ESIS New Website

Analog Input Modules for PC
with USB Port i/f


Esis Pty Ltd - Australia - Ph 02 9481 7420  Fax 02 9481 7267  www.esis.com.au 
E-mail: click here       Go to Esis main page

Prices (Australian dollars) do not include GST & may change without notice.


Voltage, Temperature Modules - Pico

Humidity probe          - for PCs

 

Related pages:
    Oscilloscope Modules with USB connection to PC - PicoScopes

    Enviromon System measures temperatures, humidity, light level and other parameters over a wide area.

    DrDaq  - low cost data logger
    Built-in: Temperature, Light & sound, Voltage, Resistance + two external temperature sensors - ext. pH sensor


USB Port: All modules now have USB ports.

  Temperature measuring modules - also measure voltage (PT-104 can also measure resistance)

  Thermocouple Information at end of this page - how they work

  Jump down to
  Downloads: User Manuals, Software


Voltage measuring modules

See also next table, the temperature modules also have voltage input modes.      7/12


Voltage Modules

Specifications

 

ADC-20
Hi Resolution
Voltage

ADC-24
Hi Resolution
Voltage

PicoLog 1012

Voltage

PicoLog 1216

Voltage

 
    ADC-20 PicoLog 1000 Series  
Resolution  

20 bits

24 bits

10 bits

12 bits

 
Channels  

4 diff/8se

8diff/16se

12 (Except in USB ADC-11 compatibility mode)

16 (Except in USB ADC-11 compatibility mode)

 
Input Range(s)  

±2500 mV
±1250 mV

7 ranges from
 ±2500 mV
down to ±39 mV

0 to +2.5 V
Single–ended, unipolar

0 to +2.5 V
Single–ended, unipolar

 
Max Sampling Rate  

Conversion time (per channel)
660 ms, 340 ms, 180 ms,
100 ms, 60 ms

1 kS/s per channel in PicoLog,
100 kS/s shared by all channels
using the supplied SDK
Real–time continuous: 1 kS/s or greater
Block mode#1: 1 MS/s single–channel
Block mode: supported by PicoScope and the PicoLog 1000 Series SDK, but not by PicoLog

 
Noise rejection  

120 dB typical at 50/60 Hz

     
Reference
Output signal
  +2.5 V ±2.5 mV
@ 2 mA
+5 V ±1.0 V
@ 2 mA
-5 V ±1.5 V
@ 2 mA
+2.5 V ±2.5 mV
@ 2 mA
+5 V ±1.0 V
@ 2 mA
-5 V ±1.5 V
@ 2 mA

Power output for sensors:
2.5 V @ 10 mA, current–limited

 
Accuracy  

0.2%

0.1%,
except ±2500 mV range:
0.2%

1% 0.5%  
Over Voltage Protection  

±30V

±30 V

±30 V to ground ±30 V to ground  
Input Impedance  

Differential: 2 MΩ
Single ended: 1 MΩ

1 MΩ

1 MΩ

 
Input Connector  

D25 female

D25 female

25–way D female 25–way D female  
PC Connection  

USB 1.1

USB 1.1

USB 2.0 USB 2.0  
Power supply  

From USB port

From USB port

From USB port From USB port  

Digital I/O

 

none

4 bi-directional
(3.3 V CMOS)

Digital output:
  2

Digital output: 4
  Except in USB ADC-11
  compatibility mode
Digital outputs (PWM): 1
  Period: 100 µs to 1800 µs
Duty cycle:
  adjustable from 0% to
  100% in 1% steps

 

PicoScope Software

 

supplied

na

PicoScope 6 for Windows PicoScope 6 for Windows  

PicoLog software

 

supplied

supplied

PicoLog for Windows PicoLog for Windows  


More information
 

 

ADC-20

More info on this page

User Manual (pdf, opens in new tab)


ADC-24

More info on this page

User Manual (pdf, opens in new tab)

Picolog 1000 Series
User Manual
(pdf file, opens in new tab)
Picolog 1000 Series
User Manual
(pdf file, opens in new tab)
 
 

Prices (Australian dollars) do not include GST & may change without notice
See below for more description & pictures of the modules

ADC-11, ADC-12 have been discontinued.

See further down the page for more description & pictures of the various ADC modules.

Temperature measuring modules (also measure voltage)

Modules:
Temperature & Voltage


When accuracy is paramount the PT-104 is what you need

 

Specifications

 

PT-104
Platinum Resistance Logger

4 x PT100 or PT1000,
resistance, voltage

 

TC-08

8 Thermocouples, or voltage ±70 mV

 

HumidiProbe
Temperature & Humidity
Probe

Formerly PP-299

   

  4 ch. Temperature, resistance & voltage

PT-104

High resolution and accuracy
Uses PT100 and
       PT1000 sensors
Supports 2,3 and 4–wire sensors
(four wire PT100 sensors are recommended for accurate measurements)
No power supply required
USB and Ethernet interfaces
Run multiple units on a single PC

 

  8 channel thermocouple data logger

       TC-08

Thermocouple types supported
      B, E, J, K, N, R, S, T
From -270 to +1820 °C
See below for chart of thermocouple temp. ranges
Up to 160 thermocouples using
    up to 20x TC-08
Automatic cold junction compensation
High resolution and accuracy
Fast sampling rate
    — up to 10 measurements per second
USB interface

 

    Temp & humidity

HumidiProbe
Records temperature and
    relative humidity
Measures 0 to 100% RH
    (non-condensing)
Measures 0 to 70 °C
Small form factor design
Connects to a PC or laptop via a USB port
No power supply required

Resolution 

Temperature: 0.001 °C
Resistance:   1 µΩ
Voltage:    0.156 µV
Converter: 24 bits

  20 bits 

0.01 °C

0.03% RH

Channels 

      4  PT100 or PT1000 sensors

Measure up to 40 using PicoLog software. You can connect up to 10x PT-104s to one PC, giving you a potential 40 channel PT100 data logger.
Alternatively you can use your PT-104 logger with up to 19 other Pico data loggers, such as the PicoLog 1000 multi-channel data logger, to create your own custom data acquisition system.

 

      8 thermocouples

Measure up to 160 thermocouples simultaneously:
the TC-08's USB connectivity allows up to 20 TC-08 USB data loggers to be used simultaneously on one PC

  2 built-in sensors:
temperature and humidity
Input Range(s) 

-200 to +800 °C
0 to 375 Ω*,   0 to 10 kΩ
0 to 115 mV,   0 to 2.5 V *
* indicates the most accurate range

 

Depends on thermocouple
see table below

±70 mV

 

Temp: 0 to +70°C

Humidity:     0-100%

Max Sampling Rate 

Conversion time:
    720 ms per channel

 100 ms (thermocouple and cold junction compensation)  Conversion time: 2 sec
Response time:
  Temp.:     5 to 30 seconds
  Humidity: 4 seconds
  


PT-104

 

 


TC-08

 


HumidiProbe

Accuracy 

Temperature: at 23 ±2 °C):  
    0.015 °C+ 0.01% of reading
Resistance: 20 ppm at 100 Ω
Voltage: 0.4%

 

Sum of ±0.2% of reading and ±0.5 °C

Voltage accuracy: sum of ±0.2% of reading and ±10 µV

 

Temp:  ±0.5°C

Humidity ±2%

Over Voltage Protection 

±30 V

 

±30 V

  –
Input Impedance  

>1 MΩ

 

2 MΩ

  –
Input Connector 

  4-pin mini–DIN

Accessory:
PT-104 Screw Terminal Adapter
The adapter allows PT100 probes that are not fitted with a mini–DIN connector to be used with the data logger without the need for any soldering.
The adapter is suitable for use with the following size of wire:
2.5 mm2 solid
1.5 mm2 stranded
14-22 AWG

 

  Miniature thermocouple connector

Accessory: TC-08 Single-Channel Terminal Board
The screw terminals allow wires to be attached to the data logger without soldering, and enable the USB TC-08 to measure voltages from 0 to +5 V, or 4-20 mA loop current.

  USB
PC Connection 

USB 2.0 (USB 1.1 compatible)
or Ethernet

 

      USB 1.1

 

USB 1.1

Power Supply  

Powered from USB port or
Ethernet

 

Powered from USB port

 

Powered from USB port

PicoScope Software 

 

 

na

 na
PicoLog software 

PicoLog data acquisition software supplied

 

PicoLog data acquisition software supplied.
PicoLog data acquisition software
      can collect up to 1 million samples.
Features include:
Multiple views - view data as a graph, spreadsheet or text
Parameter scaling - convert raw data
    into standard engineering units
Math functions - use mathematical
  equations to calculate additional parameters
Alarm limits - program an alert if a
    parameter goes out of a specified range
IP networking - transfer measurements
    via a LAN or over the Internet
      Full details on PicoLog (this page)

Software development kit: Includes drivers and example code for various programming languages including C, Excel and LabVIEW.

 PicoLog data acquisition software supplied
    Warranty 5 years   Warranty 5 years   Warranty 2 years

More info
 

Data on PT-104 (new tab)

 

TC-08-USB (this page)

 

HumidiProbe (this page)

 

Prices (Australian dollars) do not include GST & may change without notice
See below for more description & pictures of the modules

TH-03 has been discontinued.


Downloads

User Manuals for all Pico products (.pdf format)


Pico Software Demos - include simulated data

Pico Software : PicoScope, PicoLog & Enviromon Full Versions & Upgrades
Free download for existing users of PicoScope oscilloscopes, Pico data loggers and EnviroMon.
If you are not an existing user you can download demonstration versions of these programs that include simulated data. See link above.

Pico Software: Drivers and Examples for Data Loggers and Oscilloscopes
LabVIEW,  Agilent VEE,  TestPoint,  Excel,  Visual Basic,  C and Delphi
Drivers for: National Instruments LabVIEW,  Agilent VEE,  Microsoft Excel,  TestPoint,  Linux
for various Pico products.

Drivers for Linux can be downloaded from: http://www.picotech.com/linux.html


ADC-20  & ADC-24  Hi Resolution, Multi-channel Data Logger

These modules offer the ultimate in precise and accurate readings.    Larger Picture
Features such as true differential inputs, galvanic isolation and software-selectable sample rates all contribute to a superior noise-free resolution.     dln
The ADC-20 is equipped with a 20-bit A/D converter, and can maintain a gain error of 0.2%. The four true differential inputs may be configured as eight single-ended inputs or any combination in between, such as two differential and four single-ended.
The ADC-24 is equipped with a 24-bit A/D converter, and can maintain a gain error of 0.1%. The eight true differential inputs may be configured as 16 single-ended inputs or any combination in between.
Power and connection to a PC or laptop is through a USB 1.1 or USB 2.0 port.
Using the supplied PicoLog software, you can record, monitor and analyse collected data, even exporting to third-party applications such as Microsoft Excel.    Data on ADC-20 & ADC-24

 

 

ADC-20

ADC-24

Resolution

 

20 bits

24 bits

Number of channels#

 

4 differential / 8 single-ended

8 differential / 16 single-ended

Conversion time (per channel)

 

660 ms, 340 ms, 180 ms, 100 ms, 60 ms

Voltage ranges

 

2500 mV   1250 mV

2500 mV 1250 mV 625 mV
312 mV 156 mV 78 mV 39 mV

Accuracy

 

0.2%

0.1% (39 mV to 1250 mV range)
0.2% (2500 mV range)

Noise rejection

 

120 dB typical at 50/60 Hz

Input impedance

 

Differential: 2 MegOhms
Single ended: 1 MegOhms

Overload protection

 

30 V

30 V

Digital I/O

 

none

4 bi-directional (3.3V CMOS)

Reference output

 

2.5 V

2.5 V

Input connector

 

D25 female

D25 female

PC connection

 

USB 1.1 - cable supplied

USB 1.1 - cable supplied

Power supply

 

From USB port

From USB port

Environmental

 

+20C to +30C for quoted accuracy,   0 C to +45 C overall operation.   5 to 80% RH

Supplied software

 

PicoLog (data logger)
PicoLog data acquisition software can collect up to 1 million samples. Features include:
Multiple views — view data as a graph, spreadsheet or text
Parameter scaling — convert raw data into standard engineering units
Math functions — use mathematical equations to calculate additional parameters
Alarm limits — program an alert if a parameter goes out of a specified range
IP networking — transfer measurements via a LAN or over the Internet

Software development kit (downloadable): Includes drivers and example code for various programming languages including C, Delphi, Excel, LabVIEW, VEE and Visual Basic.

PC requirements

 

Processor: Pentium class processor or equivalent
Memory: 32 MB minimum
Disk space: 10 MB minimum
Operating system: Microsoft Windows 98SE, ME, 2000 or XP
Ports: USB 1.1 or USB 2.0 compliant port.
Must be connected direct to the port or a powered USB hub

Price

 

     ADC-20: $POA

     ADC-24: $POA


Prices do not include the optional
PP-310 terminal board $POA

 

 

 

PP310 Terminal Board for ADC-20/24
This has screw terminals to allow you to connect wires to all of the data logger's inputs and outputs without soldering. It also has space for voltage-divider resistors, a temperature sensor and a quad opamp.  PP310 Terminal Board for ADC-20/24: $93  8/08

The ADC-20 and ADC-24 have an LED, next to the entry point of the USB cable, that you can use to verify that the unit is working.
The LED flashes whenever the unit is taking readings.
It also flashes briefly during "enumeration", the process that PicoLog uses to detect all Pico USB devices plugged in to the computer.

  Download User Manuals:   ADC-20 & ADC-24 (.pdf file, 444KB, 43 pages)   PP310 Terminal Board (.pdf, 628KB, 8 pages)


Application software provided with Pico Devices

Pico Technology data acquisition products are supplied where applicable with application software (PicoScope, PicoLog or EnviroMon).
In addition we also supply (free of charge) a wide range of software drivers and examples.

PicoScope, PicoLog, see end of this page for descriptions.

Drivers and examples for software programmers.
For customers who wish to write their own software, or integrate Pico products into existing software both DOS and Windows drivers are provided.
For DOS: C and Pascal drivers are provided.
For Windows: 16 bit DLLs for Windows 3.1x and 32 bit DLLs for Windows 95/98/NT are provided.
Examples are included for C, Delphi and Visual Basic.
Linux drivers now available for all products.

National Instruments LabVIEW
Examples for the popular LabVIEW program from National Instruments.
The drivers and examples are compatible with LabVIEW versions 4 and 5.

Hewlett Packard HP VEE   HP VEE is attracting increasing interest, and working examples for all of our products are provided.
The drivers and examples are compatible with HP VEE versions 4 and 5.

Microsoft Excel
The example Excel macros show you how to collect data directly into the spreadsheet.

 


ADC-16: 8 Channel 16-bit - discontinued


Download Pico User Manuals (.pdf format): click here


TC-08-USB: Thermocouple Input

8 channels. Accepts type B, E, J, K, R, S, T thermocouples, or ±70mV input voltage.
Measures from -270 to +1820 °C
Any thermocouple that has a miniature size thermocouple connector.
Additionally, the TC-08 can also measure other sensors using a 70 mV range.
Automatic cold junction compensation
High resolution and accuracy
Fast sampling rate
Multiple units can be run on a single PC
High (20 bit) resolution ensures that the TC-08 can detect minute changes in temperature.
USB interface

  Data: click here (pdf file, opens in new tab)        PicoLog software included, see description below.

Thermocouple List

Ruggedised TC-08 USB is also available

TP1 Mating Thermocouple plugs K Type,  dbde
  each: $28+GST,  for 3 to 7, each $20+GST,  for set of 8: each: $15+GST (total $120+GST) 

Thermocouples, thermocouple wire: increments of 25m, please enquire

Terminal BoardUSB TC-08 Single-Channel Terminal Board

The USB TC-08 Single-Channel Terminal Board is an accessory for the USB TC-08 Thermocouple Data Logger.
The screw terminals allow wires to be attached to the data logger without soldering,
and enable the USB TC-08 to measure voltages from 0 to +5 V, or 4-20 mA loop current.

HumidiProbe - USB port

Measures Temperature & Humidity
The HumidiProbe is a replacement for the RH-02 (below).

Temp Range:    0 to 70 °C     Humidity 0 to 100% non condensing
Accuracy:         Temp.: ±0.5 °C (0 to 70 °C)   Humidity: ±2%
Resolution:       Temp.: 0.01 °C                      Humidity: 0.03%
Conversion Time: 2 seconds
Response time:    Temp.: 5 to 30 seconds       Humidity: 4 seconds (slowly moving air)
Dimensions 45 x 72 x 28 mm with 4.5m cable terminated with a USB plug.
Output connector USB    
Supplied software: PicoLog data acquisition software for Windows (98SE/ME/XP/2000)
Drivers and examples C, Delphi, Visual Basic, Agilent VEE and LabVIEW.
     A macro is also provided to collect data directly into an Excel spreadsheet.
PC requirements: Processor: Pentium class processor or equivalent
Memory: 32 MB minimum,   Disk space: 10 MB minimum
Operating system: Microsoft Windows 98SE, ME, 2000 or XP
Ports: USB 1.1 or 2.0 compliant port.   
   HumidiProbe User Manual

RH-02: Humidity and Temperature - RS232 port

       Superseded by Humidiprobe above


Download Pico User Manuals (.pdf format): click here


Software

PicoLog Data Logging Software for Windows

PicoLog data acquisition software is a powerful but flexible program for collecting, analysing and displaying data.

PicoLog can be used with the following product ranges:      7/12
    Data acquisition (DAQ) / data logging products
    Temperature and humidity converters and loggers.

PicoLog Screen

Some of the features of PicoLog are listed here.
To see for yourself just how good it is, download your free copy.

• Collect up to 1 million samples.
• Easy and intuitive to use.
• Free upgrades and technical support.
• Supports 32 bit and 64 bit# editions of Windows XP (SP2 and above), Vista and Windows 7.
    # 64 bit support is only available for USB products.
• Compatible with Pico Data Loggers (except EnviroMon).
• International language versions.
• Easy to set up and use with online help.
• Real-time data collection, analysis and display.
• Programmable alarm limits can be set for each channel.
• Data can be exported to spreadsheets and databases.
• Save multiple setups for different tests and experiments.
• Can be used with desktop or laptop PCs.
• Supports multiple loggers on the same PC.
• Uses your PC monitor to give large colour display, ideal for education and training.
• Waveforms can be saved, printed, faxed or emailed from your PC.

Multiple views

PicoLog displays data in a number of views, which can be activated as and when required. These views can be activated both during and after data collection.

Recorder View

Enables you to start/stop recording and specify recording files. It shows the current readings and alarm conditions for each channel. All settings such as scaling, channels and sampling are controlled from the recorder view.

XY graph view

Displays one parameter against another. Useful for plotting voltage against current, for example.

Spreadsheet View

Displays text data in a format that can be easily 'copied and pasted' into other applications. Data can also be saved to disk in standard ASCII text format.

Graph View

Graphs can be displayed both during and after data collection. Each channel can be displayed in its own graph or multiple channels can be displayed in the same graph. Axes can be set up manually, automatically or in chart recorder mode. Multipliers allow you to magnify areas of interest. Graphs can be 'copied' into the clipboard and then 'pasted' into reports.

Notes View

Notes view allows you to attach notes to data.

Player View

Displays previously recorded data. It enables you to scroll quickly through stored files to compare results on successive runs. The player can be used to examine old data whilst new data is still being recorded.

Program modes

PicoLog for Windows works in two modes: player mode for displaying previously recorded data and recorder mode for recording new data. You can have more than one copy of PicoLog for Windows running at once, so you can use the player to analyse old data whilst recording new data.

PicoLog can collect data from multiple converters at the same time. This not only allows a mix of voltage input units to be used on the same PC, but also allows other PC–based instruments such as the TC-08 Thermocouple Data Logger and the PicoScope 2203 USB Oscilloscope to be used at the same time.

Exporting Data

Data can easily be transferred (either as graphs or raw data) to other Windows applications by using the clipboard (copy and paste). Graphs can also be saved to disk (as Windows Metafiles) and data from the spreadsheet can be saved in text format. Current readings can be transferred using Dynamic Data Exchange (DDE). PicoLog also supports IP networking so it is possible to transfer measurements from a remote site via an existing LAN, company network, or even over the Internet.

Useful Features

Parameter Scaling can be used to convert raw data into standard engineering units. A wide range of equation and table lookup scaling options are provided.

Additional parameters can be calculated, for example to calculate power output from a boiler you may need to multiply a flow reading from one channel with the temperature difference between two further channels.

Alarm Limits can be set for each channel to alert the user should a parameter go out of a specified range.

PicoLog data acquisition software supports IP networking. This enables remote data collection from Pico's full range of data acquisition products.

New versions of PicoScope (oscilloscope software) and PicoLog (data logging software) are available for free download.

Jump to Software & Manual Downloads above


Data acquisition over the net

PicoLog and EnviroMon now have support for Internet Protocol (IP) networking. More info: click here
This enables data to be transferred from a remote site via an existing LAN, company network or even over the Internet.


PicoScope Software

PicoScope is the standard in PC Oscilloscope software, but it is much more than just an oscilloscope.

It's your complete test and measurement lab in one easy–to–use application.

PicoScope software

If you buy a traditional oscilloscope, you get an oscilloscope. Buy a traditional spectrum analyzer, you get a spectrum analyzer. Buy a traditional waveform generator, you get a waveform generator. Buy a PicoScope USB Oscilloscope and – thanks to power of PicoScope – you can get an oscilloscope, spectrum analyzer and signal/waveform generator all in one. (With PicoScope's ability to continuously stream data you can even use your oscilloscope for data acquisition.)

Free updates

You can't usually upgrade traditional test equipment. If the device is upgradable it will often need a time–consuming and expensive trip back to the manufacturer. PicoScope software is regularly updated with feature extensions and improvements. These can be downloaded for free from our website and ensure that your oscilloscope will always have the latest features and enhancements. What other test equipment gets more powerful with age?
Easy to use

PicoScope 6 is powerful yet easy to operate. You benefit from the power of the PC, and the familiar Windows interface and controls, making the software easy to use. It has tooltips and help pages to explain all the features.

Unrivalled performance

PicoScope uses the latest Windows technologies to give you the performance you need. Fast capture rates make it easier for you to see fast–moving signals, while fast data processing ensures your PC is able to process even the most complex of waveforms.

All this and it's free

PicoScope is free to download and use. Download your copy of PicoScope 6 today and experience the power and performance for yourself. (If you haven't yet purchased your PicoScope Oscilloscope you can use the demo mode to test many of the features.)

Main Functions and Features of PicoScope

Click a link and see what PicoScope can do for you:

Oscilloscope — turn your PC into a powerful oscilloscope
XY oscilloscope — plot one channel again another
Digital persistence — spot glitches the easy way
Math channels — invert, add, subtract, multiply and divide channels, or create your own functions
Automated measurements — display a wide array of measurements and statistics
Spectrum analyzer — view signals in the frequency domain
Arbitrary waveform generator — generate a waveform of almost any shape
Mask limit testing — see when a waveform or spectrum goes outside a specified area
Serial decoding — decode data from a serial bus such as CAN bus
Best of the rest — other features

Jump to Software & Manual Downloads above

Thermocouples

Thermocouples are the most popular temperature sensors. They are cheap, interchangeable, have standard connectors and can measure a wide range of temperatures. The main limitation is accuracy, system errors of less than 1°C can be difficult to achieve.

How they work

In 1822, an Estonian physician named Thomas Seebeck discovered (accidentally) that the junction between two metals generates a voltage which is a function of temperature. Thermocouples rely on this Seebeck effect. Although almost any two types of metal can be used to make a thermocouple, a number of standard types are used because they possess predictable output voltages and large temperature gradients. The diagram below shows a K type thermocouple, which is the most popular:

thermocouple

Standard tables show the voltage produced by thermocouples at any given temperature, so for example in the above diagram, the K type thermocouple at 300°C will produce 12.2mV. Unfortunately it is not possible to simply connect up a voltmeter to the thermocouple to measure this voltage, because the connection of the voltmeter leads will make a second, undesired thermocouple junction. To make accurate measurements, this must be compensated for, a technique known as cold junction compensation (CJC). In case you are wondering why connecting a voltmeter to a thermocouple does not make two additional thermocouple junctions (one for each lead), the law of intermediate metals states that a third metal, inserted between the two dissimilar metals of a thermocouple junction will have no effect provided that the two junctions are at the same temperature. This law is also important in the construction of thermocouple junctions. It is acceptable to make a thermocouple junction by soldering the two metals together as the solder will not affect the reading. In practice, however, thermocouples junctions are made by welding the two metals together (usually by capacitive discharge) as this ensures that the performance is not limited by the melting point of solder.

All standard thermocouple tables allow for this second thermocouple junction by assuming that it is kept at exactly zero degrees centigrade. Traditionally this was done with a carefully constructed ice bath (hence the term 'cold' junction compensation). Maintaining a ice batch is not practical for most measurement applications, so instead the actual temperature at the point of connection of the thermocouple wires to the measuring instrument is recorded.

Typically cold junction temperature is sensed by a precision thermistor in good thermal contact with the input connectors of the measuring instrument. This second temperature reading, along with the reading from the thermocouple itself is used by the measuring instrument to calculate the true temperature at the thermocouple tip. For less critical applications, the CJC is performed by a semiconductor temperature sensor. By combining the signal from this semiconductor with the signal from the thermocouple, the correct reading can be obtained without the need or expense to record two temperatures. Understanding of cold junction compensation is important; any error in the measurement of cold junction temperature will lead to the same error in the measured temperature from the thermocouple tip.

Linearisation

As well as dealing with CJC, the measuring instrument must also allow for the fact that the thermocouple output is non linear. The relationship between temperature and output voltage is a complex polynomial equation (5th to 9th order depending on thermocouple type). Analogue methods of linearisation are used in low cost themocouple meters. High accuracy instruments such as the Pico TC-08 store thermocouple tables in computer memory to eliminate this source of error.

Thermocouples Types

Thermocouples are available either as bare wire 'bead' thermocouples which offer low cost and fast response times, or built into probes. A wide variety of probes are available, suitable for different measuring applications (industrial, scientific, food temperature, medical research etc). One word of warning: when selecting probes take care to ensure they have the correct type of connector. The two common types of connector are 'standard' with round pins and 'miniature' with flat pins, this causes some confusion as 'miniature' connectors are more popular than 'standard' types.

When choosing a thermocouple consideration should be given to both the thermocouple type, insulation and probe construction. All of these will have an effect on the measurable temperature range, accuracy and reliability of the readings. Listed below is our (somewhat subjective) guide to thermocouple types.

Type K (Chromel / Alumel)

Type K is the 'general purpose' thermocouple. It is low cost, and owing to its popularity it is available in a wide variety of probes. Thermocouples are available in the -200°C to +1200°C range. Sensitivity is approx 41uV/°C. Use type K unless you have a good reason not to.

Type E (Chromel / Constantan)

Type E has a high output (68uV/°C) which makes it well suited to low temperature (cryogenic) use. Another property is that is non-magnetic.

Type J (Iron / Constantan)

Limited range (-40 to +750°C) makes type J less popular than type K. Main application is with old equipment that can not accept 'modern' thermocouples. J types should not be used above 760°C as an abrupt magnetic transformation will cause permanent decalibration.

Type N (Nicrosil / Nisil)

High stability and resistance to high temperature oxidation makes type N suitable for high temperature measurements without the cost of platinum (B,R,S) types. Designed to be an 'improved' type K, it is becoming more popular.

Thermocouple types B, R and S are all 'noble' metal thermocouples and exhibit similar characteristics. They are the most stable of all thermocouples, but due to their low sensitivity (approx 10uV/0C) there are usually only used for high temperature measurement (>300°C).

Type B (Platinum / Rhodium)

Suited for high temperature measurements up to 1800°C. Unusually type B thermocouples (due to the shape of their temperature / voltage curve) give the same output at 0°C and 42°C. This makes them useless below 50°C.

Type R (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/°C) and high cost makes them unsuitable for general purpose use.

Type S (Platinum / Rhodium)

Suited for high temperature measurements up to 1600°C. Low sensitivity (10uV/vC) and high cost makes them unsuitable for general purpose use. Due to its high stability type S is used as the standard of calibration for the melting point of gold (1064.43°C).

When selecting thermocouple types, ensure that your measuring equipment does not limit the range of temperatures that can be measured. Listed below is the range of temperatures that the 8 channel Pico TC-08 can measure. Note that thermocouples with low sensitivity (B, R and S) have a correspondingly lower resolution.

Thermocouple type  Overall

Range (°C) 

0.1°C

Resolution 

0.025°C

Resolution 

100..1800  1030..1800 
-270..790  -240..790  -140..790 
-210..1050  -210..1050  -120..1050 
-270..1370  -220..1370  -20..1150 
-260..1300  -210..1300  340..1260 
-50..1760  330..1760 
-50..1760  250..1760 
-270..400  -230..400  -20..400 

Precautions and Considerations for Using Thermocouples

Most measurement problems and errors with thermocouples are due to a lack of understanding of how thermocouples work. Listed below are some of the more common problems and pitfalls to be aware of.

Connection problems. Many measurement errors are caused by unintentional thermocouple junctions. Remember that any junction of two different metals will cause a junction. If you need to increase the length of the leads from your thermocouple, you must use the correct type of thermocouple extension wire (eg type K for type K thermocouples). Using any other type of wire will introduce a thermocouple junction. Any connectors used must be made of the correct thermocouple material and correct polarity must be observed.

Lead Resistance. To minimise thermal shunting and increase response times, thermocouples are made of thin wire (in the case of platinum types cost is also a consideration). This can cause the thermocouple to have a high resistance which can make it sensitive to noise and can also cause errors due to the input impedance of the measuring instrument. A typical exposed junction thermocouple with 32AWG wire (0.25mm diameter) will have a resistance of about 15 ohms / meter. The Pico TC-08 has an input impedance of 200kW so will have an error of less than 0.1% for 12 meters of such cable. If thermocouples with thin leads or long cables are needed, it is worth keeping the thermocouple leads short and then using thermocouple extension wire (which is much thicker, so has a lower resistance) to run between the thermocouple and measuring instrument. It is always a good precaution to measure the resistance of your thermocouple before use.

Decalibration is the process of unintentionally altering the makeup of thermocouple wire. The usual cause is the diffusion of atmospheric particles into the metal at the extremes of operating temperature. Another cause is impurities and chemicals from the insulation diffusing into the thermocouple wire. If operating at high temperatures, check the specifications of the probe insulation.

Noise. The output from a thermocouple is a small signal, so it is prone to electrical noise pick up. Most measuring instruments (such as the TC-08) reject any common mode noise (signals that are the same on both wires) so noise can be minimised by twisting the cable together to help ensure both wires pick up the same noise signal. Additionally, the TC-08 uses an integrating analog to digital converter which helps average out any remaining noise. If operating in an extremely noisy environment, (such as near a large motor) it is worthwhile considering using a screened extension cable. If noise pickup is suspected first switch off all suspect equipment and see if the reading changes.

Common Mode Voltage. Although thermocouple signal are very small, much larger voltages often exist at the input to the measuring instrument. These voltages can be caused either by inductive pick up (a problem when testing the temperature of motor windings and transformers) or by 'earthed' junctions. A typical example of an 'earthed' junction would be measuring the temperature of a hot water pipe with a non insulated thermocouple. If there are any poor earth connections a few volts may exist between the pipe and the earth of the measuring instrument. These signals are again common mode (the same in both thermocouple wires) so will not cause a problem with most instruments provided they are not too large. For example, the TC-08 has a common mode input range of -4V to +4V. If the common mode voltage is greater than this then measurement errors will result. Common mode voltages can be minimised using the same cabling precautions outlined for noise, and also by using insulated thermocouples.

Thermal Shunting. All thermocouples have some mass. Heating this mass takes energy so will affect the temperature you are trying to measure. Consider for example measuring the temperature of liquid in a test tube: there are two potential problems. The first is that heat energy will travel up the thermocouple wire and dissipate to the atmosphere so reducing the temperature of the liquid around the wires. A similar problem can occur if the thermocouple is not sufficiently immersed in the liquid, due to the cooler ambient air temperature on the wires, thermal conduction may cause the thermocouple junction to be a different temperature to the liquid itself. In the above example a thermocouple with thinner wires may help, as it will cause a steeper gradient of temperature along the thermocouple wire at the junction between the liquid and ambient air. If thermocouples with thin wires are used, consideration must be paid to lead resistance. The use of a thermocouple with thin wires connected to much thicker thermocouple extension wire often offers the best compromise.

Pico Products for measuring Thermocouples

The TC-08 interfaces up to 8 thermocouples of any type to a PC.



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