TITLE: DIY disc pasture meter
DATE: 2019-02-05
AUTHOR: John L. Godlee
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Disc pasture meters are used as a method of rapidly assessing grass
biomass in grasslands. They’re used a lot in rangeland ecology and
agriculture, but they’re also useful for my work. They operate very
simply by dropping a plate of a known mass and area onto a patch of
grass and recording the height at which it settles. They require
calibrating by clipping the grass below a proportion of the disc
measurements, drying it and then weighing the dry material to build
an allometric equation.

The basic design looks like this:

  {IMAGE}


I want to use a disc pasture meter in Angola when I go on fieldwork
next month, but I don’t want to have to take loads of bits of pipe
and a big disc in my luggage, which is already ridiculously bulky.
So instead, I am trying to design a meter that sources as much of
the instrument as possible from easy to find materials in Angola. I
have spent a lot of time surfing around on hardware websites and
measuring bits of pipe. The most dificult aspect of the design is
attaching the free-moving sleeve pipe to the plywood disc. I’ve come
up with a couple of designs.

The first design uses a section of flat aliuminium in an inverted-T
shape which are bolted onto the top of the plywood sheet, then the
sleeve is attached to the T section with a grounding clamp used for
pipes and electrical wires. This design is good because it
reinforces the potentially quite flimsy plywood sheet. I would take
the bolts, metal t-section, grounding clamp and the outer sleeve
pipe to Angola, and buy the rest of the piping and the plywood sheet
while I was there.

  {IMAGE}


This second design doesn’t distribute the weight as well, but
requires fewer bolts (meaning fewer drill holes) and fewer bits of
metal to take in luggage. It uses a base flange normally used to
attach a water pipe to a tank. Again, I would buy the flange, the
bolts and the outer sleeve in the UK, then get the rest in Angola.

  {IMAGE}


The flange design could potentially be made more sturdy by
sandwiching some flat pieces of reinforcing aluminium to the flange
and then going out radially in a cross shape to the edge of the
plywood disc.

I looked at a number of research papers which used disc pasture
meters to see what dimensions and materials they used. I came across
[this brilliant website] which gave me the inspiration for the
T-section design. The papers I looked at were:

  [this brilliant website]: http://phytosphere.com/gear/fallingplatemeter.html

D.I. Bransby & N.M. Tainton (1977) The disc pasture meter : Possible
applications in grazing management, Proceedings of the Annual
Congresses of the Grassland Society of Southern Africa, 12:1,
115-118, DOI: 10.1080/00725560.1977.9648818

Dörgeloh, W. G. (2002), Calibrating a disc pasture meter to estimate
above‐ground standing biomass in Mixed Bushveld, South Africa.
African Journal of Ecology, 40: 100-102.
doi:10.1046/j.0141-6707.2001.00338.x

.B. Hardy & M.T. Mentis (1985) The relative efficiency of three
methods of estimating herbage mass in veld, Journal of the Grassland
Society of Southern Africa, 2:1, 35-38, DOI:
10.1080/02566702.1985.9647996

N Zambatis, PJK Zacharias, CD Morris & JF Derry (2006) Re-evaluation
of the disc pasture meter calibration for the Kruger National Park,
South Africa, African Journal of Range & Forage Science, 23:2,
85-97, DOI: 10.2989/10220110609485891

ost of these papers used Bransby and Tainton’s (1977) design, which
suggested the following dimensions:

-   Long rod length: 180 cm
-   Long rod external diameter: 22 mm
-   Sleeve pipe length: 120 cm
-   Sleeve pipe external diameter: 27 mm
-   Disc diameter: 45.8 cm (18")
-   Total weight of free-moving parts: 1.5 kg