【2025 Temperature Meters】Insertion depth of temperature meters
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This post was last edited by The one on 2025-11-12 13:04. I. Why does the insertion depth affect the measurement results? Theoretically, thermometers are designed with the centerline of the tube as a reference for calculating the insertion depth. This method is indeed applicable in most situations, but it presents two major problems when faced with special conditions such as large-diameter pipes, tower equipment, and furnaces: one is resource waste, and the other is the inability to secure proper installation. When fluid flows through a pipe, it exists in two states depending on the flow velocity: 1. Laminar flow: When the flow velocity is low, the fluid flows in layers without mixing with each other. 2. Turbulence: When the flow velocity increases to a certain level, the streamlines become unclear, and many small vortices form within the flow field, causing the fluid to move in an irregular manner. These two flow regimes have a direct impact on temperature measurement; in a turbulent state, the thermometer only needs to pass through a thin laminar layer near the pipe wall to measure a more uniform change in the medium’s temperature.
In laminar flow, changes in upstream temperature are first detected at the center of the pipe, and then gradually propagate toward the pipe walls. Interestingly, the temperature change detected at 1/4 of the pipe has only a very short delay compared to that at the center. For this reason, the insertion depth of the thermometer does not necessarily have to be measured from the center of the pipe, especially for large-diameter pipes. If one insists on inserting it at the center point, it will result in a thermometer that is too long, which not only increases costs but also makes it more prone to damage.
This post was last edited by The one on 2025-11-12 at 13:05. II. Depth of thermometer insertion 1. Basic principle: Ensure that the sensitive part of the thermometer is fully inserted into the medium being measured. 2. Specific immersion length requirements: When measuring gas, the thermocouple should be more than 95 mm long, while thermal resistors and bimetallic thermometers should be more than 115 mm long. When measuring liquids: thermocouples, thermal resistors, and bimetallic thermometers should all be larger than 46 mm. 3. Formula for calculating total insertion depth: Total insertion depth of thermometer = length of connector + wall thickness of tube + immersion length
This post was last edited by The one on 2025-11-12 at 13:05. 4. Handling of special cases: When the diameter of the process pipeline is too small (less than 80 mm), an enlarged pipe should be fabricated. Temperature measurement in tower equipment: When the thermometer is installed horizontally, an insertion depth of 300–400 mm is sufficient. Boiler furnace temperature measurement: Excluding the thickness of the refractory bricks in the furnace, the thermocouple only needs to be inserted 150 mm into the furnace.
This post was last edited by The one on 2025-11-12 13:05. III. Why do temperature sensing elements respond slowly? The time constant and lag of the temperature sensing element depend essentially on two key factors: heat capacity and thermal resistance. Heat capacity: refers to the amount of heat required to cause a 1°C change in temperature of the temperature-sensing element. The smaller the heat capacity, the less heat is required to raise the temperature of the component, and thus the reaction proceeds more quickly. Thermal resistance: refers to the resistance encountered in the process of heat transfer. The lower the thermal resistance, the faster heat is transferred, and the quicker the temperature measurement response. Metals are good conductors of heat, but the thermal resistance of actual temperature sensing elements is often affected by the air gaps within the elements, insulating materials, and protective sleeves, resulting in a slower response. The time constant of ordinary temperature sensing elements can reach several dozen seconds or even minutes, which has a significant impact on the stability of temperature control systems!
This post was last edited by The one on 2025-11-12 at 13:06. IV. Effectively reducing measurement lag 1. Choose temperature sensing elements with low inertia When selecting temperature sensing elements, it is necessary to give priority to those with a small time constant and low lag. Generally speaking: armored thermocouples/thermoresistors have a faster response than regular types. Thin-film platinum resistors have a faster response than wound platinum resistors. Elements with a small diameter respond faster than those with a large diameter.
2. Pay attention to the installation method and location; the insertion depth must be sufficient: especially for thermal resistors, an insufficient insertion depth can lead to significant measurement errors. Generally, the insertion depth is required to be at least 8-10 times the diameter of the component protection tube.
Special treatment for pipeline installation: When the process pipelines are relatively thin, thicken the pipes locally. Prioritize installing the temperature sensing element at pipe elbows. Ensure that the temperature sensing element is aligned with the direction of fluid flow.
This post was last edited by The one on 2025-11-12 at 13:06. 3. Preferred liquid-phase measurement: When measuring the temperature of gas-liquid two-phase media, it is advisable to measure the temperature of the liquid phase. This is because the dynamic characteristics and stability of the liquid phase temperature are significantly better than those of the gas phase temperature, resulting in a faster response and more stable readings.
4. Enhance heat conduction measures: Fill the space between the temperature sensor protection tube and the temperature-sensing element with a heat-conducting material: metal shavings. High-purity magnesium oxide powder (this method is precisely used for armored thermocouples/platinum resistors). This method can significantly reduce the internal thermal resistance and improve response speed.
5. Use thermocouples with special structures. For thermocouples, the following types can be considered: exposed-junction type – the temperature-sensing end is exposed directly, resulting in a very fast response, but it is only suitable for harmless media. Shell-type: The temperature-sensitive end is in contact with the protective tube wall, and its response speed lies between that of the exposed end type and the insulated type.