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DATE: 2019-02-05
AUTHOR: John L. Godlee
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:
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.
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.
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[1] which gave me the inspiration for the T-section design. The papers I looked at were:
1: 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:
