TITLE: Reading on disc pasture meters
DATE: 2021-04-20
AUTHOR: John L. Godlee
====================================================================
Note that this information has since been expanded upon and is
available as a .pdf here.
[here](
https://johngodlee.xyz/files/dpm_reading/dpm.pdf)
Apparatus dimensions summary
Publication Long rod length (cm) Long rod
external diameter (mm) Sleeve pipe length (cm) Sleeve pipe
external diameter (mm) Disc diameter (cm) Disc
thickness (mm) Total weight of free-moving parts (kg)
---------------------------- ----------------------
--------------------------------- -------------------------
------------------------------------ ----------------------------
--------------------- ----------------------------------------
Bransby and Tainton (1977) 180 22
120 27
45.8 1.5
1.5
Dorgeloh et al. (1985) 180 NA
NA NA
45.8 NA NA
Zambatis et al. (2006) 180 22
97 27
45.8 1.5
1.5
Rayburn et al. (1998) 100 (metre stick) NA
NA NA
46*46 cm = 0.21 m^2 square 5.6 NA
Bransby and Tainton 1977 - The disc pasture meter : Possible
applications in grazing management
They call it a "single stem disc meter".
180 cm aluminium rod with an external diameter of 22 mm.
120 cm long aluminium sleeve with external diameter of 27 mm.
Internal diameter should allow it to move freely on the central rod.
Aluminium disc 1.5 mm thick, 45.8 cm diameter attached to the base
plate on the sleeve by 4 bolts.
Small weights should be attached to the base plate so that the
sleeve together with the baseplate and disc has a mass of 1.5 kg.
The disc should be released from a standard height of 60 cm above
ground level. The sleeve can also be gently placed on the grass
surface and measured when held at constant height for 15 seconds.
The "sampling cylinder" should be able to be placed over the disc
and should have the same diameter. The meter should then be removed
so that the material inside the cylinder can be sampled. Material
rooted outside the cylinder should still be sampled up to the edge
of the cylinder.
Dorgeloh et al. 1985 - Calibrating a disc pasture meter to estimate
above-ground standing biomass in Mixed Bushveld, South Africa
45.8 cm diameter base plate over a 180 cm long aluminium rod.
They took 9 measurements with the disc pasture meter per square of
grass clipping and averaged them. To the nearest mm.
Hardy and Mentis 1985 - The relative efficiency of three methods of
estimating herbage mass in veld
All samples were clipped using hand shears. Fifty disc meter
readings were taken in each plot to estimate the mean disc height
per (1 Ha) plot.
Didn't follow the grass sampling technique of Bransby and Tainton
(1977) because they were working on long grass and it was feared
that they would accidentally include material in the sampling
cylinder.
Instead, they laid out a 2 x 2 m quadrat and determined mean
settling height of 9 DPM readings inside the quadrat. The grass of
the whole quadrat was then clipped.
Zambatis et al. 2006 - Re-evaluation of the disc pasture meter
calibration for the Kruger National Park, South Africa
During fieldwork in Kruger, it was found that the DPM frequently
settles above the bulk of the grass leaf mass, at times resulting
in inflated DPM settling heights. This was particularly noticeable
in long grass, maybe because these grasses have highly lignified
culms.
The DPM was originally developed in New Zealand (Phillips and
Clarke, 1971).
The mass of the disc and rod should definitely be known as it
affects calibration, settling height.
Their measurements:
97 cm long tube of 2.7 cm external diameter attached to a 17.5 cm
diameter 0.6 cm thick base-plate, with a disc of 45.8 cm diameter
and 1.5 mm thickness. Total mass of descending parts is 1.5 kg.
Long aluminium rod of 180 cm, 2.2 cm external diameter.
Clipping should be at a height not more than 3 cm above the ground.
Bransby and Tainton (1977) say that the relationship between DPM
height and grass mass is linear.
Used a broken stick regression to show the breakpoint at which the
relationship between DPM height and mean crop mass changes, getting
more flat at height DPM heights.
More links
Phytosphere Gear - Falling plate meter
More images
Information on common pipe sizes in the UK
Pipe Ext. diam. (mm) Int. diam. (mm)
--------------------- ----------------- -----------------
Rainwater downcomer 68.0 63.8
1 1/2" waste pipe 41.4 36.6
1 1/4" waste pipe 35.0 30.0
Large overflow pipe 25.0 21.0
Small overflow pipe 20.0 16.0
My design
- Sleeve pipe internal diameter - >36.6 mm (1.15")
- Sleeve pipe external diameter - 40-41.4 mm
- Sleeve pipe length - 60 cm ?
- Long rod length - 180 cm
- Long rod external diameter - lt36 mm
- Flange internal diameter - 40 mm (FULL FACE FLANGE PN10/PN16BR1
40 mm) (237 g)
- 4 M16X70 bolts for the flange
- Plywood disc diameter - 45.8 cm
- Total weight of free-moving parts - 1.5 kg
