How to measure temperature and avoid burns in the process, part 7
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How to measure temperature and avoid burns in the process, part 7

Types of temperature transmitters

Temperature transmitters are available in distinct types of housings, which are adequate for various kinds of applications.

Hockey puck transmitters

This approach is simple to install and adequate for applications that demand compact solutions. Hockey puck transmitters are known by that name due to their resemblance and comparable size with ice hockey pucks.

These transmitters are available in two sizes, known as DIN A and DIN B. The dimensions of each type are specified in the DIN 43729 standard.

DIN A thermowell head dimensions and a DIN A type temperature transmitter

DIN A format dimensions are larger, with a diameter of 60 mm and a height of 35 mm. They usually offer more functions and have more features thanks to their larger housing.

DIN B thermowell head dimensions and a DIN B type temperature transmitter

DIN B format dimensions are smaller, with a diameter of 45 mm and a height of 20 to 23 mm. This type is ideal for installation in small spaces.

DIN A and DIN B transmitters are designed to be mounted on the head’s connection compartment of a thermowell. These devices usually have an IP rating of IP20, have exposed connection terminals and accept the connection of both thermocouples and RTDs.

They are designed to allow their installation as close as possible to the temperature sensor to minimize the need of extension or compensation cable.

If the transmitter supports HART, Foundation Fieldbus, Profibus PA or Ethernet APL communications, one of the most important uses of the diagnostic data sent by the device is its ambient temperature, therefore the use of a remote asset management solution is the fastest and simplest method to detect problems related to this issue.

Since their dimensions are standardized, they are easy to replace, whether by an identical device or by an equivalent device from another supplier.

Hockey puck transmitters are usually the first choice with their lower cost among temperature transmitters.

DIN rail mounted transmitters


DIN rail mounted temperature transmitters

This type of devices is designed to be mounted on a DIN rail, which can be installed in either a control cabinet or a field cabinet that can provide the adequate environmental protection. They are built with IP20 housings, so they should not be mounted in the field without a protective housing.

A high density DIN rail mounted temperature transmitter application

They are especially useful in applications with a high density of temperature sensors. This kind of applications do not offer ample space, therefore placing the transmitters in a nearby cabinet simplifies the installation, and the relative low cost of DIN rail transmitters is a useful advantage. DIN rail temperature transmitters can feature additional functionalities, such as intrinsically safe galvanic isolation, or/and switch outputs that can be used as alarms or as digital outputs.

Field mounted temperature transmitters

For applications that require field mounting, manufacturers offer temperature transmitters that feature robust housings adequate for the harsh environments that are typical in field installations.

These housings are typically manufactured from steel or aluminum through a casting process and can offer a level of environmental protection up to IP 67. These transmitters can be mounted wherever they may be required.

Field mounted temperature transmitter

The transmitter is usually mounted on the top of the thermowell or, if necessary due to high environmental temperatures, a temperature extension tube. If these measures are not effective enough, another option consists of using a separate remote transmitter which can be mounted in a less aggressive environment while it is still connected to the temperature sensor.

Comparison between Local and remote mounted temperature transmitter

Field mounted transmitters can use a single volume housing, but this can be problematic in case of water ingress, which could cause severe damage to the device’s electronics. To prevent this issue, watertight dual volume housings can be used.


Dual volume housing for temperature transmitters

One volume contains the electronics, and the other volume contains the connections. In this way the fragile electronics are protected by the watertight compartment.

Hazardous area applications

Although both thermocouples and RTDs are considered simple electrical devices, 4-20 mA temperature transmitters are not.

Scheme of an intrinsically safe, galvanically isolated 4-s0 mA temperature transmitter
A Phoenix Contact DIN rail mounted universal temperature transmitter with an embedded intrinsically safe galvanic isolator

Therefore, they need to either have a certification of intrinsic safety and be connected to the controller using an intrinsic safety barrier, or their housings must have a Ex d or a Ex e certificate.

Functional scheme of the Phoenix Contact universal the temperature transmitter pictured above

For these applications, DIN rail mounted transmitters can include intrinsically safe galvanic isolation. If this is the case, no additional barriers are required.

Field device manufacturers usually do not actually make different versions of their transmitters. The usual practice is to design a device with an Ex-e, Ex d or Ex de housing and make the internal electronics intrinsically safe by default. They include the corresponding certifications according to the version selected by the manufacturer.

This temperature transmitter has an Ex-d housing (flameproof), and its internal electronics are intrinsically safe. allowing the use of either one of those protection methods

This practice may not seem to be logical at first view, but when factors like economies of scale are considered, then this approach is the most efficient from the production cost point of view.

Unique features

There are many optional features that can improve the transmitter performance. For example, some transmitters can be ordered with support for dual temperature sensors. This feature enables end users to obtain a duplicate temperature signal which can be used for redundancy purposes. In the event of failure of one sensor, the transmitter will continue to operate using the second temperature sensor, while at the same time it will send an alert to the controller and the AMS (Asset Management System), if available.

Temperature transmitter with dual temperature sensors

This backup feature can also be used as a signal drift detection function. This function works by comparing the measured value by both sensors, usually by calculating the difference between them. If one of the sensors stars to drift, the other one will continue providing a correct value, and the difference signal will start to increase. Therefore, the drift will be detected.

These kinds of extended features are generally available on fully digital temperature transmitters, and in most cases are easier to configure using a software tool.

Multichannel temperature transmitters

There are some applications that require temperature measurement in different points, such as the measurement of temperature profiles, such as reactors, distillation towers, cracking furnaces, pasteurization and refrigeration of food products, fermentation, and sterilization process, among others.

In these kinds of applications, it may be convenient the use of multichannel temperature transmitters.

Moore Industries' 16-channel Temperature Concentrator System, can be equipped with a HART to Ethernet gateway, enabling its integration in Modbus TCP and HART-IP based systems. With an i.s. galvanic isolator, temperature sensors can be installed in Zone 0

These devices work like temperature multiplexers, in which a microprocessor polls sequentially the temperature sensors that are connected to it and then sends the corresponding measurement values employing a variety of digital communication protocols.

Therefore, multichannel temperature transmitters can be considered multivariable field devices. During the peak years of IEC 61158-2 fieldbuses, there were numerous manufacturers that developed this kind of devices, usually featuring support for 2 to 8 temperature sensors, which could be either thermocouples or RTDs.

Currently available multichannel devices usually work as signal multiplexers and can be integrated into control systems using either serial or Ethernet based communication protocols.

The Rosemount 848T, an 8-channel temperature transmitter for Foundation Fieldbus applications

The main advantage of this approach is a reduction in the cost per point of measurement. This is because a single temperature transmitter is used sequentially to poll more than one temperature sensor. The cost of a multichannel transmitter is higher than a single channel transmitter but is definitively lower than the price of eight single channel transmitters.

The main disadvantage is that, in the event of a transmitter failure, all the data from the temperature sensors connected to it becomes unavailable. This problem can be solved by using redundant multi-channel temperature transmitters, but a detailed cost vs benefits analysis of the investment must be done before its implementation.

With the availability of Ethernet APL technology, it is possible that the time for multichannel temperature transmitters has finally arrived.

Wireless single channel and multi-channel temperature transmitters

With the widespread availability of wireless communication protocols like Wireless HART and ISA 100, the idea of a wireless temperature transmitter with the ability to transmit the data received from the connected temperature sensors, becomes an obvious solution for applications that require temperature measurements in locations that are difficult to access.

Wireless transmitters are frequently powered by batteries, so they have a limited power budget. But since temperature sensors are either passive (in the case of thermocouples) or active but requiring only a small amount of power (in the case of RTDs), they are well suited for this kind of applications.

Temperature transmitter equipped with a WirelessHART adaptor

The first generation of wireless transmitters frequently were versions of already available field devices equipped with a wireless adaptor and powered by a battery pack. To enhance battery life the HART transmitters must be working in multidrop mode. In this mode, the 4-20 mA current loop is disabled, and the current is fixed at 4 mA. All the data is sent to the control system in digital format.

ABB's Wireless HART temperature transmitter

Wireless HART is a wireless digital communication protocol that was designed as a wireless version of the traditional HART protocol, although initially employed Wireless HART adapters to enable the use of existing wired HART devices, it has reached enough market share that it is possible to use only native Wireless HART devices in specific applications. Native Wireless HART devices send all the process data to the Wireless HART gateway in digital format.

Honeywell's OneWireless (ISA 100) wireless temperature transmitter

ISA 100 is a wireless communication protocol designed with a clean sheet of paper approach, but comparing these two wireless technologies exceeds the main subject of this article. Both are mentioned because there are temperature transmitters for both alternatives.

Both Wireless HART and ISA 100 are mesh based networks, i.e., they work better when the density of network nodes increases, so this kind of solutions look ideally matched for applications like pervasive sensing or condition monitoring.

Rosemount's 4-channel Wireless HART temperature transmitters

And since the wireless transmitter is the expensive part of the equipment in a wireless field device network, it makes perfect sense to use multi-channel wireless temperature transmitters.

Yokogawa's 8-channel ISA 100 Wireless temperature transmitter

Temperature measurement is the ideal application for these types of mesh-based networks. The sensors employed for this purpose have low power requirements, so they can be powered by the wireless transmitter, which can be powered by either a battery or by any of the various energy harvesting methods that are available. Furthermore, temperature is a variable with a slow changing ratio over time, so they are compatible with the energy saving working mode employed by wireless protocols.

Non-invasive temperature measurement

Although surface mounted temperature sensors have been available since the late 70s, their lack of precision made their use in process automation application difficult if not impossible.

But in the last few years, the advances in the processing capabilities of modern microprocessors combined with advanced algorithms has transformed this option into a viable alternative to the traditional thermowell mounted temperature transmitter.

These advances have transformed non-invasive temperature measurement into a new trend in temperature transmitter design. When we discussed the issues that appeared with the use of thermowells, like its susceptibility to get damaged if the medium being measured is or becomes aggressive. Thermowells can also be the origin of vortex formation in the process flow and this condition can damage the thermowell due to the resulting oscillations.

Finally, there is the issue of replacing a damaged thermowell. This can be troublesome under any conditions but becomes a serious problem if the replacement requires the interruption of the process. This situation can happen when the process pipe or vessel where the temperature transmitter is installed is pressurized.

The solution for this problem consists in the use of non-invasive temperature measurement.

Rosemount’s X-Well non-invasive temperature transmitter

This method requires the temperature probe to be mounted as tightly as possible to ensure its contact with the surface at the installation point.

After doing this, the microprocessor of the transmitter uses proprietary algorithms that determine the relationship between the surface and the internal process temperature and uses known thermal conductivity values to determine the flux of heat. With this last value the transmitter can calculate the process temperature.

The precision of this measurement method, under the right conditions is around 1% of the measurement obtained with a traditional thermowell mounted sensor.

ABB’s NINVA non-invasive temperature transmitter

So, although this method is not well suited for every application, its use on the adequate ones can produce notorious savings in plant operations, since it transforms temperature transmitter into portable devices, thus reducing the total number of devices required in a plant.

A Rosemount X-Well (non-invasive) Wireless HART temperature transmitter

This technology, when its used in combination with wireless communications, offers unlimited temperature measurement options.

The future of temperature measurement

Temperature measurement technology has made huge advances in the last 50 years. These advances have been achieved mostly by improvements in the microprocessors and the associated software used by manufacturers to turn their equipment into SMART devices.

Endress + Hauser iTEMP TMT 86, the first Ethernet APL temperature transmitter to reach the market

The ongoing perfect storm of technological innovations in the Process Industry, which includes deterministic Ethernet networking (TSN or Time Sensitive Networking), Ethernet for field devices (Ethernet APL or Advanced Physical Layer) and the 2-WISE (2 Wires Intrinsically Safe Ethernet) concept, the use of information models for device integration (PA-DIM or Process Automation - Device Information Model), and extended access to any device connected to the network (NOA or NAMUR Open Architecture) will work as incentives for the development of field devices that will ultimatelly allow the adoption of digitalization in the Process Industry.

And curiously, even with all the advances we have seen in these series of articles, inside the sophisticated firmware of these state-of-the-art devices, you can still find the Callendar-Van Dusen equation being used for signal linearization.

And in this unglamorous way, the temperature series comes to its end. I hope you have enjoyed reading about the topic as much as I have researching and writing it.


Mirko Torrez Contreras is a Process Automation consultant and trainer. If he had knew with anticipation that the temperature series of articles would keep him busy for three months, he would not have believed it.

So, he spent the whole Argentinian winter doing research about the history of temperature measurement. There is some irony in this endeavour.

But Spring has finally arrived, and this is the last chapter in the temperature series. Now you can help him figure out what the next topic should be in process automation variables.

This article has been sponsored by Phoenix Contact. The opinions exposed in this article are strictly personal. All the information required for and used in this article series is in the public domain.

Shahid Ahmed Kazmi

■Learning is my obsession■Teaching my passion■Instrumentation & control my profession■ Founded Instrumentation enthusiasts (exclusive group) with 29,425+ members■ Newsletter •INSTRUMENTATION REVISITED• Weekly articles.

1mo

Appreciate your thoughts Mirko Torrez Contreras Nice article. One thing I want to mention is that use of SKIN thermocouple which do not require any thermowell have been in use in industry for a long time. The tip is welded on the tubes or pipes and have a good accuracy factor. Used in heaters,furnaces and many other places.

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