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:
![DPM basic DPM basic](/img/dpm/slide_1.png)
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.
![DPM bracket design DPM bracket design](/img/dpm/slide_2.png)
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.
![DPM flange design DPM flange design](/img/dpm/slide_3.png)
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:
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