Temperature Measurement: Difficult Cases and Discussion Threads
Thread Content
During the pandemic when I was confined at home, I could take some time to surf the internet and read posts. I’ve always wanted to create a thread for discussing temperature meters, to exchange ideas with experts like you about the various tricky issues related to temperature measurement, and now I have the chance to do so. Temperature measurement is probably considered something quite ordinary by many people. Indeed, thanks to the efforts of domestic instrument manufacturers over the years, even with modest requirements, these devices can meet most application needs, and their cost is relatively low. However, there are also some special applications or conditions that require products designed specifically for those purposes. Some examples that come to mind include high-pressure thermocouples for low-density polyethylene LDPE (around 2000–3000 bar; rapid response is also required), flexible multi-point thermocouples for oil refining and hydrogenation processes, high-temperature and corrosion-resistant thermocouples for waste liquid/solid waste incinerators, wear-resistant thermocouples for vaporization furnaces, and even Profile TC thermocouples for the semiconductor industry (high precision, with a deviation of less than 1°C at 1000°C). There are also fast-response thermometers for nuclear power applications, and so on. Therefore, just like all instruments, temperature measurement devices are based on thermal resistors and thermocouples, and their design and manufacture take into account modern materials science and welding techniques; more importantly, they are developed according to actual parameters such as the process used, the medium involved, temperature, and pressure in the specific application. Speaking of ourselves, I used to work for several foreign-owned companies that had manufacturing facilities in China for temperature sensors (in fact, there aren’t many such companies in China). I held positions as a product manager and in sales, and I also visited factories in various countries – in Germany, France, the United States, and Canada. I consider myself to have a fairly good understanding of foreign manufacturers’ production processes and the materials they use ； Our two other colleagues are, one is an expert in instrumentation from China – the manager and chief engineer of a temperature measurement factory owned by a state-owned enterprise in Shanghai, as well as the vice president of an industry association ； Another person has served as production director and sales director at the factory of the largest imported temperature sensor brand in China today, and is highly familiar with manufacturing and installation processes in the petrochemical industry. No further introduction is needed; for more details, please visit our website. As a starting point, we have considerable experience in the design and production of temperature measurement devices. We are eager to exchange ideas with those of you who have extensive practical experience in this field, in order to understand the challenges, expectations, and requirements you face during use. I also hope that our discussion can bring some insight and assistance to those who read this post. Thank you!
A 100-degree Celsius thermoresistor with a length of 6.5 meters often experiences the rupture of its protective tube during use, resulting in the exposure of the insulating material and inadequate grounding. Please analyze the reasons
Let’s share some of our thoughts: First of all, with a length of over 6 meters, it should be an armored thermistor. The armored covering can burst; I’ve heard foreigners talk about this before and have also seen some photos. Its application was for temperature measurement down in oil wells, as the corrosive effects of sulfur or other components in crude oil could cause the sheath of the armored thermocouple to break, leading to the expansion of magnesium oxide inside. This is a situation that thermocouples used for underground temperature measurement often encounter. The cladding materials used by foreigners are also of high quality; aside from 316, it’s Inconel600, and even Inconel825 is used in such cases. I’m not sure what the operating conditions are in your area – is it an environment with corrosion or wear and tear that has caused damage to the shielding layer of the armor, leading to the expansion of the magnesium oxide inside? Of course, the insulation performance of the product becomes poor thereafter. Could you please provide information on the actual operating conditions: Temperature: 100°C, Pressure:, Medium:. I searched on my computer for a long time but couldn’t find those photos of magnesium oxide exploding; I remember they were quite impressive. If it’s convenient for you, please send two photos as well for us to take a look. If it’s not the case, I’ll consult our experts again
The thermal resistor is armored and made of 316 material; it is installed in a flange style. The armored resistor is protected by an outer sleeve with a diameter of φ30, which prevents corrosion. The pressure applied is likely due to the thermal expansion of the air inside the sleeve. The photo wasn’t saved to the phone
This post was last edited by Sai Xin on 2022-4-6 at 17:00. Wow, there are also protective tubes and a temperature of 100°C – in such conditions the armor still gets damaged. . I’m speechless. You can just buy a 7-meter wire-type thermistor for a few dozen yuan on Taobao, put it inside a protective sleeve, and it still works. What’s the medium outside the sleeve? Could the sheath be leaking? It’s unlikely to be due to thermal expansion and contraction; 100°C isn’t much of an issue. It is only when the temperature reaches 700, 800°C or higher that thermal expansion and contraction can have an effect on the armor material
It seems you don’t have enough experience yet. We have changed manufacturers at least five times; we’ve tried both imported and domestic ones, and the results were more or less the same.
This post was last edited by Qingfeng Hailai on 2022-4-7 09:14
Hello, the original poster. Could you talk about the details that need to be considered when selecting temperature instruments in the fine chemical industry? Thank you.
This post was last edited by Sai Xin on 2022-4-7 at 11:12. The temperature here is not high, at 100°C, so it’s completely unnecessary to use armoring. It can be changed to a wire-based structure with metal tube protection, as shown in the figure below. This way, there will be no issue of magnesium oxide exploding. If necessary, we can provide you with one or two samples without any problem. Of course, this still doesn’t solve the actual problem; if it’s convenient for you, you could also let us know the medium inside the protective tube.
Thank you! This issue covers a wide range. I think that compared to large-scale facilities such as refineries and ethylene plants, the fine chemical industry uses finer pipes, with more sophisticated designs, as well as integrated temperature control systems and multiple/micro multiple outlets; in other words, the components are relatively more delicate. Temperature measurement in thin pipes is actually a rather tricky task. We have clients whose pipes have an inner diameter of 10 mm, and temperature measurement is still required for them. If a thermistor is used, its length is only 3–5 mm. At present, the thinnest thermistors available in China have a diameter of 2 mm, which is already quite impressive; however, this not only affects the flow of the fluid but also results in inaccurate temperature readings. Alternatively, a very thin thermocouple of 1 mm or 0.5 mm can be fixed to the surface of the pipe, with an insulating layer applied, in order to measure the surface temperature ； In such cases, users sometimes are not willing to go along with it; other times, clients insist on following their requirements, and in those situations we recommend installing it at a corner, facing the flow direction of the medium ； Another time, we used a 0.5mm T-type thermocouple (which offers higher accuracy at low temperatures compared to the K-type) and inserted it into the pipeline, allowing it to drift along with the medium; this should provide a relatively accurate representation of the actual temperature. I can’t think of any other special considerations specific to fine chemical processing – are there any requirements related to sanitary polishing?
Missing process parameters such as medium pressure, medium flow rate, the relationship between insertion direction and medium flow direction, etc； However, with such a large penetration depth, it is necessary to calculate the vibration frequency, and resonance can easily occur ；