Sensing Fluid Temperatures with Flexible RTDs
Accurate temperature sensing of liquids flowing in pipes is essential to process control and energy-management systems. The traditional approach is to use probe-style sensors and thermowells inserted into the fluid stream. What are the advantages of flexible RTDs?
Using immersed sensors to sense fluid temperature of materials flowing in pipes can be costly and difficult to install, especially when retrofitted to existing facilities. Flexible RTDs are an alternative to thermowells that offer surface-mounted simplicity with no loss of accuracy.
Flexible resistance thermometers are thin, bendable temperature sensors that contain flat wire-wound elements laminated between layers of electrical insulation. This design improves thermal response in three ways:
- Flexible sensors conform tightly to sensed surfaces, leaving no air gaps to block heat transfer.
- Thin electrical insulation reduces the thermal gradient between the sensing element and the sensed surface. Response is rapid and self-heating is negligible.
- The element winding senses over an area to reduce point measurement errors.
To see how these benefits apply to fluid measurement, consider the thermal profile of fluid systems.
Thermal Profile of a Pipe
What is the temperature of a liquid flowing inside a pipe? It depends on where you sense it. Assuming the fluid is warmer than the ambient air, the highest temperature exists at the center of the flow. Temperature gradually declines as you move to the outside wall of the pipe, then drops off sharply through the insulation.
Figure 1 shows the temperature cross-section of a well-insulated pipe. Although the maximum reading is at the pipe's center, all points inside the insulation, including the pipe wall, are at nearly the same temperature. Why then the difference between immersion and surface sensing?
The problem is conduction. When you introduce a thermometer into a system, it conducts heat away from its own sensing element to the outside environment. The element actually detects a temperature somewhere between true fluid temperature and the ambient air.
Figure 2 shows a typical thermowell/sensor installation. The metal thermowell and connection head conduct heat from the sensing area to the air outside the pipe insulation. A large gradient appears along the well. Errors can result if the thermowell tip does not penetrate far enough into the fluid stream.
The major drawback to thermowells is their installation complexity. Pipes must be drained, holes drilled, fittings welded and threads tapped. In instances such as retrofit installations, the cost of hiring specialized personnel and altering piping may drive you to consider another solution. Why not install a thermometer on the pipe's surface?
In figure 3, the thermowell has been replaced with a surface sensor. It consists of a spring-loaded probe clamped to the pipe, with a junction box for leadwire connections.
This arrangement is almost guaranteed to produce errors. Ambient air directly cools the pipe surface through the opening in the insulation. Heat also flows through the probe into the junction box, which presents a large surface for radiant and conductive heat loss. The sensing element transmits an uncertain mixture of fluid and ambient temperatures. A better approach is needed for surface sensing.
Figure 4 shows a thin, flexible RTD inserted beneath the insulating blanket. Two leadwires form the only route for heat loss. In this configuration, the thermal profile is the same as figure 1. As long as the pipe has sufficient insulation, its outside surface temperature agrees with the fluid inside. Note that insulation is a must; exposing a flexible RTD to outside air will reduce its accuracy.
Is flexible RTD performance truly comparable to immersed sensors? Test data compares a one company's flexible RTD in an installation with a thermowell installation. The test conditions were:
- The RTD was mounted to the top of a 3" steel pipe with stretch tape, then covered with 3" of fiberglass insulation.
- A 12" long brass thermowell probe assembly was longitudinally installed beneath the RTD. Brass was chosen for the thermowell material because it conducts heat seven to eight times better than stainless steel.
- A bare tip-sensitive probe was inserted into the fluid as a control.
To test time response, flowing water was abruptly switched from 50 to 150oF (10 to 66oC). The results are shown in table 1.
Based on the test results, the following conclusions can be drawn.
- A flexible RTD reacts to temperature changes more quickly than a probe inside a brass thermowell. An even larger difference would be expected in comparison to a stainless steel thermowell.
- After the system reaches steady state, all three sensors give identical readings. The surface-mounted flexible RTD effectively senses true liquid temperature.
These conclusions have been verified by an independent manufacturer of energy management systems, who conducted a side-by-side comparison in a working installation.
As the test data shows, flexible RTDs work well to measure liquid temperature in pipes because they include flexible sensors that conform tightly to the surface. There are no air gaps to block heat transfer. Thin electrical insulation reduces the thermal gradient between the sensing element and sensed surface. Also, the element winding senses temperature over a sensed surface area. If you must retrofit a temperature sensor for your process piping, consider a flexible RTD.