Temperature sensor accessories
One of the most important accessories for any temperature measuring element (temperature sensors) is a high pressure sheath known as a thermowell . This may be as a thermally conductive protrusion into a process vessel or tube that allows a temperature-sensitive device to detect the processing temperature without opening a hole in the vessel or tube. Thermowells are very important for installations where the temperature element (RTD, thermocouple, thermometer, etc.) must be replaceable without putting pressure on the process. Thermowells may be made of any material that is thermally conductive, pressure-resistant, and chemically non-reactive to the process. Most thermal heaters are made of metal (stainless steel or other alloy) or ceramic materials. A simple diagram showing a thermostat in use with a temperature device is shown here
In general, it is better to install a thermowell in a tube than in a vein, because more turbulence of liquids in a tube causes heat transfer by convection and also helps to clean solid sediment from the surface of the thermola. If the temperature gauge is removed for maintenance or replacement, the thermowell maintains the integrity of the pipe pressure (without spilling process fluids and leaking air into it)
The photos of a real thermowell (made of stainless steel) are shown here, the photo on the left shows the entire length of the thermowell , and the photo on the right shows the end where the thermometer is located
A photo of a complete RTD assembly (connection head, RTD and thermola) is shown in the next photo
As useful as thermowells. All thermoses, no matter how they are installed, cause the first time delay of the temperature sensor to increase due to their mass and specific amount of heat. It should be visually obvious that a few pounds of metal does not heat and cool as much as a few ounces of RTD or thermocouple, and therefore adding a thermostat to the measuring element increases the response to any temperature measurement. What is not so clear is that such a delay, if severe, could jeopardize the stability of feedback control. A control system that receives a “delay” temperature measurement does not detect the live temperature of this process in real time because of this delay. A potential problem with thermowells is the incorrect installation of the temperature sensor. This element must be inserted with full contact at the end of the thermowell blind hole. If there is any air gap between the end of the temperature element and the bottom of the thermowell hole, this adds a second time delay to the measurement system (note). Some thermowells include a spring clamp at the bottom of the blind hole to help maintain a constant metal-to-metal contact between the sensing element and the thermola wall
Note: The air gap acts as a thermal resistor while the mass of the element itself acts as a thermal capacitor. Thus, the inclusion of the air gap constitutes a secondary “RC fixed time” thermal delay network up to the thermal delay caused by the thermola. This “command” adds another delay to the system, not just an increase in its thermal time. In general, multiple delay commands are harmful to process control because they cause a phase change in a feedback loop and may lead to oscillations
Appropriateness of process / equipment
The main consideration for selecting a suitable temperature measuring element for each application is the expected temperature range. Mechanical (non-metallic) temperature sensors and the filled system are relatively limited to the process temperature and can not move the relay signals too far from the measuring point. Thermocouples are almost the most rugged and extensive contact type temperature sensors. Accuracy varies in the type of thermocouple and the quality of installation. RTDs are more fragile than thermocouples, but do not require reference compensation and are inherently more linear. Optical sensors are not able to measure the temperature of fluids inside the arteries unless a transparent window is provided to release light to reach the sensor inside the arteries. Otherwise, the best light sensor can report the skin temperature of a vessel. Optical sensors are the only suitable solution for monitoring the surface temperature of solids, especially objects that are impractical or even dangerous to contact (for example, electrical insulators in high voltage lines). Chemical reaction is a concern of contact type sensors. If the sensing element is kept inside a thermostat, it The thermowell should be selected for minimal reaction with the process fluid (s). Bare thermocouples are particularly vulnerable to chemical reactions due to the nature of most thermocouple metals (iron, nickel, copper, etc.) and must be carefully selected for specific process chemistry to avoid reliability problems in the next step
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