Regarding temperature detection, why are there no transmitters on site?

Transmitters reduce errors and interference in line resistance. It increases costs with transmitters, but it’s convenient to use.

Typical temperature transmitters have the capability to connect thermocouples and thermal resistors. However, the terminal connections differ: thermal resistors are usually three-wire systems, while thermocouples are two-wire systems. If this is the case, it can be replaced. Of course, thermocouples require compensation wires for the metal wires, and this needs to be taken into account.

Yes, for example, the commonly used Rosemont 644

1. There are many such instruments; they are generally called integrated temperature transmitters. So why is it said that there aren’t any? For example, Ross’s 644 series. 2. A transmitter is used for remote transmission, with the signal being in current form; theoretically, this should result in slightly higher accuracy, but I don’t have any experimental data to show by how much. However, using resistance/voltage signals directly in the card module is cheaper from a cost perspective, as there is no need to purchase a transmitter; yet, thermocouple compensation wires are more expensive than ordinary wires, and three-wire resistance systems are definitely more costly than two-wire current systems. There is no actual data to show by how much it can be cheaper. Many times, it still depends on the design habits of the design institute.

Models such as Rosemount 644, 248, etc. come with either integrated or separate temperature sensors. Using a transmitter may increase costs, and it also adds an additional piece of equipment that requires maintenance; unless required by the process requirements, such a setup is generally not used

What the original poster mentioned already exists. It is called an integrated thermocouple thermal resistor; some also refer to it as an integrated temperature sensor. 　

An integrated temperature transmitter can be used, such as the Rosemount 644, 248, and others as mentioned by everyone

Transmitters are installed on-site to facilitate signal transmission and connection to the DCS. However, their use increases costs, raises the amount of maintenance required, and elevates the failure rate; therefore, temperature transmitters should generally not be used unless necessary. Thermocouples and thermal resistors are connected directly to the instrument to display temperature, which is simple and convenient. Personal opinion...........

1. Install a transmitter on-site, and then the 4-20ma standard signal can be fed into the DCS; this should also allow for temperature measurement. 2. Why aren’t thermocouples and thermal resistors required to use such transmitters? What is the difference between the two? Answering both questions together: It is possible to have the transmitter on-site output 4-20ma directly. But it is also limited. Analyze based on the specific circumstances at the site. 1. In terms of cost, transmitters are more expensive than thermal resistance thermocouples. 2. The transmitter requires power supply, while thermoresistive and thermocouple types can be connected directly with wires. (For resistive types, use compensation wires; for inductive types, use a three-wire system over long distances to eliminate wire resistance errors.) 3. In temperature measurement applications where the device is close to a heat source and the ambient temperature is high, the transmitter circuit is prone to temperature drift. 4. The requirements for transmitters in explosion-proof environments are even higher. 5. The on-site environment is complex with severe interference; there are high-power frequency converters present. It is relatively easy to deal with the interference caused by thermal resistors and thermocouples, but it is difficult to handle interference affecting the transmitter circuit itself. 6. Both DCS/PLC systems have direct-reading thermistor and thermocouple modules, and their prices are not higher than those of current modules. This is just my personal opinion; please feel free to correct me

Save costs by avoiding unnecessary installations. The difference is as follows: First, regarding the nature of the signal, a thermal resistor is itself a resistor, and changes in temperature cause positive or negative variations in its resistance value; whereas a thermocouple generates changes in induced voltage, which varies with changes in temperature. Secondly, the temperature ranges detected by the two types of sensors are different. Thermal resistors generally detect temperatures in the range of 0–150 degrees (although they can also detect negative temperatures), while thermocouples can detect temperatures in the range of 0–1000 degrees (or even higher). Therefore, the former is suitable for detecting low temperatures, while the latter is suitable for detecting high temperatures. Third, in terms of material, a thermal resistor is a metal material that exhibits temperature-sensitive changes; a thermocouple, on the other hand, is made of two different metals, and due to temperature changes, a potential difference is generated at the ends of these two different wires.