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:

  ![DPM basic](https://johngodlee.xyz/img_full/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](https://johngodlee.xyz/img_full/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](https://johngodlee.xyz/img_full/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:

  [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