[GSBN] Calibration of Balemaster probe

[Jim Carfrae]

Hello All

There have been a couple of threads here about the accuracy of straw bale moisture probes like the Protimeter 'Balemaster'.
The big question on probes of this sort (that measure the electrical resistance of the straw at the tip, and translate this into a figure for moisture content) is how far the accuracy is affected by the density of the straw.
David Eisenberg has described how, if you take a handful of straw and wrap it around the tip of the probe, the moisture reading goes up and down as you press the straw against the probe, which indicates a potential problem!
I decided to set up an experiment to try and find out how much of a problem this actually is, and here is the methodology (for those who care to know).

First I built a box with a lid that could be pressed into it.
I then measured the internal volume of the box, and weighed out some straw that if compressed to the density of a typical bale (110Kg/m3) would fill the box exactly.
The straw was taken from a bale that had been at the back of my garage for some time, in a reasonably stable environment, with wood stored next to the bale measuring (with a Protimeter 'Timbermaster') 16.1% moisture content
The straw weighed out to go in the box was loosely restrung from the bigger bale, and before I placed it in the box I calculated the volume, then measured it with the Balemaster.
I then placed the straw in the box and gradually screwed the lid down, stopping at pre-determined intervals to probe the straw through holes drilled in all sides of the box.
After the lid had been screwed down as tight as it would go (at which point the straw was too dense to easily insert the Balemaster), I took the straw out, re-weighed it, and then dried it in a fan oven until it had lost all it's moisture.

Results.
Loosely retied straw; density 68.5Kg/m3, Balemaster reading 12.2%
Straw in box; density 95Kg/m3, Balemaster reading 14.7%
Straw in box; density 108Kg/m3, Balemaster reading 15.8%
Straw in box; density 118Kg/m3, Balemaster reading 16.0%
Straw in box; density 129Kg/m3, Balemaster reading 16.3%
Straw in box; density 142Kg/m3, Balemaster reading 16.6%

Actual moisture content of straw, determined by  gravimetric analysis (weighing, drying, weighing again) 16.2%

This was a very encouraging result; wood next to the straw was 16.1%, Balemaster readings at the sort of densities found in a typical wall ranged from 15.8 to 16.3%, and the straw was actually 16.2%

An observation: I've surveyed many straw buildings with a Balemaster, and the Balemaster will give you an impression of how dense the bales in the wall are according to the resistance as you insert it. This was borne out when using the Balemaster to probe the straw in the box, with the straw at 95Kg/m3 feeling slightly loose, and the resistance gradually increasing with 129Kg/m3 feeling as tight as almost any building in my experience, and at 142Kg/m3 it was almost impossible to insert the Balemaster.

I hope this will be of interest, but I would ask you to keep these results to yourselves, as I hope to publish them as part of an academic article soon.

Regards

Jim

Jim Carfrae
PhD Research Student

Room 119, Reynolds Building
University of Plymouth
Drake Circus
Plymouth
PL4 8AA

jim.carfrae at plymouth.ac.uk
07880 551922
01803 862369
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