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The insertion depth of thermocouples, thermal resistors, and bimetallic thermometers used in the field has a significant impact on the accuracy of the measurement results. The insertion depth of the temperature sensor needs to be considered from both design and the temperature element perspectives. 1. How is the insertion depth of thermocouples, thermal resistors, and bimetallic thermometers calculated in design? The most common installation methods for thermocouples, thermal resistors, and bimetallic temperature sensors are threading, flange connection, movable flange, and installation without any fixing device. The calculation of the insertion depth of the temperature sensor varies depending on the installation method: http://yunrun.com.cn/upload/201908/02/201908022358537818.png. For more details on the insertion depth, see yunrun.com.cn/tech/2658.html. ① For temperature sensors installed using threaded fixation, the insertion depth is calculated as half the diameter of the pipe plus the size of the protrusion on the sensor used for installation, plus the thickness of the insulation layer. Since thermocouples and thermal resistors have an external seal, taking the insulation layer thickness into account ensures easy installation or removal without damaging the structure of the insulation layer. The common lengths for such protrusions are 60 mm and 120 mm (they can also be manufactured to custom specifications), while the typical thickness of the insulation layer is 120 mm. ◆Bimetallic thermometers have an internal seal; the method for calculating the insertion depth is the same. However, when determining the size of the protrusion on the temperature sensor, 30 mm should be subtracted (this is related to the structure of the protrusion; one can search for “temperature sensor protrusion” on Baidu to learn more about the dimensions and structure of such protrusions used in temperature sensors). ② For temperature sensors mounted using fixed flanges, the insertion depth is calculated as half the diameter of the pipe plus the size of the flange connection tube plus the thickness of the insulation layer. ③ For temperature sensors without fixed fixtures, those with movable flanges, or those with movable threads, the user must adjust the insertion depth. For example, in the case of thermocouples used to measure temperature inside a furnace, the insertion depth is typically calculated as the actual length inserted into the furnace plus the thickness of the furnace body (including the thickness of the furnace walls) plus the thickness of the insulation layer. 2. What other factors need to be considered when calculating the insertion depth of thermocouples, thermal resistors, and bimetallic thermometers? ①For different measurement media, the insertion depth of the temperature sensor varies. To ensure measurement accuracy, the sensitive part of the temperature sensor must be fully inserted into the medium being measured. The required immersion length for temperature sensors is generally as follows: ◆ When measuring gases, thermocouples should have an immersion length of more than 95 mm, while thermal resistors and bimetallic thermometers should have an immersion length of more than 115 mm. ◆When measuring liquids, thermocouples, thermal resistors, and bimetallic thermometers should all be larger than 46 mm. ◆The total insertion depth of the temperature sensor is: temperature sensor mounting boss + wall thickness of the tube + immersion length. ②For different devices, the insertion depth of the temperature sensor varies due to differences in installation location. In design, we often use the centerline of the pipeline as a reference to determine the insertion depth of the temperature sensor. This method is suitable for most applications, but it may not be appropriate for temperature measurement in large-diameter pipes, tower equipment, furnaces, etc. Firstly, it could lead to waste, and secondly, it makes it impossible to secure the sensor in place. For example, when measuring the temperature of ordinary tower equipment, if the temperature sensor is installed horizontally, an insertion depth of 300–400 mm is sufficient. For measuring the temperature in a boiler furnace, temperature sensors are usually installed horizontally; after accounting for the thickness of the refractory bricks in the furnace, the thermocouple only needs to be inserted 150 mm into the furnace. ③For pipes of different diameters, the insertion depth of the temperature sensor varies due to differences in flow velocity. As fluid flows through a pipe, layer flow and turbulent flow occur depending on the flow velocity; the flow velocity is highest at the center of the pipe and lowest near the wall. When the flow velocity is low, the fluid flows in layers without mixing with each other, a condition known as laminar flow. As the flow velocity increases gradually, the flow lines of the fluid begin to exhibit wavy oscillations; the frequency and amplitude of these oscillations increase as the flow velocity rises. Such flow lines are referred to as transitional flow ； When the flow velocity increases significantly, the streamlines become indistinguishable; there are many small vortices in the flow field, laminar flow is disrupted, and not only sliding but also mixing occurs between adjacent flow layers. At this point, the fluid moves in an irregular manner, with component velocities perpendicular to the axis of the flow tube; such motion is known as turbulence. In a turbulent state, the temperature sensor can measure a relatively uniform change in the medium’s temperature as long as it passes through a thin laminar layer near the tube wall. In a laminar flow state, changes in upstream temperature are first detected at the center of the pipe, and only then gradually at the pipe walls. The temperature change detected at 1/4 of the pipe length experiences only a very short delay compared to that at the center. For this reason, the insertion depth of the temperature sensor does not necessarily have to be measured from the center of the pipe, especially in the case of large-diameter pipes; otherwise, the temperature sensor would need to be very long, which increases costs and increases the risk of damage. The insertion depth of the temperature sensor is measured from below the mounting threads or flange provided with the sensor itself. If there is a nut seat on the pipe, and the height of this nut seat is 40 mm, then this value must be maintained, regardless of the diameter of the pipe. The length of the temperature sensor’s sensing portion is approximately 30 mm, and this portion must be located on the center line of the pipe. Therefore, assuming the pipe diameter is 500 mm, the insertion depth of the temperature sensor is: pipe radius of 250 mm + mounting nut seat of 40 mm + probe portion of 30 mm = 320 mm. If, from a technical standpoint, the pipe remains filled at all times and has a large diameter, then it is not subject to this requirement; it is sufficient for the length inserted into the pipe to be equal to or greater than 10 times the diameter of the protective tube ； If some pipes have a thicker insulation layer, the height of the nut seat may need to be higher. If the diameter of the process pipeline is too small (less than 80 mm), a thermometer expansion tube must be fabricated when installing a temperature sensor; this is a diameter adjustment made for small-diameter pipelines to meet temperature measurement requirements. When installing thermocouples and thermal resistors, it is necessary to ensure that this facilitates accurate temperature measurement, maintains safety, makes maintenance easy, and does not affect the operation of the equipment or production processes; all of these requirements must be met. When selecting the installation location and insertion depth of the temperature sensor, the following points should be taken into account: ① To ensure adequate heat exchange between the measuring ends of the thermocouple and thermal resistor and the medium being measured, the location of the measurement point should be chosen carefully, trying to avoid installing the temperature sensor near valves, elbows, or in dead corners of pipes and equipment ； ②Thermocouples and thermal resistors equipped with protective sleeves suffer from heat transfer and heat dissipation losses; to reduce measurement errors, these devices should have an adequate insertion depth: ◆ For thermal resistors used to measure the temperature of the fluid at the center of a pipe, their sensing end should generally be inserted to the center of the pipe (whether installed vertically or at an angle). If the diameter of the pipe containing the fluid to be measured is 200 millimeters, then the insertion depth for the thermal resistor should be 100 millimeters ； ◆For temperature measurement of high-temperature, high-pressure, and high-speed fluids (such as main steam temperature), in order to reduce the resistance exerted by the protective sleeve on the fluid and prevent the sleeve from breaking under the influence of the fluid, a shallow insertion method for the protective tube or a heat-resistant resistor of the heat-shield type can be employed. The depth to which the protective sleeve of a shallow-insertion type resistor is inserted into the main steam pipeline should be no less than 75 mm ； The standard insertion depth for heat-shrink type thermoresistors is 100 mm ； ◆If it is necessary to measure the temperature of the smoke in the flue, even though the flue diameter is 4 meters, a heat resistor with an insertion depth of 1 meter will suffice ； ◆When the insertion depth of the measuring element exceeds 1 m, it should be installed as vertically as possible, or a support frame and protective sleeve should be added. 3. How to measure the insertion depth of thermocouples, thermal resistors, and bimetallic thermometers after obtaining them? ①For thermocouples and thermal resistors mounted with fixed threads, the insertion depth is the length from the sealing surface at the end of the thread to its tip (the thread length is also included) ； http://yunrun.com.cn/upload/201908/02/201908022229468499.png ② The insertion depth of the bimetallic thermometer is the length from the sealing surface at the top of the thread to the tip of the thermometer (the length of the thread itself is not included in this measurement) ； http://yunrun.com.cn/upload/201908/02/201908022249481986.png ③ The insertion depth of the temperature sensor installed on the fixed flange is the length from the flange’s sealing surface to its top edge (the flange’s thickness is not taken into account) ； http://yunrun.com.cn/upload/201908/02/201908022242242520.png ④ For movable threaded or movable flanged thermocouples and thermal resistors, the user can adjust the insertion depth within the specified total length range. 4. What is the relationship between the insertion depth and total length of thermocouples, thermal resistors, and bimetallic thermometers? Total length of temperature sensor = Insertion depth of temperature sensor + Length at the rear end. ① For conventional assembled thermocouples and assembled thermal resistors, the total length = Insertion depth + 150 mm (150 mm is the standard value; this length can be increased or decreased according to the requirements of on-site installation). ②For thermocouples and thermal resistors with special structures, the total length = insertion depth + the manufacturer’s default structural dimensions (these dimensions are not subject to any standard specifications; the manufacturer’s product technical documentation should be referred to). ③ For bimetallic temperature sensors, the total length = insertion depth + rear end length (no standard specifications apply; the manufacturer’s product technical documentation should be consulted). Even when thermocouples, thermal resistors, and bimetallic thermometers are of good quality, proper installation procedures are essential. To obtain as accurate measurement results as possible and minimize errors, it is crucial to pay attention to the depth at which these devices are inserted.