A Comparison Between High Temperature Pressure Transducers and Cooling Elements

Pressure measurement is often considered to be a challenging task due to the harsh environments often found in industrial manufacturing. High temperatures are a specific problem for electronic components, which typically have a low tolerance for heat. The most common solution for measuring pressure in hot environments is the usage of high temperature transducers, even though reducing heat with a cooling element may be a preferred choice for a few applications. For instance, normal sensors are usually ideal for operating temperatures below 80 °C (176 °F). Measuring pressure thus tends to be a matter of choosing between a cooling element and a high temperature transducer, each with specific disadvantages and advantages.

High Temperature Pressure Transducers

Cooling Element

High Temperature Pressure Transducer

Transducers generally transform energy from one form to another, even though the energy is normally a signal. They are regularly used in automated systems, which are mostly controlled by measurements of physical quantities such as motion, force, pressure and temperature. A sensor is a specific type of transducer capable of sensing a physical property of its environment and then reporting that change, typically in the form of an electrical signal. For instance, a pressure sensor is capable of detecting pressure and reporting it to a gage that displays the pressure.

High Temperature Transducer

A high temperature pressure transducer does not contain electronic components, providing it a much greater tolerance for heat than normal pressure transducers. These devices are usually rated for room temperature up to 343 °C (649.4 °F), based on the specific model. A quality pressure transducer of this type is capable of providing highly stable measurements at high temperatures. For instance, some models have the potential to measure pressure with an accuracy of 0.25% and a thermal drift of 0.1% at 38 °C (100 °F).

The pressure range of a high temperature pressure transducer can greatly vary, from 15 pounds per square inch (psi) to more than 10,000 psi. A calibration record from the National Institute of Standards and Technology (NIST) could be available for these pressure transducers. It could be possible for Manufacturers to calibrate their transducers at different stages in their lifecycle.

This high level of performance is possible by using thin film technology, which employs sputter deposits in order to develop a molecular bond between the substrate and gage. This manufacturing technique almost eliminates changes to the transducer’s calibration, including drift, creep and shift. It is necessary for high temperature pressure transducers to have a pressure cavity produced from stainless steel and a double-isolated case to guarantee integrity of the unit in a harsh operating environment. A pressure transducer’s tolerance for physical stress can be further increased by an all-welded construction.

Amplification

High temperature pressure transducers provide a millivolt output, meaning that they need an external amplifier to transform it into a 4 to 20 mA or 0 to 10 V signal. This external amplifier will also increase the price of the system.

Mounting a DIN amplifier on a rail is a new technique for transmitting temperature from a transducer to a display device. This approach permits the amplifier to accept a number of common inputs and processes for temperature signals. The output may use only two wires, even though a 3-wire outlook will isolate the voltage. An amplifier that uses a dual-relay output should also be able to isolate the relays from each other. The output signal for this type of amplifier is normally between 4 and 20 mA. The temperature range on a rail-mounted temperature transmitter should also be linear with respect to temperature.

A temperature transmitter should allow effortless configuration through a USB port. With this feature, users will be able to connect the transmitter to a PC with a standard USB cable and upload configuration data from the transmitter. The user will then be able to use software to make the necessary changes and download the new configuration back to the transmitter. The transmitter does not need additional power during this process since it receives the necessary power from the USB interface.

This type of transmitter should also be able to accept isolated inputs from a push button, with trim adjustments in the same range as the output signal. The trim stage during this process is indicated by an LED. The trim function should be locked if it does not need to be adjusted during configuration. During normal operations, the LED indicates when the signal input is out of range.

Cooling Element

Cooling elements generally depend on the principle of convective heat transfer, which is considered to be the mechanism by which heat is transferred due to the movement of fluids. In contrast, conductive heat transfer refers to the transfer of energy due to molecular vibrations. In addition to cooling elements, convection is also used in a number of other engineering practices.

It could also be possible for a cooling element to reduce media temperature, which is generally a much cheaper solution than a high temperature transducer. This approach permits the pressure to remain unchanged, assuming that the media density is not significantly affected by temperature changes within the normal operating range. Cooling elements are capable of typically working in both water and air, but are not suitable for oil media such as hydraulic fluid. It is essential for a high temperature transducer to be used in these applications since the viscosity of this media is greatly dependent upon temperature.

A cooling element should be developed from stainless steel in order to provide maximum corrosion resistance from most process media. The nickel content of this steel is normally 1.25%, with a chromium content in the range of 0.65% and 0.8%. The cooling element should be able to withstand a maximum pressure of 5,000 psi at 38 °C (100.4 °F) and 3,500 psi at 400 °C (752 °F). It should also decrease the temperature of a liquid process from 260 to 38 °C (500 to 100.4 °F) at the sensing element.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

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