Geokon 3810A User manual

©2019, GEOKON. All rights reserved.
Document Revision: E| Release date: 3/18/19
Model 3810A
Addressable Thermistor String
Instruction Manual

WARRANTY STATEMENT
GEOKON warrants its products to be free of defects in materials and workmanship,
under normal use and service for a period of 13 months from date of purchase. If the
unit should malfunction, it must be returned to the factory for evaluation, freight
prepaid. Upon examination by GEOKON, if the unit is found to be defective, it will be
repaired or replaced at no charge. However, the WARRANTY IS VOID if the unit shows
evidence of having been tampered with or shows evidence of being damaged as a
result of excessive corrosion or current, heat, moisture or vibration, improper specifi-
cation, misapplication, misuse or other operating conditions outside of GEOKON’s
control. Components that wear or are damaged by misuse are not warranted. This
includes fuses and batteries.
GEOKON manufactures scientific instruments whose misuse is potentially dangerous.
The instruments are intended to be installed and used only by qualified personnel.
There are no warranties except as stated herein. There are no other warranties,
expressed or implied, including but not limited to the implied warranties of merchant-
ability and of fitness for a particular purpose. GEOKON is not responsible for any
damages or losses caused to other equipment, whether direct, indirect, incidental,
special or consequential which the purchaser may experience as a result of the instal-
lation or use of the product. The buyer’s sole remedy for any breach of this agreement
by GEOKON or any breach of any warranty by GEOKON shall not exceed the purchase
price paid by the purchaser to GEOKON for the unit or units, or equipment directly
affected by such breach. Under no circumstances will GEOKON reimburse the
claimant for loss incurred in removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or
software, however, GEOKON neither assumes responsibility for any omissions or
errors that may appear nor assumes liability for any damages or losses that result
from the use of the products in accordance with the information contained in the
manual or software.
No part of this instruction manual may be reproduced, by any means, without the written consent of GEOKON. The
information contained herein is believed to be accurate and reliable. However, GEOKON assumes no responsibility
for errors, omissions or misinterpretation. The information herein is subject to change without notification.
The GEOKON® wordmark and logo are registered trademarks with the United States Patent and Trademark Office.

I
TABLE OF CONTENTS
1. INTRODUCTION............................................................................................................................................1
2. INSTALLATION..............................................................................................................................................2
2.1 THERMISTOR STRING ....................................................................................................................2
2.2 HARDWARE REQUIREMENTS..................................................................................................2
2.3 TTL / USB TO RS-485 CONVERTER.....................................................................................2
3. MODBUS RTU PROTOCOL ..............................................................................................................3
3.1 INTRODUCTION TO MODBUS..................................................................................................3
3.2 MODBUS RTU OVERVIEW...........................................................................................................3
3.3 MODBUS TABLES ...............................................................................................................................3
3.4 READING 3810A SENSORS.......................................................................................................3
4. MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS...................................5
4.1 DESCRIPTION .........................................................................................................................................5
4.2 WIRING.........................................................................................................................................................5
4.3 SAMPLE CR6 PROGRAM..............................................................................................................5
4.4 SAMPLE CR1000 PROGRAM WITH REQUIRED MODEL 8020-38
ADDRESSABLE BUS CONVERTER .......................................................................................6
5. TEMPERATURE CALCULATION.................................................................................................7
5.1 STEINHART-HART COEFFICIENTS.......................................................................................7
5.2 FLOATING POINT RESISTANCE READINGS (PREFERRED METHOD)..7
5.3 RAW ADC (ANALOG TO DIGITAL CONVERTER) READINGS ........................7
6. SOFTWARE ........................................................................................................................................................8
APPENDIX A. SPECIFICATIONS ......................................................................................................9
A.1 POWER .........................................................................................................................................................9
A.2 COMMUNICATION .............................................................................................................................9
A.3 MECHANICAL .........................................................................................................................................9
A.4 MEASUREMENTS ..............................................................................................................................9

II
FIGURES
FIGURE 1: MODEL 3810A ADDRESSABLE THERMISTOR STRING ................................1
FIGURE 2: MODEL 3810A ADDRESSABLE THERMISTOR ..............................................1
FIGURE 3: MODEL 3810A-3 END TERMINATOR ............................................................2
FIGURE 4: MODEL 8020-38 ADDRESSABLE BUS CONVERTER ....................................2
FIGURE 5: PC TERMINAL PROGRAM SCREEN CAPTURE ..............................................4

III
TABLES
TABLE 1: 3810A WIRING .................................................................................................... 2
TABLE 2: RAM STORAGE .................................................................................................... 3
TABLE 3: EXAMPLE QUERY - TRIGGER MEASUREMENT ............................................... 3
TABLE 4: EXAMPLE QUERY - FLOATING POINT RESISTANCE READING ..................... 4
TABLE 5: EXAMPLE RESPONSE - FLOATING POINT RESISTANCE READING .............. 4
TABLE 6: DATALOGGERS WITH RS-485 CONVERSION .................................................. 5
TABLE 7: DATALOGGERS WITHOUT RS-485 CONVERSION........................................... 5
TABLE 8: 3810A THERMISTOR STRING TO RS-485 TO TTL CONVERTER WIRING....... 5

IV

MODEL 3810A ADDRESSABLE THERMISTOR STRING | INTRODUCTION | 1
1. INTRODUCTION
Model 3810A thermistor strings are temperature sensors that combine NTC
thermistors with digital Modbus RTU communication for ease of setup and use.
Each thermistor sensor contains a small printed circuit board assembly (PCBA)
with a thermistor and circuitry for communication and analog to digital
conversion. These sensors are assembled with cable using customer-specified
spacing. Then each sensor is encapsulated in a protective housing, creating a
finished thermistor string.
Thermistors are semiconductors that behave as resistors with a high (usually
negative) temperature coefficient of resistance: they are thermal resistors. The
thermistor beads are made from a mixture of metal oxides encased in epoxy or
glass. The beads are small in size and extremely robust, with a high degree of
stability over a long life span. The accuracy and interchangeability of the
thermistor bead used on the 3810A is ±0.07 °C, within the temperature range of
0 °C to 50 °C.
The cable consists of two twisted pairs, each wrapped with aluminized Mylar foil
and a single drain wire. There are four conductors: power, ground, and the two
differential RS-485 lines for communication. A short-circuit between any of the
four conductors will render all sensors in the string inoperable. Likewise, a break
in the cable would render inoperable all sensors below the break. However,
because the 3810A uses a bus topology, failure of any single thermistor sensor
does not affect any of the other sensors.
The sensors are polled individually via industry standard Modbus RTU (Remote
Terminal Unit) commands.
FIGURE 1: Model 3810A Addressable Thermistor String
FIGURE 2: Model 3810A Addressable Thermistor

2| INSTALLATION | GEOKON
2. INSTALLATION
2.1 THERMISTOR STRING
Uncoil the thermistor string and place it in the installation medium. For
installations in which the string will be in contact with soil, rock, etc., take care
to prevent damage to the cable and sensor. The termination assembly at the end
of the string is made with a 1/4-20 x 1/2” deep threaded hole in it. This allows for
a weight or cord/rope to be attached for assisting with the installation. The
maximum pulling force that can safely be applied to the string is 20 lb.
FIGURE 3: Model 3810A-3 End Terminator
Table 1 shows the function of each of the conductors of the instrument cable.
TABLE 1: 3810A Wiring
2.2 HARDWARE REQUIREMENTS
Communications: RS-485, half-duplex
Baud Rate: 115200
Power: 12.0V ± 20% @ 1.2mA/sensor
2.3 TTL / USB TO RS-485 CONVERTER
GEOKON manufactures the Model 8020-38 Addressable Bus Converter for
connecting addressable strings to personal computers, readouts, dataloggers,
and programmable logic controllers. The converter acts as a bridge between
USB/TTL-capable readers and the GEOKON RS-485-enabled sensor strings.
FIGURE 4: Model 8020-38 Addressable Bus Converter
For more information, please refer to the Model 8020-38 Addressable Bus
Converter Instruction Manual.
3810A String Description
Red String Power
White Communication RS-485+
Green Communication RS-485-
Black Ground
Shield Analog Ground

MODEL 3810A ADDRESSABLE THERMISTOR STRING | MODBUS RTU PROTOCOL | 3
3. MODBUS RTU PROTOCOL
3.1 INTRODUCTION TO MODBUS
Model 3810A Addressable In-Place Inclinometers use the industry standard
Modbus Remote Terminal Unit (RTU) protocol to communicate with the chosen
readout method. Modbus was designed to work on what is known as a bus
network, meaning that every device receives every message that passes across
the network. The Modbus standard does not specify a physical layer (connection
type), but it will work with any interface that can communicate asynchronously
with multiple devices (e.g., RS-485, RS-422, optical, radio, etc.). Model 3810A
strings use RS-485 as the electrical interface because of its prevalence,
simplicity, and success as a robust, industrial physical layer.
3.2 MODBUS RTU OVERVIEW
The Modbus RTU protocol uses packets (multiple sections that together
compose a message) to communicate and transfer data between devices on the
network. The general format of these packets is as follows:
1. Modbus Address (one byte) – the address of the specific device on the bus.
(Labeled on the sensors as #1, #2, #3, etc.)
2. Function Code (one byte) – the action to be carried out by the slave device.
3. Data (multi-byte) – the payload of the function code being sent.
4. CRC (two bytes) – cyclic redundancy check; a 16-bit data integrity check
calculated over the other byes in the packet.
3.3 MODBUS TABLES
The most recent sensor reading is stored in a table. The reading is presented in
different formats in different sections of the table. The register location and size
of these variables are described in the table below.
TABLE 2: RAM Storage
3.4 READING 3810A SENSORS
While Modbus RTU supports roughly 20 different function codes, the simple
functionality of a bused temperature sensor, such as the GEOKON 3810A,
eliminates the need for all but two of them. Only the Write Holding Register
(0x06) function code and Read Holding Registers (0x03) and are needed to read
the sensors.
First, use the Write Holding Register function to refresh the temperature
measurement. The command initiates multiple analog-to-digital (ADC)
conversions that are averaged to optimize precision and resolution. The updated
measurements will be ready approximately 0.25 seconds after the request. An
example request is shown in the table below.
TABLE 3: Example Query - Trigger Measurement
Variable Type (Modbus Message) HEX Decimal Description
ADC uint16 0x0100 256 Raw 16-bit thermistor reading
IC Temp. int16 0x0101 257 Low-quality 10-bit temperature reading
Resistance float32 0x0102
0x0103
258
259
Decimal thermistor reading (Ohms)
Trigger uint16 0x0118 280 Write nonzero value to initiate measurement
Device Address Function Code Data Address Register Value CRC
HEX16 02 06 0118 0001 C9C2
DEC10 2 6 280 1 51,650

4| MODBUS RTU PROTOCOL | GEOKON
Use device address 0 to “broadcast” the trigger command to all drops
simultaneously. Measurements between successive address are staggered by
50 milliseconds to prevent excess current draw from simultaneous activation.
The Read Holding Registers function can be used to read one or more 16-bit
registers (two bytes each), starting at the Data Address that was sent in the
command packet. Temperatures can be retrieved in two alternate formats; one
as a raw 16-bit ADC value, the other as a 32-bit floating point (decimal) number
that is the computed resistance of the on-board thermistor. For ease of use and
program simplicity, the 32-bit floating-point reading is recommended. An
example of this query and response is shown in the tables below.
TABLE 4: Example Query - Floating Point Resistance Reading
TABLE 5: Example Response - Floating Point Resistance Reading
FIGURE 5: PC Terminal Program Screen Capture
The table above shows the IEEE-754 floating-point response as two parts—each
one is composed of two bytes. Because of how this information is stored in
memory, the two parts are received in reverse order; the complete floating-point
number in HEX is 0x4628C87C (10,802.12 Ohms).
Device Address Function Code Data Address Number of Registers CRC
HEX16 02 03 0102 0002 6404
DEC10 2 3 258 2 25,604
Device Address Function Code Byte Count Lower 16-bits Upper 16-bits CRC
HEX16 02 03 04 C87C 4628 04F5
DEC10 2 3 4 floating point result 1,269

MODEL 3810A ADDRESSABLE THERMISTOR STRING | MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS | 5
4. MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS
4.1 DESCRIPTION
Model 3810A sensor strings can be easily read using Campbell dataloggers that
support ModBus RTU commands. Campbell dataloggers such as the CR6 and
CR1000X can read half-duplex RS-485 on select RS-485 digital I/O pairs. This
allows the 3810A string to be connected directly at the datalogger.
The Model 8020-38 Addressable Bus Converter enables Model 3810A sensor
strings to work with Campbell dataloggers that do not support direct RS-485
communications, such as the CR800 and CR1000. The Model 8020-38 is a RS-
485 to TTL converter, used with select TTL digital I/O pairs.
CRBasic is the programming language used with Campbell Scientific CRBasic
dataloggers. Campbell Scientific's LoggerNet software is typically used when
programming in CRBasic.
4.2 WIRING
TABLE 6: Dataloggers with RS-485 Conversion
TABLE 7: Dataloggers without RS-485 Conversion
TABLE 8: 3810A Thermistor String to RS-485 to TTL Converter Wiring
4.3 SAMPLE CR6 PROGRAM
The following sample program reads one 3810A string with five addressable
thermistors. The string in this example communicates with the CR6 through the
control ports C1 (green) and C2 (white), which are set up as ComC1:
'Constants used in Steinhart-Hart equation to calculate sensor temperature for
'10k thermistor
'Define Data Tables
DataTable (Test, 1,-1)
Sample (5,Celsius(),IEEE4) 'Sample Celsius for 5 sensors in string
EndTable
Campbell Dataloggers with RS-485 3810A String Description
12V Red Thermistor String Power
Digital I/O RS-485 Port TX Green Communication RS-485-
Digital I/O RS-485 Port RX White Communication RS-485+
G Black Ground
AG Shield Analog Ground
Campbell Dataloggers w/o RS-485 RS-485 to TTL Converter Description
12V V IN (12V) Converter Power
Digital I/O RS-232 Port TX TX Transmit
Digital I/O RS-232 Port RX RX Receive
G GND Ground
AG Shield Analog Ground
3810A String RS-485 to TTL Converter Description
Red V OUT Thermistor String Power
White 485+ Communication RS-485+
Green 485- Communication RS-485-
Black GND Ground
Shield Shield Analog Ground
Const A = 1.128706256E-3
Const B = 2.342327483E-4
Const C = 0.8707279757E-7
Public ErrorCode 'Result of ModBusMaster communications attempt
PublicResAsFloat 'Resistance of thermistor must be stored as Type Float
Public Celsius(5) 'Calculated Celsius for 5 sensors in string
Public Count 'Counter to increment through temperature sensors

6| MODBUS & CAMPBELL SCIENTIFIC DATALOGGERS | GEOKON
'Main Program
BeginProg
'Open COM port with RS-485 communications at 115200 baud rate
SerialOpen (ComC1,115200,16,0,50,3)
'Read all 5 sensors in string every 4 seconds
Scan (4,Sec,0,0)
'Loop to read each thermistor
For Count = 1 To 5
'Reset temporary storage for next reading
Res = 0
'Flush Serial between readings
SerialFlush (ComC1)
'Write to register 0x118 to trigger thermistor string
'NOTE: ModbusMaster won't send 0x118 unless "&H119" is entered
ModbusMaster (ErrorCode,ComC1,115200,Count,6,1,&H119,1,1,50,0)
'Delay after triggering the measurement
Delay (1,1,Sec)
'Use Modbus command to retrieve resistance from thermistor string
ModbusMaster (ErrorCode,ComC1,115200,Count,3,Res,&H103,1,1,50,0)
'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart equation
Celsius(Count) = 1/(A+B* LN(Res) + C * LN(Res)^3)-273.15
Next
'Call Table to store Data
CallTable Test
NextScan
EndProg
4.4 SAMPLE CR1000 PROGRAM WITH REQUIRED MODEL 8020-38
ADDRESSABLE BUS CONVERTER
The following sample program reads one 3810A string with five addressable
thermistor sensors. The string in this example is connected to a RS-485 to TTL
converter and communicates with the CR1000 through the control ports C1 and
C2, which are setup as COM1:
'Constants used in Steinhart-Hart equation to calculate sensor temperature for
'10k thermistor
'Define Data Tables
'Main Program
Const A = 1.128706256E-3
Const B = 2.342327483E-4
Const C = 0.8707279757E-7
Public ErrorCode 'Result of ModBusMaster communications attempt
Public Resistance As Float 'Resistance of thermistor must be stored as Type Float
Public Celsius(5) 'Calculated Celsius for 5 sensors in string
Public Count 'Counter to increment through temperature sensors
DataTable(Test,1,-1)
Sample (5,Celsius(),IEEE4) 'Sample Celsius for 5 sensors in string
EndTable
BeginProg
'Open COM port with TTL communications at 115200 baud rate
SerialOpen (Com1,115200,16,0,50)
'Read all 5 sensors in string every 4 seconds
Scan (4,Sec,0,0)
'Loop to read each thermistor
For Count = 1 To 5
'Reset temporary storage for next reading
Resistance = 0
'Flush Serial between readings
SerialFlush (Com1)
'Write to register 0x118 to trigger thermistor string
'NOTE: ModbusMaster won't send 0x118 unless "&H119" is entered
ModbusMaster (ErrorCode,Com1,115200,Count,6,1,&H119,1,1,50,0)
'Delay after triggering the measurement
Delay (1,1,Sec)
'Use Modbus command to retrieve resistance from thermistor string
ModbusMaster (ErrorCode,Com1,115200,Count,3,Resistance,&H103,1,1,50,0)
'Calculate thermistor temperature from ohms to Celsius using Steinhart-hart equation
Celsius(Count) = 1/(A+B* LN(Resistance) + C * LN(Resistance)^3)-273.15
Next
'Call Table to store data
CallTable Test
NextScan
EndProg

MODEL 3810A ADDRESSABLE THERMISTOR STRING | TEMPERATURE CALCULATION | 7
5. TEMPERATURE CALCULATION
Thermistors can be read using a GEOKON Micro 1000, Micro 800, or 8600
Datalogger, or any PC that has a USB port and GEOKON's free 3810A Utility
program.
5.1 STEINHART-HART COEFFICIENTS
A = 1.128706256 ×10-3
B = 2.342327483 × 10-4
C = 0.8707279757 × 10-7
5.2 FLOATING POINT RESISTANCE READINGS (PREFERRED
METHOD)
R= Resistance reading (from register address 0x0102)
5.3 RAW ADC (ANALOG TO DIGITAL CONVERTER) READINGS
Resistance can be found by:
R= ADC reading (from register address 0x0100)
And temperature is:
Using an example ADC reading from address 0x0100 of 0x7984 (31,108):

8| SOFTWARE | GEOKON
6. SOFTWARE
Model 3810A thermistor strings can be read by GEOKON's “Addressable String
Reader” software. You can download this software from the GEOKON website:
http://www.geokon.com/Software
The Addressable String Reader application (ASR) is designed to help the user
quickly connect to and read a GEOKON addressable string. The ASR will
automatically detect the type of string connected and will configure the display
accordingly. The three types of strings currently supported are the Addressable
Thermistor (3810A) String, the Addressable VW (Vibrating Wire) (8020-30)
Sensor String, and the Addressable MEMS (6150E and 6151E) String.
The ASR contains the following features:
Communication via Windows COM Ports
Selection of all drops, individual drops, or a combination of drops
Recording of data in rows either by button click or cyclic, at selectable
intervals
Recorded data rows are viewable in list form
Recorded data rows can be saved in CSV (comma-separated values) format
For more information, run the program and click the blue button to display
the software’s Online Help.

MODEL 3810A ADDRESSABLE THERMISTOR STRING | SPECIFICATIONS | 9
APPENDIX A. SPECIFICATIONS
A.1 POWER
A.2 COMMUNICATION
A.3 MECHANICAL
A.4 MEASUREMENTS
Power Supply: 5VDC to 15VDC
Current Per Sensor
During reading:
Idle:
8.7 mA (read)
1.0 mA (idle)
Interface: RS-485, half-duplex
Baud Rate: 115,200 bits/second
Query Response Time: 0.266 seconds (trigger write to values ready)
Min Spacing: 200 mm / 8”
Housing: Delrin / PVC
Length X Diameter: 140 x 22 mm / 5.5 x 7/8”
Working Load: 9.07 kg / 20 lbs
Breaking Strength: 22.68 kg / 50 lbs
Pressure Rating: 150 psig / 1 MPa
Sensors
Wire:
Wireless:
248 maximum
30 maximum
ADC Resolution: 16-bit
System Resolution (0 °C to 50 °C): 0.002 °C (nonlinear; largest step)
System Resolution (-20 °C to 80 °C): 0.005 °C (nonlinear; largest step)
System Repeatability (-20 °C to 80 °C): 0.002 °C (99% confidence interval)
Thermistor Type: 10KΩ NTC
Thermistor Interchangeability: ±0.07 °C
Range: -20 °C to 80 °C
Accuracy: ±0.35 °C @ -20 to -10 °C
±0.15 °C @ -10 to 0 °C
±0.07 °C @ 0 to 50 °C
±0.12 °C @ 50 to 65 °C
±0.16 °C @ 65 to 80 °C

GEOKON
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Lebanon, New Hampshire
03766, USA
Phone: [phone]
Email: [email protected]
Website:www.geokon.com
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