In the "nerve center" of chemical production, temperature is a core parameter related to reaction efficiency, equipment safety, and product quality. From high-temperature and high-pressure reaction vessels to low-temperature distillation towers, from corrosive medium pipelines to flammable and explosive process environments, chemical thermistors serve as "temperature sentinels" to accurately capture temperature signals in complex working conditions. Its' precision 'is not accidental - from the clever design of its internal structure to the scientific support of its principles, and to the precision control in practical applications, each link hides the key to interpreting the' temperature code '.
1、 The "body structure" of thermal resistance: the basic skeleton for decrypting temperature measurement
The "precise temperature measurement" of chemical thermal resistance first stems from its rigorous structural design. Unlike ordinary industrial thermistors, the strictness of chemical scenarios (such as strong corrosion, high pressure, and severe vibration) requires its "body parts" to be adapted to extreme environments, and its core structure can be disassembled into three major "password modules":
1. Temperature sensing element: the "sensing core" of temperature signals
The temperature sensing element is the "heart" of the thermistor, responsible for converting temperature changes into resistance signals. In the field of chemical engineering, metal thermistors are commonly used, with the core being high-purity metal wires (or films) that work by utilizing the physical property of "metal resistance increasing with temperature".
Platinum resistors (Pt100, Pt1000): the "choice" for high-precision temperature measurement in chemical engineering. Platinum wire purity usually reaches 99.999%, with a stable linear relationship between resistance and temperature within the range of -200~850 ℃ (in accordance with IEC60751 standard). Even in high temperature, oxidizing or reducing environments, it can maintain stable performance, especially suitable for temperature measurement of corrosive media such as nitric acid and sulfuric acid.
Copper resistors (Cu50, Cu100): the "cost-effective choice" for medium and low temperature conditions. The linearity is excellent within the range of -50~150 ℃, and the cost is only 1/5 of that of platinum resistors. However, copper is prone to oxidation and is not corrosion-resistant, and is mostly used in non corrosive media such as cold water pipelines and lubricating oil circuits.
Nickel resistors (Ni100, Ni500): High sensitivity (temperature coefficient of resistance is twice that of platinum), but narrow linear range (-60~180 ℃), and poor long-term stability. They are only used in temporary temperature measurement scenarios in chemical engineering.
The packaging form of temperature sensing components also needs to be adapted to chemical scenarios: for example, in dusty working conditions, a "ceramic insulation skeleton+glass sintering seal" is used to prevent dust from entering and affecting the resistance value; On the reaction vessel with severe vibration, use "spring wire winding" instead of ordinary winding to reduce metal wire breakage caused by vibration.
2. Protective casing: "protective armor" for extreme environments
The media in chemical sites often carry an "aggressive" nature - strong acids, strong alkalis, high temperature and pressure, and even the erosion of granular media. If the temperature sensing element directly contacts the media, it may cause performance drift or instant damage. The protective sleeve is the key to "blocking the gun" for it.
The material selection of the casing should closely follow the "characteristics" of the working conditions:
Corrosive media (such as hydrochloric acid and chlorine gas): Choose Hastelloy C276 (resistant to wet chlorine and hypochlorite) or Titanium Alloy TA2 (resistant to seawater and dilute sulfuric acid);
High temperature and high pressure conditions (such as synthetic ammonia reactor, temperature 300 ℃+pressure 10MPa): use 310S stainless steel (resistant to high temperature oxidation) or silicon carbide ceramics (resistant to high temperature 1600 ℃, high hardness and erosion resistance);
Viscous/scaling prone media (such as resin reaction vessels): Choose thin-walled seamless sleeves (to reduce thermal hysteresis)+polished inner walls (to avoid media adhesion), and if necessary, use "anti sticking coatings" (such as polytetrafluoroethylene, pay attention to temperature resistance limit ≤ 200 ℃).
In addition to material, the structural design of the casing also has a "gateway": for example, on high-pressure pipelines, "flange connection" is used instead of threaded connection (to avoid leakage under high pressure); In conditions prone to blockage (such as coal powder pipelines), use "oblique cut casing" (to reduce medium accumulation) or reserve "blowing ports" (to regularly introduce nitrogen for cleaning).
3. Lead system: a "noise free channel" for signal transmission
The resistance signal of the temperature sensing element needs to be transmitted to the secondary instrument through the lead wire. If the lead wire resistance changes with the ambient temperature (such as temperature fluctuations in chemical workshops), it will directly "contaminate" the measurement signal - this is also a common cause of accuracy deviation in chemical thermal resistance. Therefore, the design core of the lead system is to "eliminate lead resistance interference".
There are three commonly used lead methods in chemical engineering:
Three wire system: Connect one lead at each end of the temperature sensing element, and connect the third lead to the midpoint of one end of the lead. Balance the lead resistance with temperature through a bridge, suitable for scenarios with moderate accuracy requirements (such as ordinary pipeline temperature measurement, with an error of ≤ 0.5 ℃);
Four wire system: Two leads are connected to each end of the temperature sensing element (two for current flow and two for voltage measurement), completely avoiding the influence of lead resistance. It is a "standard" for high-precision temperature measurement (such as precise temperature control in the reaction kettle, with an error of ≤ 0.1 ℃);
Integrated lead: Integrate the lead with the sleeve (such as "armored thermistor"), protect the lead with a metal armor layer (such as stainless steel), and enhance its resistance to vibration and impact. It is suitable for installing chemical equipment with narrow space (such as heat exchanger tube bundles).