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.

ISO 9001:2008 CERTIFICATION FOR KD2 PRO

Last Friday was a big day here at Decagon--after a year of intense effort, we finally got ISO 9001:2008 certified. If you're not involved in a manufacturing business, you may not know what that means. Here's what it has meant for us:

QUALITY STANDARDS
Quality standards for all components that are built into your KD2 Pro. The KD2 Pro's components are tested when they come in the door to exacting specifications.

MANUFACTURING STANDARDS
Manufacturing standards that detail assembly for every step of the process to manufacture an instrument.

TESTING IS CRITICAL
Testing at critical points within the manufacturing process to be certain everything is made and working together correctly.

There are lots of other details to ISO that are important as well. In the end, I don't think this will change the quality of the KD2 Pro--we've always been pretty stringent about our manufacturing and testing procedures. Decagon's growing fast and we want to maintain quality and consistency as we get bigger.

KEEP LEARNING

THERMAL RESISTIVITY WORKSHOP
If you are interested in improving your thermal resistivity knowledge and skills, this workshop will be of interest to you.

"Measuring RHO (Thermal Resistivity) in soils and backfill materials for power cable engineering."

First RHO International Workshop
Held in Barcelona, Spain
26 thru 27 April 2010

Oral Sessions
* Basic principles of making correct measurements.
* Discussion of factors affecting thermal resistivity measurements.
* Discussion of the current state of standards for measuring thermal resistivity.

Ampacity Calculations
* 2-D numerical approaches for ampacity calculations.
* 3-D modeling for cable crossing.

Lab Exercises
* Use current equipment to measure thermal resistivity.
* Interpret thermal resistivity data.
* Compile useful and relevant modeling data.
* Understand limitations of computer design.
* Click here for more information.

REVISING IEEE 442 Thermal Resistivity

Well, the process of revising IEEE 442 has begun. This standard affects anyone making thermal resistivity measurements who needs to cite IEEE compliance. As a result, many people have strongly held—and divergent—opinions about what the result should be.

It reminds me of this quote: “I have come to the conclusion that the making of laws is like the making of sausages—the less you know about the process the more you respect the result.”

Work on the standard is being done by the C-25 work group on thermal resistivity under the Insulated Conductors Committee. The ICC is a professional organization within the Power Engineering Society (PES) of the Institute of Electrical and Electronics Engineers (IEEE).

This workgroup was responsible for originally drafting IEEE 442. The standard was reaffirmed in 2003, but its last major update was in 1981. Quite a lot has changed since the early 80’s, especially in the world of computing and microprocessors. The committee has a lot to consider in bringing elements of the standard up to date with current science and technology.

We look forward to seeing the results of their work.

Nimesh Patel of Geotherm is the committee chair.

KD2 Demonstration Video

ASTM STANDARD

ASTM 5334 was updated in 2008 to reflect the current state of the art of thermal properties measurements. Its last serious revision (not minor update or reaffirmation) took place a few decades ago, so it was time for a major overhaul.

INFINITE LINE HEAT SOURCE
We got involved with standards change not because we thought the KD2 didn’t conform to ASTM D5334. The KD2 has always conformed to the spirit of the ASTM standard—the idea of using the infinite line heat source method to determine thermal properties from a heated needle.

EXTRA WORK AND DEGRADED DATA
But D5334 still caused problems for some of our customers. The KD2 uses complex numerical methods to get an accurate resistivity, and that didn’t use to conform to the letter of the standard. In order to conform completely, our customers had to extract raw data and draw a graph. It created a lot of extra work and degraded the quality of their data.

OBSOLESCENCE
ASTM D5334-05 was obviously obsolete. It had survived most of the computer age intact. It called for heating a thermal needle for 1000 seconds, plotting log time vs. temperature, picking two points that looked good, eyeballing a straight line, and then getting resistivity from the slope of that line.

PENCIL AND PAPER NOT GOOD ENOUGH
Specified needle sizes were too big to give accurate data unless they were calibrated, but the standard didn’t call for calibration. And I’d be pretty surprised to find any professional engineer trying to get accurate resistivities from a crude pencil and paper graph.

TIME FOR AN UPDATE
I discovered that updating standards is a chore—even when everyone involved thinks it’s past time for an update. But the resulting standard, ASTM D5334-08, conforms to best practices in thermal properties measurements. Even if you don’t use our instrument, you should spec and use the new standard. It’s a leap forward for thermal properties measurements.