How to measure temperature and avoid burns in the process, part 6
Mirko Torrez Contreras
Freelance Technical Translator and Writer | Certified Profibus and Profinet Engineer and Trainer I Explosion Protection Consultant and Trainer | Technology evolution storyteller
Temperature transmitters
Connecting temperature sensors to controllers
There are numerous differences between RTDs and thermocouples, each kind of device having its own advantages and disadvantages depending upon the application. In a typical process industry application both kinds of temperature measurement devices are connected to either a PLC (Programmable Logic Controller) or a DCS (Distributed Control System).
Control systems employ I/O (input/output) modules or cards to connect field devices. These modules consist basically of an ADC (analog to digital converter) which converts the analog signals sent by the field devices into digital values that are sent to the controller’s I/O map where they are evaluated and, depending on their value, provide the information required by the controller to operate the final control devices, thus creating a closed control loop.
The controller’s I/O system
The controller’s I/O module arrangement is known as the I/O system. This system features diverse models of I/O cards adequate for equally diverse types of input and output signals. The most frequently used type of I/O signals are digital inputs and outputs, which are used for the actuation of solenoid valves and the monitoring of their position using the feedback of proximity sensors. Two position sensors (for either on/open or off/closed valve positions) are usually associated with one digital output.
For analog signals, early systems used 1-5 V analog signals, but this type of signal is too sensitive to voltage drops in most applications. For this reason, 1-5 V I/O analog modules are now usually found in small and/or inexpensive controllers, while the most used type of analog signal in the industry is the 4-20 mA current loop.
The 4-20 mA current loop
4-20 mA current loops are much less susceptible to voltage drop and electrical noise or EMC disturbances, therefore they are adequate for longer cable runs. Additionally, they can deliver power to the field devices through the current loop (if the power required is between a minimum of 9.6 mW to a maximum of 480 mW).
Temperature I/Os
Controller manufacturers offer specially designed input modules that can receive either mV signals in the case of thermocouples or m? variations in the case of RTDs.
These special modules are more expensive than 4-20 mA input cards and therefore are used in special applications where the temperature sensors are located near the controller’s I/O system and/or high precision temperature measurement is required. The reason of their higher price is due to the high sensitivity they must have to detect with enough precision the small signal variations that temperature sensors deliver in an application.
The hidden costs of temperature sensors
Additionally, as we discussed in prior articles, temperature sensors are not well suited for long cable runs, because these signals are very sensible to voltage drops and EMC issues and in the case of thermocouples, cabling costs can become too expensive due to the need to use either compensated or extension cables.
For these reasons, in most applications the best approach consists of using 4-20 mA temperature transmitters.
How transmitters work
Temperature transmitters are devices that take the signal corresponding to the variable being measured by a transducer, and then pass it to an ADC (Analog to Digital Converter), where it is converted into a digital value.
This value is processed by the transmitter’s microprocessor which evaluates the signal and performs tasks such as linearization, temperature compensation and other tasks. Afterwards the processed digital value goes through a second DAC (Digital to Analog Converter) that converts the digital value into an analog 4-20 mA current signal.
The advantages of using temperature Transmitters
There are several advantages in the use of temperature transmitters for temperature sensors:
Therefore, the best practice for industrial temperature measurements is to convert any thermocouple and/or RTD signals to a 4-20 mA current loop as close to the measurement point as possible.
Some details to take care of before
One factor that may make this practice difficult to implement is the presence of high temperature environments.
Temperature transmitters have the same general operating temperature range as other kinds of electronic field devices, which is usually I the -40 ?C to 85 ?C range
Higher or lower temperatures than the specified by the manufacturer can have negative effects in the transmitter’s electronics lifespan. In these cases, the solutions available are based on the increase of the separation distance between the temperature transmitter and the temperature sensor.
To accomplish this requirement, there are different options available, depending on the type of transmitter. We will discuss them accordingly when we describe each type later.
Earlier temperature transmitters lacked precision
The first generations of temperature transmitters were analog devices that used potentiometers to set functions such as calibrations, ranging, and zero and span setting. Technicians had to use a screwdriver and a multimeter to setup these early transmitters and potentiometers are prone to suffer wear.
Additionally, the signal conditioning necessary to transform the temperature sensor output into a 4-20 mA signal was done using analog components.
In many cases this mode of operation was the cause for imprecise or even false signals
Microprocessor based temperature transmitters solved that problem
Digital transmitters have enhanced measurement precision, device performance and accuracy to levels that were not possible with earlier designs.
In a digital transmitter, the first DAC takes the mV or m? signal from the temperature transducer and converts it into a digital value of typically sixteen bits (earlier versions used eight bits and newer versions can use up to 24). This conversion offers a range of 2^16 values (0 to 65536).
The corresponding values are sent to the transmitter’s microprocessor which perform various signal conditioning and mathematical operations, like linearization, cold junction compensation or the calculation of a 3 wire RTD signal among others.
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Once these operations are performed, the digital value is sent to the second DAC, which transforms it into a current value between 4-20 mA.
Currently, 4-20 mA analog transmitters have become fully digital, and incorporate digital communication protocols like HART, Foundation Fieldbus, Profibus PA or Ethernet APL.
This transforms the plain temperature sensors into a smart field device, and allows them to be configured, parameterized and commissioned remotely using asset management software solutions. Remote diagnostics become available, a timesaving functionality that for practical purposes eliminates the need to get access to the field mounted device. This practice is equivalent to the IT practice of remote network infrastructure maintenance.
Digital transmitters offer the capability of sending more than one variable to the control system. As an example, HART transmitters can send up to four variables, the first (called PV or Primary Variable) can be send either as a 4-20 mA signal with the secondary, tertiary and fourth variables sent as SV, TV and QV variables. These three additional variables are always sent as digital values via the HART communication protocol superimposed on the 4-20 mA loop.
digital transmitters also improve the signal accuracy and precision because they use a single DAC to digitalize the signal provided by the sensor transducer and directly send the digitalized value to the controller. They can even deliver the measured value in the required engineering units. Fully digital transmitters became widely available during the IEC-61158-2 era, and their use is expected to gain popularity as Ethernet APL technology becomes available in the market.
Thermowells
Thermocouples and RTDs are usually employed to measure temperatures in aggressive environments. To protect the sensors, they are not exposed directly to the application’s environment, instead they are mounted inside protective probe like housings known as thermowells.
Thermowells are made from diverse types of stainless steel, depending on whether the measurement environment contains corrosive substance or not.
For high temperature applications, thermowells may be built using special alloys or ceramics.
The use of metals is the best way to ensure that the temperature sensors are exposed to the same temperature as the measured environment. They also allow installing the sensors as close as possible to the core of the measurement environment.
One undesired possible consequence of the use of thermowells is that the transmitter may become exposed to high temperatures.
A temperature transmitter that is exposed to higher temperatures than the maximum specified for the device usually starts to disappear randomly from the live list of devices connected to the controller.
After prolonged exposure to temperatures above the device’s nominal range, the electronics start to age at a higher rate than expected.
To avoid this issue, the usual practices are based on increasing the distance between the temperature sensor and the transmitter. This can be accomplished in different ways.
One of the simplest ways to protect the head’s connection compartment of the thermowell when it is exposed to temperatures that exceed the maximum allowed temperature of the transmitter, is the use of an extension tube.
Sensor extensions are metal tubes with threaded ends that increase the distance between the connection point of the thermowell and the corresponding transmitter, thus preserving the electronics from harmful temperature levels which may negatively affect their projected lifespan.
Types of thermowell connections
Thermowells can be mounted using a threaded connection, a welded connection or different types of flanged connections that enable additional flexibility. They allow end users to easily replace a damaged or worn out RTD or TC without having to interrupt the process.
A factor that must be considered in the selection of the thermowell connection method, is that all of them have an impact on the pressure resistance capacity of the process pipe, or vessel whose interior temperature we are trying to measure. For high pressure applications, welded thermowell connections are recommended.
Threaded connections are the most commonly used because they are the simplest to install and allow for a quick installation or replacement of the temperature sensor. But they are not used in high pressure applications or for use with toxic, explosive or corrosive substances because they are the most prone to suffer leaks.
Flanged connections are designed to be bolted to a mating flange installed on the vessel, they offer high pressure resistance, easy installation, and replacement, but they are the costliest installation method
More details to consider before installation
Care must be taken in the calculation of the thermowell insertion length.
Although the ideal position for the thermowell tip would be in the middle of the vessel or pipe where we are installing the accessory, this is not adequate for applications where there is a fast flow of the substance since the flow affects the heat transmission from the thermowell to the temperature sensor.
Another factor that can affect the insertion length is the presence of solids in suspension in the flow, which could damage the probe..
Spring loaded sensors allow their easy replacement in case of sensor failure and also ensure a good contact between the sensor and the thermowell.
The next instalment of this series of articles will cover the types of temperature transmitters currently available in the market, some interesting application cases that have become commonplace with the availability of new technologies and a taste of what the industry seems to be preparing for the near future.
Mirko Torrez Contreras is a Process Automation consultant and trainer. After dealing with the coldest winter in decades, he is really glad that the Spring has finally arrived. Especially because the heating equipment he used to withstand the worst part of the winter did not survive the experience.
As the end of this series approaches, he has realized that the topic of temperature measurement is way larger than what he initially thought. But it was an interesting way to expand his admittedly little knowledge about this branch of knowledge.
This article has been sponsored by Phoenix Contact. The opinions exposed in this article are strictly personal. All the information required for and employed in this article is of public knowledge.