Temperature measurement and control systems!

Data Acquisition Temperature

How can I connect a temperature sensor to my control system?

Today, you have tons of ways to connect your instrumentation to your control system, but as always, all the options have good points and bad. With an application where you have to connect temperature sensors to your system, which option(s) will work best for you?

Of course, your choice will depend on many factors, some of them unique to your system. However, once you know the pros and cons of the available options, you can narrow down the list and make your choice easier.

Technology constantly evolves. You can connect your sensor in many ways – direct cables, field transmitters, HART, wireless, and so on. If you lack in-depth knowledge of these possibilities, then you’ll naturally choose types you know like direct wires or analog. We at Visaya want to expand your knowledge to expand your options!

We at Visaya want to expand your knowledge to expand your options!

Failure again?!

Let me share a real case with a metal company. A process control problem damaged all the cables that connected the field sensors to the system. However, this issue happened more than once, and each time, they wound up working without an important measurement for far too long.

Hoping to fix this wagon for good, the engineer asked vendors to suggest solutions. And every vendor suggested wireless. They even explained and showed off their devices, as they do when they smell a hot sale in the air. In the end, one of the vendors won, and the customer yanked the junk cables and set up wireless devices to transmit all the process data.

Today, you can buy multi-input transmitters that give you updates in seconds and come with strong batteries to boot. New technology solves multiple old issues, but the customer has to know about it first. So let’s discuss some of the ways you can connect your temperature measurement to the control system!

Direct connection

Your control system can use a card to read the sensor without a field transmitter. This sort of installation can save money by skipping the transmitter, but it takes a little work. For instance, some resistance temperature detectors (RTDs) need cables with certain insulation materials, such as glass or polyvinyl. Thermocouples also need special cables, those that match the sensor type.

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If you have only a short distance to cover, then direct connection gives you a simpler setup than a field transmitter. But for longer distances, installation needs will cost more than transmitters. Moreover, you’ll sometimes have problems with external noise, like electromagnetic interference (EMI), frequency interference (RFI), or electrostatic discharge (ESD). Large sensor leads can act like antennas, causing measurement errors from the noise.

Cons

  • High maintenance
  • No diagnostics or performance analysis
  • Tendency toward interference
  • High installation cost

Field transmitter

Many processes use field transmitters to connect temperature sensors to their systems. The transmitter translates the sensor signal and sends it to the system in different ways.

Depending on the communication with your control system, you can have just the temperature measurement or more. Analog setups will deliver only the temperature. Many companies in different segments still prefer this option, but you lose a lot of data from the diagnostic functions of the transmitters. Still, this installation is robust and suffers from less than direct wires.

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Or you can connect the field transmitter using a digital protocol such as FOUNDATION Fieldbus, PROFIBUS, or HART.  These protocols will bring you diagnostic information and other smart functions from the transmitter, and you’ll get an accurate and reliable measurement in your system.

Pros

  • Diagnostics and smart functions
  • Wide range of field protocols and vendors
  • Good noise resistance

Remote input/output (I/O)

Depending on your system’s structure, you could install remote I/O control. Sensors connected this way need fewer wires, all converting happens in the field, and it offers digital communication.

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Like the transmitter, this setup will reduce interference. Many systems can support it, and you can connect not only temperature sensors but also other transmitters and sensors locally.

Cons

  • Proprietary architecture
  • No diagnostics or advanced functions

Wireless communication

Wireless has become a standard option today. The tech evolved a lot recently, so now you can get data within seconds and battery life of more than a year, depending on the setup and device.

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It also has a much easier setup compared to cables, but you need to size your network to ensure reliability. You’ll need to change the battery every now and then too, but by getting all your data through the wireless, you can plan that exchange. Yay planning! On the down side, wireless doesn’t provide the fastest speeds.

Pros

  • Low installation cost
  • Long battery life in certain applications
  • Smart diagnostics and functionalities

Multiplexers

You can use local multiplexers to connect your temperature sensors as well. They do all the conversion locally and can communicate with the control system using a proprietary protocol or open protocols like MODBUS or PROFIBUS.

You’ll need a structure to install the multiplexer on and cables to connect the sensor to the structure, but this setup will decrease the chance of  EMI/EDS/RFI too.

Cons

  • Slow updates
  • Limited accuracy
  • Dated technology

Conclusion

We’re just skimming the surface here. You need to check your process and see which method will give you the data you need. You also have to decide whether you want smart functions to avoid unscheduled downtime. Start digging, folks!

Related tags: Control system Field temperature transmitter HART Multiplexer Remote IO Temperature Temperature measurement temperature transmitter Visaya Recommends Wireless communication
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