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4 Channel RTD Interface

Instrument cover photo.

This documentation covers part number 10-0000094

Overview

Resistance temperature detectors (RTDs) are precision temperature measurement devices whose resistance varies with the temperature of the probe. RTDs are generally used for higher resolution measurement of smaller temper- ature ranges than thermocouples and are not dependent upon the same Seebeck voltage as a thermocouple. Our 4 channel RTD Interface is a quick, economical, and accurate way to interface RTDs to a variety of data acquisition systems and test environments. Unlike many other signal conditioning solutions on the market, the RTD Interface produces a digital output via a USB connection as well as an analog voltage output that can be read and digitized by external systems. The range of the voltage output is adjustable as well as the span of temperature represented making it easy to scale data to areas of interest and utilize lower bit depth measurement systems. This unit is designed to work with PT100 type RTDs. If your system uses PT500 or PT1000 devices please contact the factory to discuss options to connect your system to the 4 Channel RTD Interface device.

Front Panel

The front panel of the instrument has four LED indicators on it.

  • Power - Green LED indicating the device is receiving power and on.

  • Read - Yellow LED indicating that the device is reading temperatures from the RTDs.

  • TX - Blue LED indicating the device is sending data over the serial connection to a host device.

  • RX - Blue LED indicating the device is receiving data over the serial connection from a host device.

What's in the Box

Upon receipt of your unit, unpack the contents of the box and inspect all parts for any damage incurred during shipping. Immediately report any missing parts or damage to Leeman Geophysical for replacement. Note that there are many optional accessories available, see the accessories section of the manual for details and usage notes.

  • RTD Interface in anodized enclosure

  • 5 Position plug-in terminal block

  • 4 Position plug-in terminal block (X4)

  • USB Cable

  • Factory amplifier calibration documentation

Specifications

Parameter Min Typ Max Unit
Environmental
Operating Temperature -40 - 80
Physcial
Weight - 0.43 - kg
Width - 127 - mm
Length - 166 - mm
Height - 35 - mm

Hookup

USB Power and Communications

Power is provided via the USB mini-B port on the back of the unit. If the interface will not be plugged into a computer for digital communications, it can be powered with a simple USB wall power supply. The USB port also provides communications to a host device as a serial connection using FTDI Technologies interface devices. If your system does not already have FTDI Virtual Communications Port (VCP) drivers installed, they may be downloaded at no cost from FTDI at https://ftdichip.com/drivers/vcp-drivers/. The default baud rate of the device is 9600, though this is a user modifiable setting. For the details on serial communications and commands, see the Serial Command Interface section.
If the interface is to be plugged into a USB hub device we recommend a powered hub. Un-powered hubs may not be able to provide adequate supply power and result in unstable behavior. If powering the interface from a wall supply, we recommend a 500mA or greater supply to avoid issues.

Using the Plug-In Terminals

The plug in terminals provided make creating your connections easy as they can be connected in a convenient position and then simply plugged into position on the back of the RTD interface. This also makes switching out a damaged unit or troubleshooting much faster. To use the terminals, open the connection block with a screwdriver. Strip and insert the wire, then tighten the terminal. Pull to check for a secure connection. Ensure there are no stray strands of wire adjoining adjacent connections and no exposed conductor which could be hazardous. Also be careful to grip the actual conductor in the terminal, not the outer insulator as this can cause high resistance and intermittent connections.

Good and bad connections to plug-in terminal blocks.

Connection positions on the back panel of the instrument as seen from the back.

2-Wire RTD

Two wire RTDs are the simplest, though least accurate RTD model available. The two wires carry the excitation current and are used to measure the RTD voltage. Therefore some of the lead resistance is included in the mea- surement, so keeping lead wires as short as possible is essential. To connect a 2-wire RTD to the interface, one wire is connected to the Force + terminal and another to the Force - terminal. Jumper wires (we recommend new, clean 22 gauge stranded wire) are then inserted between Force - and RTD - as well as between Force + and RTD +.

Connection of a 2-wire RTD to the interface with a 4 position plug in terminal block.

3-Wire RTD

Three wire RTDs are more accurate than two wire RTDs as lead resistance concerns are partially mitigated by the third conductor in the system. First, determine which two of the three wires are connected together and which is the third wire. Often these are color coded by the manufacturer, but this can also be determined with a multimeter by measuring the resistance between wires. The pair with the lowest resistance (a few Ohms at most) is connected. Insert one of the pair of connected wires into the Force + terminal. Insert the other wire of the pair into the RTD + terminal. Insert the third wire into Force - and install a jumper (we recommend new, clean 22 gauge stranded wire) between Force - and RTD -.

Connection of a 3-wire RTD to the interface with a 4 position plug in terminal block.

4-Wire RTD

Four wire RTDs are the most accurate RTD configuration and removes most lead resistance errors. Two wires are connected to each end of the resistance element in the sensor probe. Determine which wires are paired by measuring resistance between wires with a multimeter. The pairs will have a few Ohms at most of resistance while opposite ends will read near 100 Ohms. Many manufacturers color code these pairs as well. Connect one of the low resistance pairs (either one) to Force + and RTD +. Connect the other pair to Force - and RTD -.

Connection of a 4-wire RTD to the interface with a 4 position plug in terminal block.

Analog Output

The RTD interface can produce an analog output voltage in the range 0-10 VDC which is linearly proportional to temperature over a user selected voltage and temperature range. The output connections are ground referenced and available on the 5 pin plug-in terminal block. The outputs should be buffered if driving any load other than a high input-impedance device such as an analog to digital converter system.

Serial Command Interface

Settings of the RTD Interface are modified via a simple serial command interface. For more information on using a serial terminal to communicate with instruments, be sure to read our how-to post on the company blog. The instrument is set from the factory to 9600 baud. Settings are stored in non-volatile memory and kept through power cycles of the instrument.

Command Listing

Commands are all followed by a newline character.

Command Description
SETVMIN C XX.XXX Set the minimum output voltage on channel C
SETVMAX C XX.XXX Set the maximum output voltage on channel C
SETTMIN C XX.XXX Set the minimum temperature on channel C
SETTMAX C XX.XXX Set the maximum temperature on channel C
SETWIRES C X Set the RTD to 2, 3, or 4 wire configuration on channel C
SETFILT C X Set the filter on channel C to 50 or 60 Hz
SETRREF C XX.XXX Set the reference resistance on channel C
SETRNOM C XX.XXX Set the nominal RTD resistance on channel C
SETGAIN C XX.XXX Set the gain of the output amplifier on channel C
SETOFF C XX.XXX Set the offset of the output amplifier on channel C in volts
SETBAUD X Set the baud rate to X
READ Force a reading regardless of mode or time settings
RESET Restart the instrument
SHOW Show the current configuration
HELP Displays a help menu with a list of available commands
DEFAULTS Resets all stored values to the factory default values

Command Descriptions

SETVMIN sets the minimum output voltage allowed on a given channel. This value must be equal to or greater than zero and less than or equal to ten. No matter the temperature on the sensor, this output voltage will be the minimum allowable analog output on that channel.

SETVMAX sets the maximum output voltage allowed on a given channel. This value must be equal to or greater
than zero and less than or equal to ten. No matter the temperature on the sensor, this output voltage will be the maximum allowable analog output on that channel.

SETTMIN sets the minimum temperature to be registered on a given channel. When the temperature is at this minimum the output voltage will be at the value set by VMIN. This value must be greater than or equal to -200◦C and less than or equal to 850◦C.

SETTMAX sets the maximum temperature to be registered on a given channel. When the temperature is at this maximum the output voltage will be at the value set by VMAX. This value must be greater than or equal to -200◦C and less than or equal to 850◦C.

SETBAUD sets the baud rate of the device to a new rate. This change takes effect immediately and the serial terminal utility will need to be disconnected and reconnected at the new baud rate. Valid rates are 1200, 2400, 4800, 9600, 19200, 38400, 57600, 74880, and 115200 baud.

SETWIRES sets the RTD wiring configuration to be 2, 3, or 4 wires on the given channel. Though readings may be taken with an inaccurate setting of this parameter, they may be incorrect.

SETFILT sets the power line interference filter to be 50 or 60 Hz on the given channel. In North America 60 Hz prevails and most European and Asian counties are on 50 Hz power. This filter simply reduces this high frequency noise on generally slowly changing temperature data.

SETRREF sets the reference resistance for the given channel. This parameter is set at the factory and should only be altered in advanced tuning of system accuracy after consultation with the factory.

SETRNOM sets the nominal resistance of the RTD for a given channel. This parameter is generally set to 100 Ohms for PT100 devices, but can be altered based on calibration data from your sensor’s manufacturer.

SETGAIN sets the output amplifier gain on a given channel. This parameter is set at the factory and should only be altered during factory calibration.

SETOFF sets the output amplifier offset on a given channel. This parameter is set at the factory and should only be altered during factory calibration.

READ takes a reading from all RTDs.

RESET takes a reading from all RTDs.

SHOW Display the current instrument configuration.

HELP displays a condensed version of the command help menu. This is very helpful in the field as a quick reference without the instrument’s manual.

DEFAULTS resets all user select-able settings to their factory defaults. The instrument will be set at 9600 baud and all RTDs will be in 3-wire mode with 60Hz filters applied. Factory set amplifier gains and offsets will not be altered.

Examples

  • Set the minimum voltage on channel 1 to 1.5 VDC SETVMIN 1 1.5
  • Set channel 3 to use a 4 wire RTD SETWIRES 3 4
  • Change the baud rate to 115200 baud SETBAUD 115200

Configuration

RTD Settings

For each RTD connected to the unit, it is very important to set the correct configuration settings. The SETWIRES command should be used to set the correct number of wires (2, 3, or 4) for each RTD connected. There can be any combination of 2, 3, or 4 wire RTDs connected to the unit. The SETFILT command should be used to set the power line filter to the appropriate AC wall power frequency for your region of operation (50 or 60 Hz). Generally all other settings such as nominal and reference resistances are left at their default factory set values unless calibration information is available from your RTD manufacturer and you find it necessary to modify the RTD nominal resistance.

Analog Output

The analog output of the unit is set on a channel by channel basis. At the value of temperature set by SETTMIN the analog output will be that value set by SETVMIN. At the value of temperature set by SETTMAX the analog output will be that value set by SETVMAX. In between the voltage output will vary linearly with temperature. An example calculation is provided below on how to determine the calibration and output of the device.

Analog Output Example

The experimental setup considered in this example is designed to measure temperatures of -100◦C to 80◦C. The RTD Interface analog outputs are connected to a digitizer and a user set 1-5 VDC output range. Setting TMIN to be -100, TMAX to 80, VMIN to 0 and VMAX to 5 we can calculate the following calibration from volts to temperature.

\[ Temperature = Voltage~Out~({T_{max}-T_{min} \over V_{max}-V_{min}})+(T_{max}-{T_{max}-T_{min} \over V_{max}-V_{min}}V_{max}) \]

Resolution Calculation Example

Assuming the signal from the example problem above is sent to a ideal zero noise 16-bit converter with an input range of 0-5 VDC we can calculate the ideal minimum temperature resolution. First we must calculate the change in voltage represented by one bit of the converter.

\[ Volts~per~Bit={V_{max}-V_{min} \over 2^n-1}\]

Setting \(n\) to be 16 (the bit depth) and \(V_{min}\) and \(V_{max}\) are set to 0 VDC and 5 VDC respectively we determine that the analog to digital converter has an ideal resolution of 0.076 mV. Next we must calculate how much change in temperature is represented by a volt of output change, the slope of our calibration.

\[{T_{max}-T_{min} \over V_{max}-V_{min}}\]

This number comes out to be 45 degrees Celsius per volt. The product of this and our ADC resolution of 0.076 mV yields an ideal resolution on the digitization of 0.003°C. Of course noise in the digital to analog and analog to digital conversion process will reduce this, but it may be used as a guideline when deciding the range and resolution requirements of the RTD Interface and attached hardware. Note that the RTD Interface itself uses a 12-bit digital to analog converter for the output, so in this instance the resolution will be limited by the output resolution (roughly 2mV or in this case 0.09°C).

Warm Up

To reduce drift, we recommend powering up the instrument at least 15 minutes ahead of data collection to ensure the most accurate results. This may depend on the ambient conditions and is generally very small error and the device stabilizes quickly.

Data Interpretation

Output of the serial READ command indicating the temperatures from each RTD in degrees
Celsius.

The data from the READ command is plain text ASCII. There are four tab-delimited fields, each representing an RTD temperature. Temperatures are output in calibrated degrees Celsius. These values can be recorded and interpreted by any user written program or serial terminal program.

Accessories

There are a variety of accessories available to make installing, deploying, and using your interface easier! Contact us with any questions on which accessories are appropriate for your deployment or for custom integration services.

4 Position Plug-In Terminal Block

1-0001141
Extra 4 position terminal blocks make having spare pre-wired RTDs on hand possible and allow for rapid swap out of sensing elements.

5 Position Plug-In Terminal Block

1-0001149
Extra 4 position terminal blocks make having spare pre-wired RTDs on hand possible and allow for rapid swap out of sensing elements.

Revision History

Date Changes
January 2023 Initial Release
Febuary 2023 Update company address
April 2024 Moved Documentation to MkDocs Format