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http://www.omniflex.com		 2004-11-12 00:00:00
No.13

 

Omnifacts is published by Omniflex Pty Ltd. This newsletter is designed to inform and educate in the areas of monitoring and control.
This publication may be freely redistributed if copied in its entirety.  Portions of this newsletter may be reprinted with permission
Thermocouples - A Primer
Ian Loudon

  Thermocouples are temperature sensors, which consist of two different wires made of specially selected metals or alloys. The wires are joined at one end and this end forms the actual sensor and measurement point. Often this joint is placed in the tip of a specially constructed temperature probe for protection. The open circuit voltage, or "emf", measured across the other "open" ends of the two thermocouple wires is approximately proportional to the temperature difference between the location of the probe (the joined end of the two wires) and the point where the "open" end is located.

Thermocouples are inexpensive, simple, durable, extremely repeatable, and relatively accurate. They can measure temperatures from nearly absolute zero to about 2500 degrees C (about 4500 degrees F). A few important comments will help "understand" thermocouples and the terminology, namely:

  1. The metals used for the wires sometimes have names that are not common at all. It is therefore easier to refer to thermocouples constructed of specific metal combinations by a capital letter (K, J, B, E, R, S.....etc.). The letter symbols are internationally accepted as names for thermocouples.
  2. The wires used for specific thermocouple types often have insulation which is colour-coded. It is then easy to tell different types of thermocouple apart in practical installation.
  3. Maximum thermocouple voltages are in the region of 20 to 80 milliVolts, over the temperature range that typically spans 0-1000 or 0-2000 degrees C. This means that the measured voltage changes only a few to a few tenths of microvolts for every degree.
  4. Since ambient temperature at the measurement location (the open ends of the thermocouple wires) can vary, the measured voltage will also vary. In order to obtain an absolute reference, thermocouples are calibrated with the open end at exactly 0 degrees C. The open end is often referred to as the "Cold Junction" since traditionally an ice bath (mixture of water and ice) was used in order to maintain this temperature.
  5. In modern instruments, the thermocouple reading is automatically adjusted for the fact that this "reference" (Cold Junction) temperature is not actually immersed in an ice bath and therefore differs from 0 degrees C. This is referred to as "automatic cold-junction compensation (CJC)". However, thermocouple wires must run right up to the instrument terminals. They cannot be terminated at the terminal block rail, with copper wires running to the instrument, as the instrument would then read the temperature of copper wires, and this could be at a different temperature to the open end of the thermocouple wires.
  6. If thermocouple wires have to be extended, this must be done with wires made of the same (or similar) metals or alloys. These are normally referred to as "compensation cables".
  7. Over any temperature range, the thermocouple measured emf is not exactly proportional to temperature. Fortunately this non-linear behaviour is repeatable, and is well documented in the standards for thermocouples. This nonlinearity can spoil potential accuracy of the temperature measurement, if not taken into account. Modern instruments are typically able to linearise the readings and make them exactly proportional to temperature.
  8. Thermocouple linearisation is based on Calibration Tables. These tables characterise each thermocouple type, giving the exact reading in microvolts, at one degree C steps, when the Cold Junction is exactly at 0 degrees C. Always refer to the reference tables when in doubt about the accuracy of thermocouple readings.


SMS OPC Server for GSM Cellular Networks

Omniflex has developed an OPC server for GSM cellular networks to support Short Message Service (SMS) transmission of data from remote outstations that require monitoring. The suitability of GSM networks has become a reality due to their coverage, reliability and low cost of ownership.  There is no infrastructure cost to consider as you have with in house radio based systems and you can extend your monitoring country wide – in fact it is possible to use the technology outside the countries borders too. A simple SIM card purchase with suitable GSM provider package is all that is required.

The Omniflex OPC server and Maxiflex RTU  Outstations Integrate to form an easy to use front end for SCADA monitoring systems. Reporting by exception (Change of State) or on a time cycle basis the Maxiflex outstations are capable of interfacing to Digital and Analogue signals or even Intelligent devices with serial protocols. eg Hart, Modbus or custom etc. the Maxiflex outstations can be used in Telemetry mode or as Controllers at the front end. Setpoint parameter changes may be down loaded via the OPC server where and when required. Forced outputs are also a feature of the system.

The OPC server provides the interface to any windows based application supporting the OPC standard. For example SCADA systems or Asset management systems. A feature of the system is a call count monitoring system to keep a check on call traffic and provide early indication of over use. This is especially important where change of state inputs (eg Door Alarms etc) on the outstation can trigger unnecessary calls if a fault develops on a door alarm. The OPC server allows individual selection of digital input for change of state and these can be changed on-line without going out to the RTU.

The Maxiflex RTU has a dedicated GSM Network Interface Module (NIM) which fits into the Maxiflex Base like any other module providing a compact and neatly integrated solution. The GSM antennae is simply attached directly to the front of the module.


Omniterm TTP Universal Trip Amplifier

 OVERVIEW

Use the Omniterm TTP Dual Trip Amplifier to monitor any dc instrumentation signal, including thermocouples, resistance bulbs, mVolts, Voltage and Current; and output two high or low alarms/trips via Form C output contacts.

This instrument uses easy software configurability without the need for any calibration to set any input signal type and range. Setpoints are set directly in engineering units using the same software configuration tool.

BENEFITS

  • Reduce stockholding by only needing to hold one type in stock
  • No field calibration means lowest ranging, installation and commissioning costs.
  • Software configuration means range setting can be done in minutes, an configuration can be stored for effective asset management.



Last Month:

Last month we talked about several important topics including:
 

  • Thermocouples - A Primer
  • SMS OPC Server for GSM Cellular Networks
  • Omniterm TTP Universal Trip Amplifier

If you missed these or other key discussions,
you can find the back issues on the newsletters page of our website:

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This publication may be freely redistributed if copied in its ENTIRETY.  Portions of this newsletter may be reprinted with permission.

(c) Copyright 2003 OMNIFLEX PTY LTD 

  
In this Issue
• Thermocouples - A Primer
• SMS OPC Server for GSM Cellular Networks
• Omniterm TTP Universal Trip Amplifier
•
   
• Last Month's Issue
• Subscribe/Unsubscribe

Omniflex Omniterm B series Products cater for all Themocouple and RTD types. If you have one thats not on our list ask us and we will add it.

All Omniflex Product Linearize Thermocouples and RTDs to Temperature

Omniterm B configuration software is free - download it from our website.

www.omniflex.com

Omniflex has developed OPC servers for :

  • SMS service GSM Networks
  • Ethernet
  • Modbus Serial RS232/485
  • Conet LAN

The Maxiflex GSM solution provides a low set up cost infrastructure for telemtry over wide areas

Maxiflex and it's OPC server delivers the Data directly to Windows based  supervisory systems

Omniterm TTP Programmable Limit Alarms with Universal Input.

  • Thermocouple
  • RTD
  • Voltage
  • millivoltage
  • Current
  • Two Wire Transmitter

 

Omniterm TTP can also be used to power a field or head mounted two wire transmitter and provide intrument loop limit alarms

A single TTP covers many limit alarm applications and thus can reduce dramatically critical spares holding requirements