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6 February 2014

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Data Acquisition Intelligence
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Monitor Issue 5: How to Measure Temperature with a PC

CONTENTS

How to use your PC to measure temperature: Choosing Sensors | Brief comparison of thermocouples and RTDs | Getting the temperature data into your PC | Thermocouples | RTDs, Thermistors, PRTs, Pt100 | Glossary M-O


How to Use Your PC to Measure Temperature: Choosing Sensors

Temperature monitoring is central to the majority of data acquisition systems, be it to save energy costs, increase safety, reduce testing time. Whatever your reasons, you will need a device to measure the temperature—a temperature sensor. Thermocouples, resistance temperature devices (RTDs), thermistors and infrared thermometers are all types of temperature sensor. Which you choose depends on factors like your expected maximum and minimum temperatures, the accuracy you need and your environmental conditions.

The most popular sensors are thermocouples and RTDs. We've covered these in more detail below, concentrating on potential problems when using them in computerised temperature measurement.


Brief Comparison of Thermocouples and RTDs

ADVANTAGES                          DISADVANTAGES
                       Thermocouple
* Low cost                  |  * Accuracy        
* Wide temperature range,   |   (typical off-the-shelf 
  eg -200 to 1200 oC        |    accuracy +/-1 to 2 oC)
* Rugged                    |  * Ageing              
 (can be glued to a variety |   (Accuracy decreases with 
  of substrates and with-   |    age, as the electro and  
  stand harsh environments) |    chemical properties   
* Fast response time owing  |    change. Ageing speeds up
  to small size             |    in harsh environments and
                            |    when the thermocouple is
                            |    regularly heated and
                            |    cooled to the extents of
                            |    its range.
                            |  * Noise
                            |   (tend to produce noisy 
                            |    signals over long 
                            |    distances)
                            |
               Resistance Temperature Device
* Stable                    |  * Expensive
 (less prone to ageing      |  * Larger and so slower to 
  than thermocouples)       |    respond
* Accurate as standard      |  * Narrower temperature 
  +/- 0.25 oC               |    -200 to 650 oC

For a table of thermocouple and PRTD accuracies see the Microlink site.


Getting the Temperature Data into your PC

OK, let's suppose you've chosen a sensor - how do you get data from it into your PC? The answer is a data acquisition interface with suitable software such as Windmill. The interface hardware might plug into a port on your PC, for example the USB or Ethernet port. Older systems were often cards which plugged into the expansion slot of your computer. You wire the sensor to the data acquisition unit and the computer can now monitor temperatures. (Other methods of connecting the hardware to the PC were discussed in our second newsletter.)

The next two sections go into more detail about thermocouples and RTDs - concentrating on things you need to be aware of when connecting them to a compturised system.


Thermocouples - Rugged, Versatile and Low Cost

Thermocouples are popular temperature sensors because they are cheap, versatile and sturdy. They consist of two dissimilar metals joined together, making a continuous circuit. If one junction has a different temperature to the other, an electromotive force (voltage) is set up. This voltage varies with the temperature difference between the junctions. If the temperature at one junction is known, the temperature at the other junction can be calculated.

Types of Thermocouple
There are several types of thermocouple, labelled with letters according to their constituent metals. A K-type thermocouple, for example, is made up of chrome and Alumel. The metals give the thermocouples differing properties, such as temperature ranges and accuracy.

Potential Pitfalls in a Computerised Thermocouple System

  1. The "Cold Junction" Reference Measurement
    The system depends on knowing the temperature of one of the thermocouple junctions (the cold junction). Housing this junction in an isothermal box will keep the temperature constant, and a cold junction sensor in the box will tell the system the temperature. In our plug-in card example, the isothermal box sits outside the computer. You would connect the thermocouple wires to screw terminals in the box, and connect the terminals to the card with a ribbon cable.
  2. Attaching the Thermocouples to Metal Surfaces
    If the thermocouples are directly attached to a metal surface, particularly one carrying its own voltage such as a heating element, you need to isolate the signals. This will prevent high voltages in the monitored item damaging the data acquisition equipment. It will also make the measurements "floating", letting you record the small thermocouple voltage in the presence of high voltages.
  3. Linearisation
    The voltage produced by a thermocouple does not change linearly with temperature - presenting a problem for the data acquisition system. A good solution is to use software to obtain the correct temperature in, say, oC or oF. Windmill will do this for you automatically, with built-in linearisation for B, E, J, K, N, R, S and T type thermocouples.
  4. Using the Wrong Type of Thermocouple Lead
    You need to connect the thermocouple to the data acquisition equipment using the correct type of extension or compensating lead. This is made of either the same material as the thermocouple metals, or material with similar characteristics.
  5. Long Thermocouple Leads - Noisy Signals and Added Wiring Costs
    Thermocouple leads are often many metres long, and have a higher resistance than normal copper wire. This means that the leads can act as aerials, picking up environmental electrical noise that contaminates the voltage signal. It might also mean expensive wiring costs. In this case you need either to take precautions against noise, or distribute data acquisition units - placing them close to the thermocouples on Modbus, RS485 or Ethernet networks for example.

We recommend the Windmill 751-TC package (USB) or Microlink 851-TC package (Ethernet / TCP-IP) for thermocouple measurement.


RTDs - Accurate and Stable

Resistance temperature devices (or detectors) rely on the principle that the resistance of a metal increases with temperature. When made of platinum, they may be known as platinum resistance thermometers (PRTs), and when specified to have a resistance of 100 ohm at 0 oC, as Pt100.

Potential Pitfalls in a Computerised RTD System

  1. Errors Arising from Lead Resistance
    When the resistance to be measured is small, the resistance in the leads to the RTD can significantly affect accuracy. Several methods exist for monitoring RTDs, which address the problems associated with lead resistance. These methods include balanced bridges and constant current sources.

    Constant current source measurements give excellent results for all wiring configurations, including 2 wire, 3 wire, 4 wire and 4 wire compensated. The most accurate results are obtained using a 4 wire arrangement. Each RTD requires the data acquisition hardware to provide a constant current source. The current flows through the RTD and the voltage drop over the RTD is measured. Using Ohms law the value of the resistance of the RTD can be calculated.
  2. Converting the Resistance to a Temperature
    Software like Windmill automatically converts the resistance measurement to a temperature in your choice of engineering units.

We recommend the Windmill 752-RTD package for RTD temperature measurement.


By Jill Studholme

Ordering a Temperature Monitoring System
You can order a temeperature monitoring system from our on-line shop. Our USB thermocouple system is currently on offer with 49% off. We also offer thermocouple monitoring over Ethernet and Internet.


GLOSSARY OF DATA ACQUISITION TERMS: M-O

(For letters A-L please see our web site Glossary.)

M
Symbol for mega (thousand).
Mains
Source of electrical power, normally the electricity supply system.
Mains Frequency
Electricity ac supply frequency; 50 Hz in UK, 60 Hz in US.
MES
Manufacturing Execution System.
Metre
The SI fundamental unit of length, equal to 1.093 yards. (Meter in US.)
MIS
Manufacturing Information System.
MMI
Man Machine Interface. Also known as human machine interface. The communication between the computer system and the people who use it.
Modular
Form of construction in which hardware or software units, often with differing functions, are quickly interchangeable.
Multiplexing
Where each signal is switched in turn to a single analogue-to-digital converter. As opposed to where one A-D converter is used for each signal in simultaneous sampling.
Modbus
An industrial networking system that uses peer-to-peer communications. Developed by Modicon
N-Type Thermocouple
Nicrosil-Nisil thermocouple with a temperature range of -200 to 1200 oC.
Network
The physical interconnection of devices sharing a communications protocol.
Node
A device with a direct point of access to a network.
Noise
Any unwanted electrical signals contaminating the signal to be measured.This noise may be electronic noise which is an artefact of semiconductor construction techniques and can be reduced.
Alternatively the noise may be caused by environmental factors. This type of noise can be the result of poor positioning or screening of signal wiring. This may result in mains frequency or RF pickup contaminating the required signal.
Non-Conforming
A product outside manufacturing limits but not necessarily defective.
Non-Destructive Testing
Method of inspecting materials and products without affecting their subsequent properties and performance. Abbreviation NDT.
Nyquist Theorem
The Nyquist theorem demands that a signal be sampled at at least twice its maximum frequency. To get an accurate picture of a waveform however, a sampling rate of 10 - 20 times the highest frequency is better.
OEM
Original Equipment Manufacturer. A company which makes basic computer hardware for other manufacturers to build into their products.
Offset Error
If you get a reading other than zero for a zero condition then you have an offset error: every reading will be inaccurate by this amount.
Ohm
SI unit of resistance, such that one ampere through it produces a potential difference of one voltage.
On-Off Control
A simple control system which is either on or off.
Output
Information leaving a device.

Copyright Windmill Software Ltd * Reprinting permitted with this notice included * For more articles see http://www.windmill.co.uk

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