Pressure transducers can be used in many challenging environments where they may be exposed to extreme temperatures but are expected to operate reliably and predictably.
These challenging temperature exposures can come from a few sources such as the surrounding environment (ambient temperature), radiation (sunlight), or the temperature of the process media being measured (media temperature).
Media temperature can have large and unwanted impacts on the reliability of pressure transducers that are not always obvious. So, what can be done to minimize these impacts and keep these instruments safe from damage?
I’ve seen many customers deal with this problem during my nearly 20 years at Ashcroft. This article will take you through how process media temperature can impact pressure transducers and discuss methods that can help minimize the unwanted effects to keep sensors, and your equipment, working.
Media Temperature Impacts on Pressure Transducers
In order to directly sense media pressure, a pressure transducer must have some way to be exposed to or make contact with the media. Many pressure transducers utilize a sensing diaphragm, which responds to (deflects) changes in media pressure.
The deflection of this sensing diaphragm can be measured by a variety of methods like strain gauging and capacitance.
As the media is in contact with the sensing diaphragm, the pressure can be measured. The sensing diaphragm is also exposed to the media and, therefore, the media temperature.
High or low temperatures will cause metal to expand or contract, respectively, which can induce measurable changes in the sensing diaphragm.
In strain gauge-type pressure sensors, the temperature can change the resistances of strain gauges, as well as influence the output of the sensor. In capacitive-type sensors, the properties of the capacitor itself (such as the dielectric) can influence the output of the sensor.
Figure 1:
One question I’ve heard many times before is, “Can a pressure sensor be temperature compensated?”
Yes, it can! Let’s look at how.
Temperature Compensation
When high or low temperatures affect pressure instruments, manufacturers utilize temperature compensation to counteract the effects. Temperature can influence the sensing element but also affect the accuracy of electronic components on the sensor PC board.
The process of temperature compensation involves putting an electronic signal into the device that will sense the internal temperature and provide a counteracting signal to offset the effects of temperature.
Although it helps a great deal, temperature compensation can be imperfect when the temperatures are not the same across the entire pressure transducer. Rapid changes in temperature at different parts of the transducer can create inaccuracies until a consistent and equalized temperature profile is reached.
In applications where temperature changes take a long time (hours, days) even if they are very large, it is much more likely that a consistent and equalized temperature profile is achieved, and the temperature compensation is working well.
In applications with large changes in media temperature that happen quickly while the surrounding environment stays the same, a variety of solutions can mitigate the extreme temperature effects.
Mean Time Between Failure
Temperature changes and extreme temperatures can produce unwanted impacts on reliability. Temperature cycling induces stress on mechanical and electronic components. High operating temperatures are also known to reduce the long-term reliability of electronic components.
Mean Time Between Failure (MTBF) calculations are commonly employed to estimate how long a device will last on average before it fails. A common rule of thumb is that the MTBF is cut in half for every 10 degrees Celsius change in operating temperature.
To mitigate the effects of temperature, you need to find a solution to make the equipment closer to ambient temperature, which will help with accuracy and reliability.
Mitigating Temperature Effects
The media temperature shouldn’t exceed the specified operating temperature of the product, but these measurement instrument accessories can help protect your instruments from damage:
Capillaries
A capillary provides distance between the process and the transducer, keeping it away from dangerous temperatures.
One part of the capillary connects to the process and the other connects to the transducer allowing you to mount the transducer somewhere safe away from the extreme temperatures of the process.
Capillaries help lower (or increase) the temperature of the media by minimizing the media volume while maximizing the surface area exposed to the surrounding (ambient) temperature.
A few inches/centimeters can bring the media temperature back to a safe operating range relatively quickly (see Figure 2 below).
Figure 2:
Capillaries work well in applications involving gas or liquids that are not expected to have rapid changes in pressure or large amounts of media flow in/out. When flow or rapid changes are expected, there are other solutions to consider.
Diaphragm Seals
Diaphragm seals are used to protect pressure measuring instruments like pressure transducers when process compatibility is required. They seal and isolate a connection to prevent process media from passing through but still allow pressure to be measured.
However, keep in mind that diaphragm seals can impact the accuracy of your reading because they require additional pressure to displace their fluid.
This impact on accuracy will be greater with diaphragm seals having higher spring rates, fill fluid with higher thermal expansion coefficients, and on lower pressure range instruments.
It is a best practice to recalibrate the pressure transducer with the diaphragm seal attached at the expected working media temperature.
Steam Siphons
When there is a need to measure steam process line pressure, you should consider special precautions. Steam carries a great deal of energy and accessories like long capillaries may be unable to reduce temperature due to steam flow.
Steam siphons are designed for these applications. They help protect transducers from high-temperature steam by creating a condensate barrier and isolating the instrument from the rapid temperature changes due to direct exposure to steam.
We don’t like to pressure you, but we have more information.
Now that you understand the impact that media temperature can have on your measurement instrumentation, you can research the solution that’s right for your application.
If you want to learn more about measurement instrument accessories and designing assemblies, view our assembly guide.
You can also check out some of our other blog posts about accessories:
- Do Measurement Instrument Accessories Affect Accuracy?
- What’s the Right Pressure Instrument Mounting Assembly for Me?
- Isolation Ring Assemblies for the Water/Wastewater Industry
Feel free to contact us today to talk to one of our industry experts and get all your measuring instrumentation questions answered.