Temperature Transmitter – How to Select The Right One
What makes a good temperature transmitter?
Well, that’s a good question, and it depends on the application. In example, one that works well in a freezer may not stand up to the conditions inside a chemical reactor.
There are a number of factors to consider when selecting a temperature transmitter, and I’m going to touch on some of the ones I consider to be most important. For the purposes of this discussion, I will be covering TT’s that use one of the most common sensor types, which is a resistive temperature detector (RTD). I will cover the differences of RTD’s vs thermocouples (the other most common sensor type) in another post. But for now let’s consider TT’s that use the RTD type sensor, keeping in mind much of the selection process will be the same for either sensor type.
First, I’d like to make a distinction between a temperature transmitter (TT) and a temperature indicator (TI). A TI will often be called a thermometer and is basically a device that has a mechanical or electrical temperature sensing element and also has a scale or graduated markings on it that show temperature in ºF or ºC: no signal is transmitted. A TT is a device that transmits (fancy word for sends) a temperature signal to another device or display, TT’s usually use a method of measuring temperature that involves sensing changes in potential (voltage). The voltage part is beyond the scope of this article and will be covered in another post. A classic example of a TT is your thermostat that is wired to your heater or air conditioner, and an example of a TI is a meat thermometer, as you can see, nothing too mysterious about them at all.
Now, on to what to consider when selecting a TT.
1 – Temperature Range
The most obvious place to start are the maximum and minimum temperatures that need to be measured. Are you looking at a heating thermostat with range in the ball park of 50 ºF to 90 ºF (10 ºC to 30 ºC), or an industrial application with cooling and heating in the range -40 ºF to 300 ºF (-40 ºC to 150 ºC)? Or maybe something all together different?
When getting into industrial applications, TT’s will often have adjustable ranges. While a TT that is set to a wide range will also be able to measure within a smaller range, the accuracy of the measurement will go down. This has to do with how many increments the signal from the TT is split into, a larger range means each increment has to be larger as well.
For example, if measuring from 0 ºC to 100 ºC with uncertainty of 1%, the transmitter may be off by as much as 1 ºC. If measuring from 0 ºF to 50 ºF, with the same uncertainty of 1%, the transmitter can only be off by a maximum of 0.5 ºC. This becomes important when demonstrating a process is staying within specified guidelines, like in chemical processing or pharmaceutical manufacturing – commercial applications will likely have looser acceptable uncertainty but commonly used transmitters for these projects often come with a preprogrammed, nonadjustable range.
2 – Pressure Range
Another important criteria is the pressure range that the instrument can reliably operate within. If you have a scenario where you’re measuring the temperature of a high pressure vapor such as steam, you want to make sure the sensor and it’s housing can stand up to it. In Canada, there are requirements that each fitting (IE, where you insert the TT into the process for measuring, this may be a threaded connection, welded, a flange, etc.) is rated with a Canadian Registration Number (CRN). And of course, the sensing element of the transmitter itself may be capable of withstanding the pressure. Sensors and transmitters are often listed with acceptable limits listed in both temperature and pressure.
3 – Chemical Compatibility
If you’re looking for a TT to measure process temperature in an industrial setting, chemical compatibility is something that should be looked at carefully. I suggest doing a Google or Bing search for chemical compatibility using the chemicals used in the process in the search, or you can search by what materials your transmitter is made from. Either way, there are many great resources such as those offered by Cole-Parmer, The Engineering Tool Box, as well as a host of others. Usually the materials of construction are listed vs the type of chemical within a table, and there will be a rating given as something like: suitable, somewhat suitable, unsuitable. Pay attention to these ratings as the life of your transmitter can be greatly shortened by selecting materials incompatible with the process.
4 – Requirements For Hazardous Environments
The Canadian Electrical Code (CEC), used in Canada, and the National Electrical Code (NEC) used in the US, define the classes, divisions and groups that hazardous areas are classified within. The CEC and NEC gives criteria that equipment must conform to in order to operate safely in these hazardous environments. Three acceptable means of keeping electrical equipment from producing a potentially damaging and dangerous explosion are making a device intrinsically safe, explosion proofing it, or installing it in a purged and pressurized enclosure.
Intrinsically safe circuits are those that produce less energy than is needed to ignite the mix of vapors, gases, dust or flyings in their easiest to ignite ratio with oxygen. Meaning there isn’t enough heat generated to cause an explosion.l
Explosion proof equipment, while capable of producing enough energy to ignite a fuel air mixture in a harmful environment, is built with rugged and tight construction so that the probability of an ignition is extremely small. In addition to this, in the upset condition that there happens to be an explosion within the equipment, the robust construction will contain it and prevent any damage to other equipment, surrounding areas or persons in the vicinity.
A third option when electrical equipment is needed inside a hazardous environment is installing it inside a pressurized enclosure and using an inert gas, like nitrogen, to “purge” oxygen out of the enclosure. This eliminates the possibility of an explosion by lowering the air to fuel ratio inside the enclosure, and thus as seen by the electrical device, below the threshold for ignition.
A full review of the electrical classifications and their requirements is beyond what this article will cover, and is quite intensive, but I wanted to provide some guidance on what options are out there. Check out the Underwriters Laboratories of Canada website for tons more info (or your local codes), or reach out to me directly in the comments section below!
The most important thing to keep in mind is when there are flammable gases, vapors, dust, or flyings present, there is a need to review the hazardous area classifications in your country.
The Right Temperature Transmitter For The Job
Hopefully you found this post insightful. When selecting a proper transmitter there is a lot to consider. Using the tips above regarding temperature range, pressure range, chemical compatibility and hazardous area requirements and you’ll be well on your way to making the right choice for your application.