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At CAS DataLoggers, we have the ideal monitoring solution for recording temperature, humidity, current/voltage, or just about any value. Learn about the benefits and specifications of our data loggers and real time data acquisition systems. Record your data at high accuracy and store it on large internal memory, setup alarms with email, text and phone alerts, and send data wirelessly for remote viewing. CAS DataLoggers is ready to meet any need for our datalogging solutions, data acquisition devices and specialized software. Read more on our General News page.

T&D RTR-500GSM Basic GSM Modem Setup Guide

Enable Remote Monitoring with T&D Cellular Base Station
CHESTERLAND OH—January 26, 2012

The RTR-500GSM is a Mobile Base Station with built-in wireless communication via GSM phone network capabilities. It can be used as a Base Unit for T&D’s popular RTR-500 wireless datalogger series, and when in communication range for GSM cellular phone service, the device can transmit data and warnings in remote locations where a LAN connection is impossible, making it ideal for tracking data while in transport. Warning reports can also be sent via e-mail or SMS to specified e-mail addresses, enabling the user to always stay appraised of changes in measurement readings. Read the entire article on our T&D tutorials page.

Strain Gauge Measurements With Current

Utilizing the Bestselling dataTaker Line of Data Loggers

CHESTERLAND OH—August 31, 2011

Wheatstone bridge circuits are extensively used for measuring the output of strain gauges and for measuring other sensor outputs where a relatively small change in resistance must be detected. Bridge circuits have the advantage of high measurement sensitivity and also provide a significant degree of temperature compensation. dataTaker data loggers provide comprehensive support for the measurement of Wheatstone bridge circuits in full, half and quarter bridge configurations. The dataTaker supports two methods of Wheatstone bridge measurement:

• excitation of sensors using a constant current source
• excitation of sensors using a voltage source

Both of these methods of bridge support can have a number of options, depending the number of active arms in the bridge and the number of wires used to connect bridges to the dataTaker datalogger.

Constant Current Excitation of Bridges
The constant current excitation method of bridge measurement utilizes a current of 2.500 mA or 200.0 μA for excitation. The advantage of using this method is that the bridge sensitivity and zero offset is independent of the length of leads used to connect the bridge to the dataTaker, so this method is particularly useful if long lead wires (25 feet or greater) are used to connect the logger to the sensor. In some cases the bridge output can have greater linearity and reduced temperature sensitivity for constant current excitation than for voltage excitation.

The bridge excitation current is supplied between the Excite (*) terminal and the Return (#) terminal of the analog input channel during measurement. Note that the current is only applied for approximately 50 milliseconds so that it will be difficult to measure using a standard meter. The current is applied for approximately 10 milliseconds before the bridge voltage measurement is initiated. In the case of long cables which might have significant capacitance, this delay can be increased using the MDn channel option where n is the delay time in milliseconds. By default, the bridge excitation current is 2.500 mA, but may be set for 200.0 μA if required. The 2.500 mA current provides the best resolution for typical 350 ohm strain gauges, but the smaller current can be used if the strain gauge resistance is greater than 700 ohms or if self-heating of the bridge sensor is a concern. If the 200.0 μA excitation current is required, then this is specified as a channel option in the channel specification using the I channel option.

The Arm Resistance
The data that is returned by the data logger from bridge circuits excited with a constant current is the ratio of the change in arm resistance to the nominal arm resistance. The data is returned in units of ppm (see below). The arm resistance for the bridge being measured must be known by the dataTaker, and is specified as a channel option in the channel specification. The default arm resistance is 350 Ohm, which is typical for many types of strain gauges. For higher accuracy, if the actual arm resistance is available from the data sheet or if the bridge resistance is not 350 ohms, the arm resistance can be specified as a channel option Ohms.

Full Bridge with Constant Current Excitation
The dataTaker can provide excitation and measure the output from a full bridge of devices such as strain gauges, pressure cells, load cells, etc. This configuration is a 4 wire input, and supports 1, 2 or 4 active arms. Any of the bridge arms can be active arms. The configuration also provides compensation for cable wire resistance, allowing long cable wires to be used. Bridge arms which are not active must have bridge completion resistances. These can be an inactive device of the same type as the devices on the active arms, or these can be a resistor with the same resistance value as the active devices at rest, and ideally have a temperature coefficient that is similar to that of the active devices. The entire bridge circuit is external to the dataTaker – the logger does not provide any bridge completion for partial bridges. The full bridge configuration with constant current excitation is connected to the dataTaker as a 4 wire input:

Full Bridge with Constant Current Excitation
where the Excite terminal provides current excitation of 200.0 μA or2.500 mA, which returns via the Analog Return (#) terminal. The bridge output is read between the + and – terminals.

Interpreting the Data from a Full Bridge with Current Excitation
The data returned from full bridges is the ratio of change in measured resistance to the arm resistance, expressed in parts per million.

Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned data from units of ppm to units of Microstrain.

Half Bridge with Constant Current Excitation
The dataTaker can also provide excitation and measure the output from a half bridge configuration of strain gauges, pressure cells, etc. This configuration is a 3 wire input which supports 2 active arms. This configuration compensates for cable wire resistance and temperature difference. The half bridge configuration with constant current excitation is connected to the dataTaker as a 3 wire input.

In this case, Ra and Rc are the 2 active arms of the half bridge. Note that a jumper is required between * and + terminals. As before, the result of the measurement is returned in ppm which is the effective change in resistance of the bridge multiplied by 106. When using stain gauges in half bridges and current, it may be desirable to convert the returned data from units of ppm to units of Microstrain.

Quarter Bridge with Constant Current Excitation

The quarter bridge configuration for measuring bridges is a variation of the half bridge configuration, where there is one active device (such as a strain gauge) and a bridge completion resistance to balance the bridge. The bridge completion resistance can be an inactive device of the same type as the active device or can be a resistor with the same resistance value as the active device, and ideally has a temperature coefficient similar to that of the active device. The entire bridge circuit is external to the dataTaker – the logger does not provide any bridge completion for partial bridges.

In this case, for accurate results it is important that the bridge completion resistor Rc have an resistance that is equal to the active arm of the bridge at rest to properly balance the bridge (measured voltage = 0.0 with no load applied).

Voltage Excitation of Bridges
The alternative method for measuring bridge circuits with the dataTaker is the voltage excitation with ratiometric measurement. The principle of the method is that the bridge is excited by a constant voltage source and the bridge output voltage is measured as a ratio of the measured voltage to the excitation voltage. In practice, resistance of the cable wires connecting the bridge to the logger reduces the excitation voltage that is actually applied to the bridge, which in turn results in a proportionate loss of output signal voltage from the bridge. To correct for this error, the actual voltage applied across the bridge is measured using a second channel.

The Bridge Excitation Voltage Source
The bridge excitation voltage, also often referred to as the bridge power supply, can be supplied from a number of sources:
• the internal voltage excitation source of the dataTaker via the Excite (*) terminal of the analog channel, which can output a nominal 5 Volts (actually nearer 4.5 Volts);
• the switched 12 Volt sensor power supply terminal of the dataTaker, which is limited to 100 mA total current draw;
• the switched 5 Volt sensor power supply terminal of the dataTaker (Series 3 only) which is limited to 25 mA total current draw;
• or an external voltage source either directly connected to the bridge or switched through the EXT */# terminals.

The bridge excitation voltage must be switched on during the period of measurement. If the internal voltage excitation source is used, it will be automatically switched on by the logger at appropriate times. If an external voltage source is used for excitation, the bridges can be either permanently powered or can be powered only during measurement by using the external excitation terminals of the data logger. The default for the internal voltage source for bridge excitation is the unregulated 5 volt supply from the Excite terminal, and is automatically selected when bridge inputs with voltage excitation are specified. However if the bridges are powered from external sources connected directly to the bridge, then the Excite terminal voltage should be disabled using the N channel option. If the bridge is to be powered from external sources connected through the external excitation terminals, EXT */#, then the external excitation option should be enabled using the E channel option.

Measuring the Bridge Excitation Voltage
In practice the resistance of the cable wires connecting bridges to the dataTaker reduce the excitation voltage that is actually applied to the bridge. This results in a proportionate loss of output signal from the bridge. To correct for this error, the actual excitation voltage across the bridges is also measured. The bridge excitation voltage is connected as a differential or single ended voltage input to any analog input channel, and must be measured immediately before the output of any bridge is measured. This measurement is referred to as the ‘bridge reference voltage’, and is measured on the bridge reference channel that is identified to the dataTaker by the BR channel option for the particular channel. This can be on the same channel as that being used for the actual bridge signal voltage or on a different channel.

Note that if the excitation voltage is greater than 3.0 volts, it might seem possible to use the HV channel type to measure the bridge excitation voltage, but this is not recommended since the measurement accuracy when using this channel type is normally not sufficient for bridge applications. In this case, it is better to use the bridge itself to attenuate the voltage being sourced and then use a scale factor to allow the data logger to calculate the actual excitation voltage.

To measure the excitation voltage in dEX, use the appropriate channel in the channel wiring screen or use Manual Channel Type with the following commands:

bridge output reference excitation measurement
3V supply 1*V(BR,W)
6V supply 1+V(2,BR,W)
5.00V supply Not Required

Notes:
1. The bridge reference channel must precede the bridge measurement channel(s) in the dataTaker program because the bridge reference voltage is used to calculate the bridge data for the subsequent bridge measurement channels.
2. If bridge measurements are included in more than one Schedule, then the bridge reference channel(s) must be declared in each Schedule.
3. If a bridge reference channel is not declared, then the bridge reference voltage defaults to 5 Volts.

Full Bridge with Voltage Excitation
The full bridge with voltage excitation configuration is the more traditional method for the measurement of bridge outputs. However, fully implementing this requires more resources than any of the constant current methods, requiring two channels for each bridge if each bridge has a separate bridge excitation. If a shared excitation source is used, two channels are required for the first bridge, with one channel for each additional bridge that is excited by the same power supply. This configuration is only appropriate if all cable wires are the same length, such that all bridges receive the same voltage excitation as measured for the first bridge. This configuration supports 1, 2 or 4 active arms. Any of the bridge arms can be active arms. Bridge arms which do not have active devices must have bridge completion resistances to balance the bridge. These can be inactive devices of the same type as the active devices, or can be a resistor with the same resistance value as the active devices at rest, and ideally have a temperature coefficient that is similar to that of the active devices. Also, the entire bridge circuit is external to the dataTaker – the logger does not provide any bridge completion for partial bridges.

Interpreting the Data from a Full Bridge with Voltage Excitation

Data returned from full bridges with voltage excitation is calculated as the ratio of the change in bridge output voltage to bridge excitation voltage, expressed in parts per million.

Calculating Microstrain for Full Strain Gauge Bridges
When using stain gauges in full bridges, it may be desirable to convert the returned data from units of ppm to units of Microstrain.

This full bridge method of strain gauge measurement has a resolution of approximately 0.2 Microstrain.

Half Bridge with Voltage Excitation

Half bridges with two active arms and voltage excitation are commonly used when a large number of bridges need to be located in close proximity. The dataTaker supports this configuration by using single- ended inputs. Half bridges with two active arms require two bridge completion resistances to balance the bridge. The two bridge completion resistances can be either inactive devices of the same type as the active device, or can be a resistor with the same resistance value as the active devices, and ideally have a temperature coefficient similar to that of the active devices. This half bridge configuration with voltage excitation can be used to measure a single half bridge, or to measure a number of half bridges which share the same bridge excitation voltage supply, and share the same set of bridge completion resistors. Multiple half bridges that are excited from a single excitation voltage source and that share bridge completion resistors are illustrated below.


Interpreting the Data from a Half Bridge with Voltage Excitation

Data returned from half bridges with voltage excitation is calculated as the ratio of the change in bridge output voltage to bridge excitation voltage, expressed in parts per million.

Calculating Microstrain for Half Strain Gauge Bridges

When using stain gauges in half bridges, it may be desirable to convert the data from units of ppm to units of Microstrain.

Converting Bridge Outputs to Engineering Units

So far, methods have been provided to convert measured bridge output in ppm to units of Microstrain for the various bridge configurations. However units of Microstrain apply to strain gauge bridges which are measuring deformation. Many sensors available today employ a bridge circuit to sense the parameter they are designed to measure. For example some pressure cells, load cells, micro-displacement transducers, etc. in fact contain a diaphragm or similar structure which has a full strain gauge bridge bonded to one surface. The diaphragm is mechanically distorted by the pressure or load, which is measured by the strain gauge bridge. This distortion is calibrated to units of pressure, load, etc. by the manufacturer. Supporting these types of sensors with the dataTaker is quite simple, as shown by the following examples.

Pressure Transducer
A pressure transducer that is constructed as a full bridge device with a 4 wire connection, is connected to the dataTaker as a full bridge with constant current excitation (type BGI). The transducer has an output of 0.05 V full scale at 10 VDC excitation for a full scale pressure of 100 PSI. The dataTaker will measure:
At minimum bridge output (0.0 psi) = 0.0 ppm
Therefore 1ppm = 100 PSI/5000 ppm = 0.02 PSI. This transducer calibration can be used in a dataTaker program to return the data in units of PSI, so that it is a simple matter of constructing a scaling conversion to convert ppm to PSI.

Load Cell
A load cell that is constructed as a full bridge device with a 4 wire connection is connected to the dataTaker as a full bridge with constant current excitation (type BGI).The load cell measures a load of 100 lbs full scale and has an output of 2.0006 mv/V at full scale. The dataTaker will measure:
At minimum bridge output (0.0 pound load) = 0.0 ppm
Therefore 1ppm = 100 lbs / 2000.6 ppm = 0.049985 lbs. This transducer calibration can be used in a dataTaker program to return the data in units of lbs.

Measurement Ranges and Accuracy
The dataTaker measures all bridge inputs as a low level voltage, with a resolution of 0.25 μV, and a nominal accuracy of 0.1%. The accuracy for particular applications can be calculated from this information, and the excitation current or voltage used.

Error Messages

There are no specific error messages for bridge inputs. However, input voltage signals which fall outside the voltage range of the dataTaker will produce an over-range reading of –99999.9 ppm or +99999.9 ppm. The dataTaker also reports the error condition with the error message ‘E11–input(s) out of range’ if the Messages Switch /M is enabled.
For further information on the popular dataTaker family of dataloggers, other data logging devices, or to find the ideal solution for your application-specific needs, contact a CAS Data Logger Applications Analyst at (800) 956-4437 or visit the website at www.DataLoggerInc.com.

Contact Information:
CAS DataLoggers, Inc.
12628 Chillicothe Road
Chesterland, Ohio 44026
(440) 729-2570
(800) 956-4437
sales@dataloggerinc.com
http://www.dataloggerinc.com

Configuring DT8x Data Loggers to Interface with Maple Displays via Modbus

Modbus Data Loggers Assist with Multiple Devices in the Field and Onsite

CHESTERLAND OH—August 3, 2011

Customers occasionally need to perform the task of interfacing their data loggers with the popular line of Maple displays, whether viewing their data on the plant floor or out in the field. Following is a quick tutorial on configuring the bestselling dataTaker DT8x series data loggers to interface with Maple displays using Modbus.

First configure the Modbus interface on the data logger. Users have several options here: the logger can be configured as a Modbus Master in which case all data must be explicitly written to or read from the display by the data logger. If more than one display will be used, for example, a local display by the logger and a remote display for an operator, the logger should be configured as a master to allow it write to multiple devices. If there’s just one display, it’s possible to set the display as the master and logger as the slave, which simplifies the programming on the logger side.

Once the choice of making the logger a master or slave device has been made, the next step is to configure the hardware interface of the data logger, as the DT82I, DT80 and DT85 models support Modbus communication via either the serial sensor port on the front of the logger (1Serial) which allows RS-232, RS-485 or RS-422 communications or the host serial port on the side of the logger (2Serial) which is RS-232 only. When using the front serial port, a custom cable will need to be built; in the case of RS-485 communications the Tx terminal is the Data- and the RTS terminal is the Data+. If the host port is used, a standard dataTaker IBM-6 cable can be used to connect to a display that uses the standard 9 pin wiring configuration (pins 2,3 and 5). Following the instructions in the manual in the section for the Modbus interface, the appropriate PROFILE settings will be need to made in the logger for the serial port being used. These include: Baud Rate, Parity, Databits, Stopbits, Flow, Function (MODBUS or MODBUS_MASTER), Mode (for RS232/422/485 on the serial sensor port only) and MODBUS_SERVER for the appropriate port to set the bus address of the logger.

After the data logger has been configured using one of the above options, the final configuration step is to set-up the Maple display. Like the logger, the display can be configured as a Modbus Master or Slave device. The display will normally be configured as the opposite of the logger, but there can only be one master on the bus. If there is more than one display, the most common configuration is to set them all to be Slaves. If there is only one display, it can be set to be a Master if the logger is set as a Slave. If the display is set to Master, the display will be configured to use the Mobus RTU master driver. To do this, in the EasyBuilder 5000 software, go to Edit -> System Parameter Settings -> Device Properties, create a new device and set the PLC type to Modbus Master. If the display is set to Slave, use Mobus RTU slave driver. In the EasyBuilder 5000 software, go to Edit -> System Parameter Settings -> Device Properties, create a new device and set the PLC type to Modbus RTU/TCP Slave. Now configure the other parameters to match the logger communications settings: Baud rate, parity, data bits, stop bits, flow, and PLC I/F type. If the display is a slave, the Station # in the configuration screen is the Modbus Address of the display, so this needs to be set appropriately as it will be the address that users write to from the data logger.

To exchange data with the Display, proceed according to whether the display has just been set as a Master or Slave. If the Display Is a Master, this is the simplest configuration: in this case, the logger simply writes the data to a CV which it automatically maps to a Modus register. For example, 1CV is read from the display at register 30001 or from a read/write at register 40001. The default format is as a signed integer, but the loggers SETMODBUS command can be used to change the format of the data for a CV or group of CVs to unsigned integers or floats. Be aware that in the case of floats, the data is stored across 2 Modbus registers. If the Display Is a Slave, then to display data, the data logger has to explicitly write the data to the display using the nMODBUS(ADm, Rx:y)=zCV command where: n is 1 for the serial sensor port of the logger and 2 for the host port; m is the address of the display (set as the station # above); x is the register set = 4 for read/write; and y is the register number. Also note that the register number configured on the display is 1 less than the register number written to from the logger, i.e. the logger writes to 40001 and the data appears on the display in LW 0; and z is the number of the CV holding the value to be written.

When Reading data, use the command nModbus(ADm,, Rx:y, =nCV). The parameters are the same as above, except that x can be 3 for read-only registers. Again, the standard data type is a signed integer, but the logger supports other formats via the MBx modifier in the write command. As before, be aware that 32-bit values will span 2 registers. When writing ASCII data, text data can be written as the equivalent decimal value. To send 1, send 49(the ASCII decimal value of 1).

Check out the DT8x family of data loggers here at our dataTaker product page.

For further information on the bestselling dataTaker line of DT8x data loggers, other data loggers supporting Modbus, or additional technical questions on any application-specific solution, contact a CAS Data Logger Applications Analyst at (800) 956-4437 or visit the website at www.DataLoggerInc.com.

Contact Information:
CAS DataLoggers, Inc.
12628 Chillicothe Road
Chesterland, Ohio 44026
(440) 729-2570
(800) 956-4437
sales@dataloggerinc.com