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Connecting temperature sensors to fieldbus

29 November 2008

Getting temperature measurements and sensor parameters into digital format is the thrust of recent announcements by major instrument manufacturers.

Yokogawa's wireless prototype
Yokogawa's wireless prototype

Temperature measurement may be the oldest and most widely measured process variable, and today it is solidly embracing a new type of transmitter: one that communicates using HART, Profibus, or Fieldbus Foundation protocols.

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PHOTO: Just for show: Yokogawa brought this wireless prototype instrument to its European users’ conference in September 2008.

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To be sure, the digital format is more expensive than the standard 4 – 20 mA current loop wiring that has become pervasive over the last 40 years. But today digital instruments (especially in the case of HART-based instruments) are manufactured in such large volumes the price difference is all but eroded. In fact, with some instrument makers, it would be difficult to order an instrument *without* HART electronics built in.

WIRELESS TEMPERATURE

Almost every discussion of fieldbus these days includes mention of the hottest topic: wireless data transmission. The new thrust into wireless sensors heralds a new era for temperature and pressure measurements. At the present it is confined to HART, as Wireless HART has become an industry standard and all manufacturers are scrambling to provide conforming products for what they see as potentially a big market. There are millions of HART temperature instruments already in place and a simple, low cost, loop-powered radio adapter will bring their operation into the wireless world. Honeywell and Emerson are the early wireless instrument adopters, and Yokogawa has now announced it will have wireless products in the first quarter of 2009.

All of these major companies have long pedigrees in temperature measurement, with high accuracy and well constructed instruments. Yokogawa, for example, boasts its YTA temperature transmitters are ‘SIL 2 right out of the box.’ so that no optional hardware of specialised software are necessary as it is already is certified by TÜV. With mean-time-between-failures (MTBFs) of 172 years, these instruments are for all practical purposes, lifetime investments.

‘Wireless enables improved plant asset management,’ explained Henk A. van der Bent, of Yokogawa’s IA Field Networks, at a recent users’ conference in Barcelona. ‘It is a cost-effective way to increase process and asset monitoring, and it frees up cable resources for higher priority measurements.’ He says :Yokogawa will offer wireless HART evaluation kits which will include its YTA temperature transmitters for two-wire RTD and thermocouples. The transmitters will have typical battery lives of five years. The kits will also include a loop powered HART wireless adapter to unlock ‘secondary data.’ in the thousands of HART-based YTAs that are already installed.

He makes it obvious that Yokogawa much prefers the wireless ISA100 standard, which, unlike the wireless HART standard, isn’t finished yet. ‘Our focus is on field to control room integration, and we recognise that different types of wireless must co-exist. Wireless is not limited to process automation,’ Mr. van der Bent said. ‘We must have a wireless system that accommodates not only HART, but also FF, Profibus, Modbus, and others,’ he said.

He notes that ISA100 is more secure than wireless HART, and that it potentially will allow orders of magnitude more devices in the network. And not just more devices, but more types of networks than HART’s mesh network. And in this sense, it will be more efficient: ‘Higher speed, longer battery life, time synchronisation, lower latency,’ he says.

FIELDBUS INTEGRATION

There are basically three types of temperature transmitter products in today’s market: temperature head transmitters, complete field-mounted transmitters, and rail-mounted transmitters designed to be housed inside small cabinets located close to the process. Head transmitters are closest to the sensors but they may not be practical if vibration or ambient temperature conditions are too harsh, in which case field or rail mounted transmitters are preferable. Manufacturers are making all of them today in fieldbus-ready formats.

Virtually all of the primary instrument manufacturers—ABB, Emerson, Endress+Hauser, Moore Industries, Siemens, Yokogawa, and others—offer complete lines of temperature transmitters products: head, field, and rail-mounted. In this brief survey we will look at what has been introduced in recent months.

Temperature head transmitters resemble hockey pucks and are installed at the top of the thermowell housing. Endress+Hauser’s loop-powered TMT84 and TMT85 transmitters are out-of-the-box ready for connection to Profibus or Fieldbus Foundation networks.

And along with the communications capability comes a surprising amount of digital intelligence packaged into these small units. They have two sensor inputs for redundant operation and have all international approvals for operation in hazardous areas. Internal diagnostics are capable of detecting short circuits, wiring errors, cable open circuits, and even corrosion detection on the sensor cables. There is a backup function and drift detection during two-sensor operation. The accuracy of the overall thermometer can be ‘significantly’ improved with sensor-transmitter matching using the Callander Van Dusen coefficients from the thermometer calibration. Technicians who have to install dozens of these transmitters will undoubtedly appreciate the TM85, which uses spring clip technology for safe and rapid connection.

E+H's fieldbus-enabled temp head transmitter
E+H's fieldbus-enabled temp head transmitter

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PHOTO: Rapid and easy connection, Endress+Hauser’s TMT85 spring clip technology for its fieldbus-enabled temperature head transmitter.

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ABB has introduced what it calls a ‘new generation of HART temperature transmitters’ in its field mounted TTF350, which is an upgrade to the TTF300. The TTF350 is now the high-end model in the TTx300 HART family of temperature transmitters; it features dual compartment housing technology and three cable entries with two sensor inputs and a large LCD display with configuration options. The dual compartment housing technology separates the connection area from the system electronics. As is pretty much standard in all temperature products today, the TTF350 has high measuring accuracy (±0.1 degree with long-term stability of ±0.05% per year), sensor drift detection, sensor input redundancy, a portfolio of diagnostic functions, and SIL 2 and ATEX protection features.

Longtime temperature instrument maker Moore Industries in California recently introduced its TFZ programmable FF transmitter which it says requires only 10.5 mA for normal operation, so it can be networked with up to 32 field devices on one FF H1 segment.

Operating on a FF network, it can be remotely programmed or interrogated over the segment using standard FF tools. To give readers and idea of what this means, here are some example FF function blocks that can be used with the instrument:

Resource Function Block (RB)—

Contains diagnostic information, hardware and electronics information (memory, manufacturer identification, device type, software tag) and display configuration parameters.

Temperature Transducer Block (TB)—

Contains temperature measurement data, including sensor and terminal temperature. It also includes information about the sensor type, engineering units, linearisation, re-ranging, damping, temperature compensation and diagnostics.

Analogue Input Block (AI)—

Processes measurements from a sensor and makes them available to other function blocks. The output value from the AI block is displayed in engineering units and contains a status indicating the quality of the measurement.

Siemens’ Sitrans TH400 is a head-mounted temperature transmitter that communicates by either FF or Profibus, but most of Siemens’ digitally communicating temperature products have been HART-based.

It has recently launched two new temperature transmitters which use a two-wire system and are designed for rail mounting: the Sitrans TR200 and Sitrans TR300. Sitrans TR200 is configured using a PC, whereas Sitrans TR300 can be parameterised via a HART interface.

The mounting rail system also offers the advantage that the measuring points can be set up centrally away from any harsh environmental conditions. The devices are thus simple to access and can be easily protected against heat and vibration.

Both devices feature a diagnostics LED, which indicates the function status. In this way, open circuits or short circuits can be quickly detected and corrected especially in the case of a large number of measuring points. The temperature transmitters have a measuring input for resistance thermometers or thermocouple elements and, in addition, can evaluate resistance-based sensors and millivolt signals. The new transmitters are thus suitable for universal use in all sectors.

VERY HIGH TEMPERATURES

When very high temperatures are encountered in the manufacturing industries, special equipment must be used to accurately record these values. There are many potential applications for high temperature measurement, including ovens, dryers, heat treatment lines in the metal and glass industries, paper, plastic, textiles and semiconductor manufacturing.

Micro-Epsilon's
Micro-Epsilon's

A significant proportion of the activity of the French company Pyro-Contrôle involves manufacturing specific sensors for the most demanding process industries. It has developed a special flame-temperature sensor specially for studying the combustion and flame temperatures of gases in the range between 400 and 1,700°C.

This bare-wire thermocouple is characterised by a very small measuring point with a diameter of only a few microns. As a result, it has no effect on the flame temperature and significantly reduces measurement errors by minimising losses due to radiation and heat conduction.

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PHOTO: This "Optris CT Hot" infrared temperature measurement system from Micro-Epsilon not only handles high temperatures, it can work in environments up to 250°C without any cooling of the sensor head.

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The main application for this product, which is manufactured under licence from Gaz de France, is temperature measurement at the heart of flames to meet the needs of scientific laboratories and facilitate the adjustment of certain burners, as well as the development of special welding procedures.

ABB has developed a whole range of high temperature sensors, which it introduced them at Hannover Fair this year. The new generation is called SensyTemp and can measure temperatures up to 1650°C. The basic difference in the three levels of sensors offered is the type of thermowell they fit into.

The SensyTemp TSH210 has a metal thermowell with optional ceramic inner tube, and is suitable for media temperatures from 600 to 1300°C depending on the protection tube material and thermocouple. Common applications can be found in reheating and hardening furnaces, smelting operations, blast furnaces, air-circulation furnaces, waste incineration and flue-gas desulfurisation.

The TSH220 temperature sensors feature ceramic thermowells as well as additional ceramic inner tubes and are intended for media temperatures up to 1800°C and are typically equipped with precious metal thermocouples. They are used in cement and brick manufacturing, porcelain and ceramics industry, garbage and hazardous waste incineration, glass industry and steel industry.

Finally at the top end are the SensyTemp TSH250 for applications in the glass industry. These are designed with a ceramic thermowell at whose tip a platinum sleeve is attached. A platinum-coated ceramic thermowell qualifies these temperature sensors for measurement in molten glass at a temperature of up to 1650°C. Due to the high temperatures, they are available only with precious metal thermocouples.

IR FOR HIGH TEMPERATURES

Measuring high temperatures takes special equipment for direct readings and often a better idea is to keep your distance and measure it with non-contact infrared technology. The trouble is, as anyone who has ever been in a steel mill knows, even if you are some distance away from the molten steel, the ambient temperature can be very hot, and this can be troublesome for delicate instruments—even noncontact IR temperature measurement systems.

For this reason Micro-Epsilon has developed an infrared thermometer which they say is ‘ideal for use in high temperature production lines and process environments,’ because it can work in high ambient temperatures. The instrument, called Optris CT hot, enables temperature measurements from –40°C up to 975°C in ambient temperatures of up to 250°C without any cooling of the sensor head. This is made possible due to several new design features: new detector materials, a new mechanical design and new high temperature cabling. Micro-Epsilon says it has has used its in-house development capabilities to design new semiconductor electronics and connectors, as well as a more robust, compact sensor housing that allows the unit to be adapted to suit standard industrial mountings, flanges and brackets.

Multiple CT hot sensors can be used inside heat treatment lines with several process chambers to measure temperatures of the web or fabric at different points over the length and width. The benefits are that information can be obtained about the drying curve of the material, moisture content, and non-uniform application of coatings and bonding.

In addition, process engineers can optimise energy consumption, speed and product quality, by reducing the process temperature once the product is at its desired temperature. The narrow beam optics on the CT hot enables oblique aiming, which means the user can now measure thick fabrics or light fabrics using the same sensor without any calibration adjustment.

Ulrich Kienitz, General Manager at Micro-Epsilon, explains that the latest heat treatment lines are often built inside a machine in a very compact design, in order to optimise process efficiencies. In these applications, sighting tubes for temperature sensors can no longer be positioned outside the process chamber. Compact, non-contact, infrared sensors are therefore required inside the chamber itself. The CT hot with a compact head and no requirement for additional cooling, fits easily into the restricted space.

Mr Kienitz comments: ‘In textile or fabric drying processes, for example, companies can position multiple CT hot sensors along the inside of an oven or drying machine, which is typically around 30 metres in length. The sensors are able to monitor the temperature of a complete sheet or strip of fabric. By analysing the data from the sensors and looking for unusually high temperature gradients and fluctuations, operators can gauge the humidity of the material and change the process operating conditions such as speed.’


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