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CONTENTS
CHAPTER NO: TITLE PAGE NO
1 PROJECT DESCRIPTION
2.1 Moisture Control for mixer 1 and 2 2
2.1.1 Introduction 2
2.1.2 Existing System 5
2.2 HYDRO mix VII sensors 6
2.2.1 Positioning the sensor 8
2.2.2 Installing the sensor 9
2.3 HYDRO control 10
2.4 Sensor calibration 13
2.5 Moisture Control 15
3 CONCLUSION 18
3. 2
2.1 Moisture Control for mixer 1 and 2
2.1.1 Introduction
A batch plant or batching plant, is a device that combines various ingredients to
form concrete. Some of these inputs include sand, water, aggregate (rocks, gravel,
etc.), fly ash, potash, and cement. A batching plant can have a variety of parts and
accessories, including: mixers(either tilt-up or horizontal or in some cases both),
cement batchers, aggregate batchers, conveyors, radial stackers, aggregate bins,
cement bins, heaters, chillers, cement silos, batch plant controls, and dust collectors
(to minimize environmental pollution). The center of the concrete batching plant is
the mixer. There are three types of mixer: tilt, pan, and twin shaft mixer. The twin
shaft mixer can ensure an even mixture of concrete and large output, while the tilt
mixer offers a consistent mix with much less maintenance labor and cost.
Mixer
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With the growing popularity of specialist concretes such as self-compacting concrete
(SCC), the requirements for information about the raw materials has increased
considerably for manufacturers. The processes used to produce these concretes have
also had to be considered to ensure they allow the correct proportioning, mixing and
placing at the plant or on site. It is now clear to most concrete producers that the
need for adequate quality control is much more critical with SCC than in the case of
conventional concretes. Further, the production of SCC’s requires greater
competence from those involved as well as good control of materials and
equipment’s used for production.
In a time when most organizations are looking to maximize profits while reining in
costs and maintaining quality, it is no longer economical to overlook the benefits
that moisture measurement can bring to the manufacturers of concrete, both ready-
mix and precast.
One of the major sources of inconsistency in concrete production is the ever varying
moisture content of the sand and aggregates. Water content changes can originate
from different parts of the concrete production process mainly due to:
Natural moisture content variation in aggregates.
Inaccuracy of water feeding system into the concrete mixer.
Lack of moisture control in production operation.
Inadequate water set points.
Dirty moisture probe.
Uncovered material transport or storage systems allow increments of water
content by rainfall or their reduction by evaporation
The most significant source of moisture variation is the natural aggregate moisture
content. A variation of 1 percent moisture content in a dry aggregate by weight
results in a change in 10 kg of aggregate loaded into the mixer for every 1000 kg of
dry aggregate weighed.
Therefore, the greater the variation of moisture, and the greater the weight of
aggregate used, the more serious this condition becomes. Water content in
aggregates can be as high as 16 percent, which has large repercussions to the
economics of trading in this material.
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This engineering note explains moisture control methods for batching systems. A
batching system usually consists of a number of raw material silos or hoppers and a
mixer combining these materials to create a final product. Here the final products
obtained are concrete pavers.
Both mixer 1 and mixer 2 are used to batch different recipes of batch products. All
these recipes varies by the pigment added. The different pigment added can be Red,
Yellow, Black or Tan.
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2.1.2 Existing System
The Existing system consist of two different Mixers, Mixer 1 and
Mixer2 which are used for the batching process. The end product being
concrete pavers.
Each mixer combines a calculated amount of aggregates and cement and
a fixed amount of water and pigments as per calculation and mixes them
up. The end product is pressed to make pavers.
However the existing system had no adequate moisture control methods
used due to which the product developed was either too wet or too dry.
The prewet and Final water setpoints were made by past corrections.
This method was inadequate and resulted in a bad production of pavers.
Even moisture from the aggregates were not considered creating a
difference in the expected results.
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2.2 HYDRO MIX VII sensors
Introduction
The Hydro-Mix VII digital microwave moisture sensor with integral signal
processing provides a linear output (both analogue and digital). The sensor may be
easily connected to any control system and is ideally suited to measure the
moisture of materials in mixer applications as well as other process control
environments.
The sensor reads at 25 times per second, which enables rapid detection of changes
in moisture content in the process, including determination of homogeneity. The
sensor may be configured remotely when connected to a PC using dedicated
Hydronix software. A large number of parameters are selectable, such as the type
of output and the filtering characteristics.
The sensor is constructed to operate under the most arduous conditions with a wear
life of many years. The Hydro-Mix VII should never be subjected to unnecessary
impact damage as it houses sensitive electronics. In particular, the replaceable
ceramic faceplate, although extremely hardwearing, is brittle and may crack if
subjected to severe impact.
Suitable applications
The Hydro-Mix VII microwave moisture measuring sensor may be successfully
used in the following applications:
• Static pan mixers
• Planetary mixers
• Turbo mixers
• Single and twin shaft horizontal mixers
• Ribbon mixers
• Flush mounted in chutes or similar applications
Sensor connection and configuration
As with other Hydronix digital microwave sensors, the Hydro-Mix VII may be
remotely configured using a digital serial connection and a PC running Hydro-Com
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sensor configuration and calibration software. For communication with a PC,
Hydronix supply RS232-485 converters and a USB Sensor Interface.
There are three basic configurations by which the Hydro-Mix VII can be connected
to a mixer control system:
• Analogue output – A DC output is configurable to:
• 4-20 mA
• 0-20 mA
• 0-10 V output can be achieved using the 500 Ohm resistor supplied with the
sensor cable.
• Digital control – an RS485 serial interface permits direct exchange of data and
control information between the sensor and the plant control computer or Hydro-
Control system. USB and Ethernet adapter options are also available
• Compatibility mode - this is a legacy mode which allows a Hydro-Mix VII to
connect to a Hydro-Control IV or Hydro-View unit.
The sensor may be configured to output a linear value of between 0-100 unscaled
units with the recipe calibration being performed in the control system.
Alternatively it is also possible to internally calibrate the sensor to output a real
moisture value.
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2.2.1 Positioning the sensor
The sensor may be installed into many types of mixer or application.
In most cases the sensor will work exceptionally well with the standard filtering
parameters. Some mixer types and certain applications may require further
adjustments to the internal filtering parameters of the sensor.
For installations in flat surfaces, the top of the sensor must be flush with the floor
of the mixer.
When installing the sensor in curved surfaces, ensure that the centre of the ceramic
is flush with the radius of the mixer wall.
In all installations, it is recommended that the sensor is fitted in an area where it is
away from any possible collection of ‘sitting’ water. It is also necessary to monitor
the position of the sensor over time as the mixer floor wears, and adjust the sensor
as necessary to maintain the recommendations above. This is usually best done as
part of the standard maintenance procedure at the site where the sensor is installed.
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2.2.2 Installing the sensor
The Hydro-Mix VII is fitted to the mixer using a Fixing Plate (part no 0021)
welded to the permanent floor or side wall of the mixer and the Adjustable Clamp
Ring assembly (part no 0033) which is supplied with the sensor.
The Adjustable Clamp Ring Assembly facilitates the correct positioning and
subsequent height adjustment of the sensor.
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2.3 HYDRO- CONTROL
Hydro-Control V is an easy to use control system for controlling water addition
during concrete mixing. In conjunction with the Hydronix Hydro-Mix V/VI or
Hydro-Probe Orbiter it is designed to precisely achieve the required moisture target
without the need for metering the water, although use of a water meter is preferred.
From software version HS0035 v. 4.20, the Hydro-Control V has temperature
compensation which enables the control of water addition to achieve the required
consistency throughout the year, regardless of variation in material temperature.
It is simple to install and can be fitted to both new and existing plants. It uses the
latest Hitachi H8 microprocessor with SMD technology to achieve a compact and
reliable unit.
The Hydro-Control V can be connected to a batch controller via an RS232 serial
link to allow transfer of mix cycle information and remote recipe selection. The
RS232 port is also used to send software upgrades from a service computer.
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Preset Mode
A fixed amount of water defined by the recipe is added during both the pre-wet (if
required) and final-wet phases of the mix cycle regardless of the current moisture
reading, this mode can also be operated without a sensor being connected.
Auto Mode
An amount of water defined by the recipe is added during the pre-wet phase (if
required) and the sensor moisture reading is used to control the water addition up
to a target defined by the selected recipe during thefinal-wet phase of the mix.
Calc Mode
An amount of water is added during the pre-wet (if required) and then the system
calculates the amount of water to add during the final-wet phase from a ‘Calculated
Moisture Target’ and recipe Dry weight parameter.
The calibration method used in the Hydro-Control IV requires that the operator
chooses an optimum mix from the mix log display. The results recorded by the
Hydro-Control during this mix cycle are then transferred to the relevant recipe
when the ‘Water OK’ button is pressed, thereby providing a template of the Hydro-
Mix sensor response for future mixes.
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This approach has the main advantage of being easy to use - the operator uses
his/her judgement to identify a ‘good’ mix and simply identifies this to the system.
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2.4 SENSOR CALIBRATION
Calibration defines the relationship between the raw output (‘Unscaled’) from a
Hydronix moisture sensor to actual moisture in the material.
Every material has different electrical properties, therefore it is not possible to
produce a sensor that can measure true moisture for all materials without having to
be calibrated.
The raw ‘Unscaled’ output from a Hydronix sensor increases linearly with moisture
in the material.
This linear relationship is shown below, this is a straight line graph. The line is
unique for each material. Once correctly calibrated any moisture value can be
determined from any unscaled reading.
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2.5 MOISTURE CONTROL
Although there are lot of other variables effecting the final mix moisture, final set
points for probes in mixture 1 and 2 have a major contribute. The first step is to
calculate the calibration coefficients. This was achieved using the equations
provided in Hydronix manual.
V = Total amount of water in the mix (lit)
W Dry = Total mix dry weight (Kg)
US Wet (A) = Probe reading shown on the batch computer before the prewet is
added
US Dry (A) = Probe reading shown on the batch computer at the time of discharge.
US Wet = Calculated probe reading to be maintained at discharge.
All the above values can be obtained from the batch panel except for WDry which
can only be obtained by knowing the moisture content in the aggregates before
every batch and subtracting the amount of water present.
M and Grad values calculated for various batches during the last week can be seen
in detail below.
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According to the information provided in Hydronix manual, calibration coefficient
(Grade) is a constant. That means it is a fixed value for each probe which can be
used to calculate the final probe reading at discharge using which an ideal total mix
moisture can be maintained. For example, take the tabulated values of mixture 2 and
plot a graph with Grad on X axis and Total Moisture on Y axis. From this graph the
correct value of Grad can be obtained which in this case for mixture 2 is
(0.012763979). Now take the values from first row in table 2 and calculate the final
set point by using the equations provided below.
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M = 0.0667381498188272
Grad = 0.0127639787765828
US Dry (A) = 36.9
US Wet (A) = 50.5
US Wet = US Dry (A) + (M)/(Grad) = 42.13
This should be the probe reading at discharge in order to achieve total mix
moisture as 6.5 %. This same equation was used to calculate the final set point for
all the readings in table 2 which can be seen below.
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3. Conclusion
Moisture calibration was carried out and the corresponding values for probe
readings were found out. Recommended final probe readings at discharge for
mixture 1 is 31.23 and for mixture 2 is 41.72. These values are only effected when
the empty probe reading changes.
Thereby both the mixers are made to run in the above preset values.
