrical considerations. The first electrical
consideration is our typical thermocouple. It is a bare wire connection, and so if you’re
connecting to a metal motor or a cabinet, it is a perfect path for ground loops. So
you want to pick an instrument to measure with that has good common mode rejection to
avoid those ground loops. The other option is to use one of the insulated type thermocouples.
When measuring a thermocouple, it creates a very small voltage based on the temperature
gradient across it, so noise will be an issue. So it’s important to use short wires to
avoid picking up as much noise as possible. The other thing you can do is most measurement
instruments have a way of trading speed off for accuracy, so the idea is to always pick
a slower measurement. Make one good measurement instead of lots of fast measurements to reject
that noise. Moving on, the next source of error would be a thermocouple would connect
up to our reference junction, and so you want to pick a good reference junction because
it’s going to provide the absolute temperature. After the reference junction, our next source
of errors will be the algorithm inside of our data logger. So the algorithm actually
has to remove the errors produced when we connect our thermocouple up to our reference
junction. We actually create two more dissimilar metal junctions. We’ve got to back that
out. And then the algorithm has to convert the voltage to a temperature, and so how well
it does it is the accuracy of that algorithm. So to give you a real life example, on our
34970 and our 34972, it actually specifies 1° of accuracy for a J type thermocouple
from the reference junction to the display. So if our thermocouple has about 1° of accuracy,
that means our system error is going to be 2°C. So there are certainly more accurate
sensors than a thermocouple, but thermocouples are really known for their ruggedness and
their large temperature range they can measure across. So when measuring thermocouples, the
things you want to keep in mind again are those thermal accuracies, make sure you have
good adhesion, and then noise will be an issue with thermocouples because it’s such a small
voltage, so make sure your wires are short, and that you make one good reading instead
of many fast readings. I hope this is helpful. Thanks for watching.