The power supply for the temperature transmitter must be free of voltage spikes; otherwise, the transmitter may be easily damaged. Transmitter calibration should be performed 5 minutes after power-up, and the ambient temperature at the time of calibration should be noted. When measuring high temperatures (>>100°C), the sensor cavity and the terminal box should be thermally isolated using a filling material to prevent the terminal box from overheating and damaging the transmitter. When using the sensor in environments with severe electromagnetic interference, the housing should be firmly grounded to prevent interference; power supply and signal output connections should utilize shielded cables (typically Ø10mm), and the cable gland nuts must be tightened to ensure an airtight seal. Only the RWB-series temperature transmitter offers a 0–10 mA output (utilizing a three-wire configuration); however, at output levels below 5% of the full scale, non-linearity may occur due to the cutoff characteristics of the transistor circuitry. Temperature transmitters should be calibrated once every six months. If a DWB-series unit cannot undergo linear correction due to circuit limitations, it is advisable to select an appropriate measurement range-in accordance with the product manual-to ensure optimal linearity.
Causes of Inaccurate Data Display
1. Long cable runs resulting in signal attenuation;
2. Impedance mismatch within the signal lines;
3. Signal interference due to a lack of shielding.
Integrated Temperature Transmitters
An integrated RTD (Resistance Temperature Detector) transmitter is a compact device designed to be installed directly within the terminal head of an RTD sensor. Typically, an integrated temperature transmitter consists of a temperature-sensing probe (either a thermocouple or an RTD sensor) and a two-wire solid-state electronic module. By utilizing a solid-state module form factor, the sensing probe is mounted directly inside the terminal head, thereby creating a single, integrated transmitter unit. Integrated temperature transmitters are generally categorized into two main types: RTD-based and thermocouple-based.
An RTD temperature transmitter comprises several functional blocks, including a reference unit, an R/V (resistance-to-voltage) conversion unit, a linearization circuit, reverse-polarity protection, current-limiting protection, and a V/I (voltage-to-current) conversion unit. After the RTD signal is converted and amplified, the linearization circuit compensates for the inherent non-linear relationship between temperature and electrical resistance; finally, the V/I conversion circuit outputs a constant-current signal (typically 4–20 mA) that is linearly proportional to the measured temperature.
A thermocouple temperature transmitter typically consists of various circuit blocks, such as a reference source, cold-junction compensation circuitry, an amplification unit, a linearization processor, a V/I converter, open-circuit detection/handling circuitry, reverse-polarity protection, and current-limiting protection. This device functions by taking the thermoelectric potential generated by a thermocouple, compensating for cold-junction effects, and amplifying the signal. Subsequently, a linearizing circuit is employed to correct for the inherent non-linearity between the thermoelectric potential and temperature, before the signal is finally amplified and converted into a standard 4–20 mA current output. To prevent accidents resulting from temperature control failure-caused by a broken thermocouple wire during measurement-the transmitter is equipped with a fail-safe protection circuit. Should the thermocouple wire break or suffer from a poor connection, the transmitter will output a maximum signal value (28 mA), thereby triggering the associated instrumentation to cut off the power supply.
Integrated temperature transmitters offer numerous advantages, including a simple structure, reduced wiring requirements, a robust output signal, strong immunity to interference, excellent linearity, compatibility with simple display instrumentation, and the use of solid-state modules that provide resistance to vibration and moisture. Furthermore, they feature reverse-polarity protection and current-limiting protection, ensuring reliable operation.
The integrated temperature transmitter produces a standardized 4–20 mA output signal, making it fully compatible with computer-based control systems as well as other conventional instrumentation. Upon user request, the device can also be manufactured as an explosion-proof or fire-resistant measuring instrument.