Decagon Devices MAS-1 User manual
MAS-1
4-20 mA Soil Moisture Sensor
Operator’s Manual
Decagon Devices, Inc.
Version: October 22, 2014 — 10:19:59
MAS-1
Decagon Devices, Inc.
2365 NE Hopkins Court
Pullman WA 99163
Phone: 509-332-5600
Fax: 509-332-5158
Website: www.decagon.com
Trademarks
c
2014 Decagon Devices, Inc.
All Rights Reserved
ii
MAS-1 CONTENTS
Contents
1 Introduction 1
1.1 Customer Support . . . . . . . . . . . . . . . . . . . . 1
1.2 About This Manual . . . . . . . . . . . . . . . . . . . 1
1.3 Warranty ......................... 2
1.4 Seller’s Liability . . . . . . . . . . . . . . . . . . . . . . 2
2 About the MAS-1 3
2.1 Specifications ....................... 3
3 Integrating the MAS-1 5
3.1 Wiring........................... 5
3.2 Testing the Sensor . . . . . . . . . . . . . . . . . . . . 8
4 Installing the MAS-1 9
4.1 Procedure......................... 9
4.2 Orientation ........................ 10
4.3 Removing the Sensor . . . . . . . . . . . . . . . . . . . 10
5 Calibration 11
5.1 MineralSoils ....................... 11
5.2 Potting Soil/Peat . . . . . . . . . . . . . . . . . . . . . 11
5.3 RockWool ........................ 12
6 Troubleshooting 13
7 Declaration of Conformity 14
iii
MAS-1 1 INTRODUCTION
1 Introduction
Thank you for choosing the MAS-1 4-20 mA Soil Moisture Sensor.
This innovative sensor enables you to monitor soil moisture accu-
rately and affordable with a standard 2-wire, 4-20 mA analog inter-
face for use with many data acquisition and control systems. The
MAS-1 cannot be used with the standard Decagon data loggers.
1.1 Customer Support
There are several ways to contact Decagon if you ever need assis-
tance with your product, have any questions, or feedback. Decagon
has Customer Service Representatives available to speak with you
Monday through Friday, between 7am and 5pm Pacific time.
Note: If you purchased your sensor through a distributor, please con-
tact them for assistance.
Email:
Phone:
509-332-5600
Fax:
509-332-5158
If contacting us by email or fax, please include as part of your mes-
sage your instrument serial number, your name, address, phone, fax
number, and a description of your problem or question.
1.2 About This Manual
Please read these instructions before operating your sensor to ensure
that it performs to its full potential.
1
1 INTRODUCTION MAS-1
1.3 Warranty
This sensor has a 30-day satisfaction guarantee and a one-year war-
ranty on parts and labor. Your warranty is automatically validated
upon receipt of the instrument.
Note: The one year service plan activates when Decagon ships the
instrument and not at the time of purchase.
1.4 Seller’s Liability
Seller warrants new equipment of its own manufacture against de-
fective workmanship and materials for a period of one year from the
date of receipt of equipment.
Note: We do not consider the results of ordinary wear and tear,
neglect, misuse, or accident as defects.
The Seller’s liability for defective parts shall in no event exceed the
furnishing of replacement parts “freight on board” the factory where
originally manufactured. Material and equipment covered hereby
which is not manufactured by Seller shall be covered only by the
warranty of its manufacturer. Seller shall not be liable to Buyer for
loss, damage or injuries to persons (including death), or to property
or things of whatsoever kind (including, but not without limitation,
loss of anticipated profits), occasioned by or arising out of the instal-
lation, operation, use, misuse, nonuse, repair, or replacement of said
material and equipment, or out of the use of any method or process
for which the same may be employed. The use of this equipment con-
stitutes Buyer’s acceptance of the terms set forth in this warranty.
There are no understandings, representations, or warranties of any
kind, express, implied, statutory or otherwise (including, but with-
out limitation, the implied warranties of merchantability and fitness
for a particular purpose), not expressly set forth herein.
2
MAS-1 2 ABOUT THE MAS-1
2 About the MAS-1
The MAS-1 measures the dielectric constant of the soil in order to
find its volumetric water content. Since the dielectric constant of
water is much higher than that of air or soil minerals, the dielectric
constant of the soil is a sensitive measure of water content. The
MAS-1 supplies a 70 MHz oscillating wave to the sensor prongs that
induces an electromagnetic field in the medium (soil) surrounding
the sensor. The charging and discharging of the sensor is controlled
by the dielectric of the surrounding soil.
A microprocessor on the MAS-1 measures the charging of the sensor,
and therefore the dielectric constant of the soil which is related to
the water content of the soil. The microprocessor makes a dielectric
measurement and updates the transmitted current once per second.
The transmitted 4-20 mA current can be converted to the water con-
tent of the soil using a simple calibration function.
We designed the MAS-1 to be used with standard 4-20 mA con-
trollers and monitoring systems. It cannot be used with Decagon
logging systems. For more information about using Decagon logging
systems please contact Decagon’s customer support representatives.
2.1 Specifications
Electrical
•Interface: Standard 4-20 mA, 2-wire analog transmitter
•Supply voltage: 12 to 32 VDC continuous
•Output current: 4-20 mA
•Overvoltage protection: Yes
•Reverse polarity protection: Yes
•Settling time: 4 seconds
•Wiring:
3
2 ABOUT THE MAS-1 MAS-1
1. Red wire: (+) supply
2. Black wire: (-) output
3. Shield: Not connected
Measurement
•Type: Volumetric water content (VWC)
•Range: 0 to 100% VWC typical
•Resolution: Depends on current measurement (data acquisi-
tion) device
•Accuracy: ±6% VWC with generic calibration for supported
growing media up to 65% VWC, above which accuracy lessens.
Increased accuracy can be achieved with a medium specific cali-
bration. For more information on how to perform your own me-
dia specific calibration, or to have Decagon’s calibration service
perform one for you, visit us online at http://www.decagon.com
•Output: 4-20 mA current proportional to VWC
•Sensor measurement interval: 1 second
•Operating Environment
•Temperature: −40 to 60 ◦C
Sensors can be used at higher temperatures under some condi-
tions. Contact Decagon for more details
Physical Properties
•Dimensions: 8.9 cm x 1.8 cm x 0.7 cm
•Cable: 2 m or 5 m (standard), 3 wire (22 AWG tinned Red and
Black wires, 24 AWG tinned bare wire); (Custom cable length
available upon request)
4
MAS-1 3 INTEGRATING THE MAS-1
3 Integrating the MAS-1
A 4-20 mA system generally consists of a sensor, transmitter, power
supply, and device to read the current being transmitted through
the current loop. The MAS-1 is an integrated sensor and 4-20 mA
transmitter. When the MAS-1 is powered by the Power Supply, it
transmits a current though the loop that is proportional to the soil
dielectric permitting and therefore the soil volumetric water content.
In figure 1, the current loop is shown by the dotted line labeled I=3
to 20 mA the arrows indicate the direction of the current.
Figure 1: 3 to 20mA current loop diagram
The MAS-1 uses a microcontroller to regulate the interval at which it
takes measurements. It takes one second from the time it is powered
up to take its first measurement and transmit current though the
loop. The trasnmitted current will reach a stable value within four
seconds of power up. After the initial four second startup, measure-
ments are taken every one second, while the current in the loop is
continuously maintained. Since the measurement intervals are con-
trolled by the MAS-1 itself, there is no need to pulse the excitation
voltage. A constant supply voltage should be applied in order for
the MAS-1 to function as it is designed
3.1 Wiring
Conventional (PLC)
A programmable Logic Controller (PCC) is typically used to read
5
3 INTEGRATING THE MAS-1 MAS-1
the current transmitted though the MAS-1. The red wire (see Fig-
ure 2) of the MAS-1 is connected to a voltage output terminal that
is able to supply 12 to 32 VDC. The black wire is connected to an
input terminal that accepts a current input ranging from 4 mA to
20 mA. For the MAS-1 to function properly, the voltage drop from
the red to the black lead must be 12 V or greater.
Figure 2: Typical Wiring Connection
Non-Conventional
When using a device, such as a data logger, that does not have
an input capable of measuring current a pickoff resistor can be used
as shown in figure 3. Assuming that the single ended input has an
input impedance, or resistance, much larger that of RV olt, then all
of the current in the 4-20 mA loop passes through RV olt. If the data
logger can measure the voltage drop over the RV olt, then the current
can be calculated as:
I=Vmeasured
RV olt
(1)
where I(mA) is the 4-20 mA current, RV olt (ohms) is the resistance
of the pickoff resistor, and Vmeasured (mV) is the voltage drop over
RV olt.
6
MAS-1 3 INTEGRATING THE MAS-1
Figure 3: Wiring Connection for Devices Without Current
The optional 100 uF capacitor shown in parallel with the RV olt re-
duces measurement noise. It should have a voltage rating higher
than the largest supply voltage. Be sure to observe correct polarity.
The MAS-1 requires a voltage of at least 12 V. This limits the value
of RVolt since part of the total voltage drop will be across the re-
sistor. Equation 2 can be used to determine the maximum value for
RVolt. Table 1 shows time resistance values.
Vsupply −12 = 0.02RV oltM ax (2)
Table 1: Maximum resistance values for RV olt at specified voltages
Supply Voltage Load
13 V 50 ohms
24 V 600 ohms
32 V 1000 ohms
The MAS-1 sensor has several advantages over voltage-output sen-
sors, even for voltage-input data loggers.
1. The MAS-1 supply voltage does not need to be regulated for
the sensor to work properly; it can be any value between 12
and 32 volts, without affecting sensor output.
2. When using a current-based sensor like the MAS-1, the signal
is not affected by electrical resistance in the cable, so the sensor
output is not affected by cable length or wire gauge.
7
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