The TR-1 Needle Performs Beautifully

The TR-1 needle used to be one of my least favorite needles. Customers who were using the IEEE 442 standard needed to use it, but I always felt like it didn’t work very well, and I was pretty up front with people about that. I actively tried to convince people to spec the ASTM standard instead so they could avoid the TR-1 needle. If they couldn’t do that, we had workarounds using uncalibrated data that we felt gave people results that were close for most soil samples to the ones they would get using the smaller single needle.

The TR-1 needle violates certain assumptions of the infinite line heat source theory. That theory is an essential part of heated needle thermal properties measurements. I felt pretty pessimistic about ever being able to make good measurements with such a large needle. So, as more and more of our customers needed comply with IEEE-442, I felt the best option was to work to get the IEEE standard changed. Long time readers of our blog and website will know that this has been a goal for a couple of years now.

Noting the pace of institutional change, however, I started to wonder if it might not be faster to change the theory. So last summer we started a big project in-house to look at the theoretical and practical issues involved with using a large needle.

A physics graduate intern devoted his summer to looking at the problem mathematically, then I spent several weeks drilling down to try to understand what was going on. What we discovered surprised us and changed our whole approach to the TR-1 needle.

I could write a lengthy article about what we uncovered, but to sum it up briefly, when you’re measuring thermal properties in soil using heated needles, you walk a tightrope. You’re trying to minimize heat input to minimize moisture migration, and minimize read time to minimize temperature drift errors. These factors push towards a short read time. But you need longer read times to get around contact resistance errors and to overcome theoretical constraints imposed by using a large, fat needle. Much of our work involved changing how we balanced all these factors to try to improve results in dry soils for the TR-1 needle.

We had an aha moment that dramatically changed our approach. The KD2 Pro firmware has now been rewritten. If you’re a current customer, you will have received a firmware update and new information about calibrating your TR-1 needle.

And now that we’ve worked it all out, the TR-1 needle performs beautifully. It was originally the needle I liked the least, but after this work I like it best. I think it’s really nicely suited for measuring soil, concrete, and rock. It’s even able to perform well in granular materials with large particle sizes. It should never be used, however, to measure liquids. It will not give accurate results, and the KS-1 needle should be used instead.