This document discusses various methods for measuring soil water and flow. It begins by describing common flow measurement devices like orifice plates, venturis, and turbines and their accuracy, rangeability, advantages, and disadvantages. It then covers methods for monitoring soil water content including soil feel and appearance charts, gravimetric sampling to determine water content by weight, and instruments like tensiometers, porous blocks, and neutron probes. The last section provides a brief overview of different types of workshop measuring instruments for tasks like measuring length, angles, surface finish, and deviations.

1. AET 314 INSTRUMENTATION
HND 1 AGRIC & BIO- ENGINEERING
(FARM POWER & SOIL + WATER)
R R Dinrifo, Ph.D
MODULE 1V
Flow Measurement
Soil Water Monitoring & Measurement
Workshop Measuring Instruments:

2. Sensor Rangeability1 Accuracy2 Dynamics Advantages Disadvantages
orifice 3.5:1 2-4% of full span -
-low cost
-extensive industrial
practice
-high pressure loss
-plugging with slurries
venturi 3.5:1 1% of full span -
-lower pressure loss than
orifice
-slurries do not plug
-high cost
-line under 15 cm
flow nozzle 3.5:1 2% full span -
-good for slurry service
-intermediate pressure
loss
-higher cost than orifice
plate
-limited pipe sizes
elbow meter 3:1 5-10% of full span - -low pressure loss -very poor accuracy
annubar 3:1
0.5-1.5% of full
span
-
-low pressure loss
-large pipe diameters
-poor performance with
dirty or sticky fluids
turbine 20:1
0.25% of
measurement
-
-wide rangeability
-good accuracy
-high cost
-strainer needed, especially
for slurries
vortex shedding 10:1
1% of
measurement
-
--insensitive to variations
in density, temperature,
pressure, and viscosity
-expensive
positive
displacement
10:1 or greater
0.5% of
measurement
-
-high reangeability
-good accuracy
-high pressure drop
-damaged by flow surge
Flow Measurement devices

3. Orifice flow meter
venturi
Elbow meter
Pitot tube

4. SOIL WATER MONITORING & MEASUREMENT
• Efficient irrigation requires a systematic water management program.
Such a program answers the questions of when to irrigate, how much
water to apply during irrigation and how best to apply the water (rate
of application, method, etc.).
• Accurate assessment of the soil water-holding characteristics along
with periodic soil water monitoring and measurement are required.
Monitoring and measuring soil water available to irrigated crops is
part of an integrated management package.

5. Among the most common approaches are:
• soil feel and appearance
• gravimetric sampling
• tensiometers
• porous blocks
• neutron scattering

6. Soil texture
Available water
Coarse (sand, loamy
sand)
Moderately coarse
(sandy loam, fine
sandy loam)
Medium (loam) Fine (silt loam, clay loam)
100% (field
capacity)
Leaves wet outline on
hand when squeezed.
(0.0)
Appears very dark,
leaves wet outline
on hand; makes a
short ribbon. (0.0)
Appears very dark,
leaves wet outline on
hand: will ribbon out
about 1". (0.0)
Appears very dark, leaves
slight moisture on hand
when squeezed: will ribbon
out about 2". (0.0)
70%-80%
Appears moist: makes a
weak ball. (0.2-0.3)
Dark: makes a hard
ball. (0.3-0.4)
Quite dark: makes
tight plastic ball:
ribbons out 1/2". (0.4-
0.6)
Quite dark ribbons and slicks
easily: makes plastic ball.
(0.5-0.7)
60%-65%
Appears slightly moist:
forms weak brittle ball.
(0.4)
Fairly dark: makes a
good ball. (0.8)
Fairly dark: forms firm
ball: barely ribbons.
(0.8)
Fairly dark: forms firm ball:
ribbons 1/4"-1/2". (0.9)
50%
Appears dry: forms very
weak ball or will not ball.
(0.5)
Slightly dark: forms
weak ball. (0.8)
Fairly dark: will form
ball: slightly crumbly.
(1.0)
Balls easily: small clods
flatten out rather than
crumble: ribbons slightly.
(1.1-1.2)
35%-40%
Dry: will not ball. (0.6-
0.7)
Light color: will not
ball or forms brittle
balls. (0.9-1.0)
Slightly dark: forms
weak ball: crumbly.
(1.2-1.3)
Slightly dark, forms weak
balls: clods crumble. (1.4-1.5)
Less than 20%
(wilting point)
Very dry: loose, flows
through fingers. (0.8-1.0)
Dry: loose, flows
through fingers.
(1.3-1.6)
Light color: powdery,
dry. (1.6-2.0)
Hard, baked, cracked, light
color. (1.6-2.3)
Soil feel and appearance chart for estimating available soil water.

7. • Soil sampling involves taking soil samples from each of several
desired depths in the root zone and temporarily storing them in
water vapor-proof containers. The samples are then weighed and the
opened containers oven-dried under specified time and temperature
conditions (104°C for 24 hours).
• Percent soil water content on a dry mass or gravimetric basis, Pw, is
determined with the following formula
Pw = [(wet sample weight-dry sample weight) / dry
sample weight] x 100

8. Tensiometry
Soil water tension, soil water
suction, or soil water potential
are all terms describing the
energy status of soil water. Soil
water potential is a measure of
the amount of energy with
which water is held in the soil.
A soil water characteristic or
water release curve shows the
relation between soil water
content and soil water tension

9. Sketch showing typical electrical resistance block installation.
Porous blocks are made of materials
such as gypsum, ceramic, nylon, and
fiberglass. Similar to tensiometers, the
blocks are buried in intimate contact
with the soil at some desired depth and
allowed to come to water tension
equilibrium with the surrounding soil.
Once equilibrium is reached, different
properties of the block which are
affected by its water tension may be
measured.

10. Neutron Scattering
Neutron scattering is a time-tested technique
for measuring total soil water content by
volume. This method estimates the amount of
water in a volume of soil by measuring the
amount of hydrogen present. A neutron probe
consists of a source of fast or high energy
neutrons and a detector, both housed in a unit
which is lowered into an access tube installed
in the soil. The probe is connected by cable to a
control unit which remains on the surface.
Clips on the cable allow the probe to be set at
pre-selected depths in the soil profile. Access
tubes should be installed at least to the depth of
the expected rooting zone. The control unit
includes electronics for time control, a counter,
memory, and other electronics for processing
readings

11. WORKSHOP MEASURING INSTRUMENTS
classification of measuring instruments:
• Length measuring Instruments.
• Direct measuring.
• Indirect measuring.
• Angle measuring Instruments.
• Instruments for surface finish.
• Instruments for deviations.

12. WORKSHOP INSTRUMENTS