Thermistors (such as PT100, PT1000) are commonly used temperature sensors in the industrial field, and their temperature measurement accuracy is affected by various factors. The reasons for temperature measurement deviation are analyzed from three aspects: hardware, installation, and environment, and targeted solutions are proposed.
1、 Errors caused by hardware factors
Resistance temperature characteristic deviation:
The nominal resistance value of a thermal resistor (such as 100 Ω for PT100 at 0 ℃) may have initial deviations due to differences in production processes. If not calibrated at the factory or aged during use, it will cause the temperature measurement value to deviate from the true value.
Solution: Select a precision (± 0.15 ℃) thermistor and regularly send it to a measuring institution for calibration; Intelligent transmitters with temperature compensation function are used for key equipment to automatically correct the resistance temperature curve.
Lead resistance interference:
The lead resistance of a three wire or two wire thermistor will be added to the measurement circuit, especially during long-distance transmission (such as>50 meters), and the lead resistance (about 0.1 Ω/meter) may introduce several degrees Celsius errors.
Solution: Adopt a four wire connection method to eliminate the influence of lead resistance; If a three wire system must be used, it is necessary to ensure that the material and length of the three leads are consistent, and to correct the lead resistance through hardware compensation (such as bridge balancing) or software.
Poor connector contact:
Oxidation, looseness, or virtual soldering of wiring terminals can cause an increase in contact resistance, resulting in fluctuations or underestimation of measured values.
Solution: Regularly check the connector and sand the oxide layer with sandpaper; Choose gold-plated or spring-loaded connectors to improve contact reliability; Avoid using welded connections in vibrating environments.
2、 Installation and usage issues
Insufficient insertion depth:
The thermal resistance is not inserted into the measured medium (such as pipelines or furnaces), resulting in the temperature measurement point being affected by the ambient temperature, causing the measurement value to lag or be lower.
Solution: Determine the insertion depth based on the characteristics of the medium (usually 1/3~1/2 of the pipeline diameter), and install protective sleeves to reduce thermal conduction losses.
Long response time:
The material of the protective sleeve (such as stainless steel) has poor thermal conductivity, or the thermal resistance probe is too thick, which will prolong the thermal response time (such as taking more than 10 seconds to rise from 25 ℃ to 100 ℃) and cannot capture rapid temperature changes.
Solution: Choose thin-walled casing (such as 0.5mm wall thickness) or thin-film thermistor; In dynamic temperature measurement scenarios, compensate for response delay through algorithms.
3、 Environmental interference and lack of maintenance
Electromagnetic interference:
The electromagnetic field generated by devices such as frequency converters and motors may couple to the measurement circuit, causing signal fluctuations.
Solution: Thread the signal line through a metal tube for shielding, or use a thermal resistance cable with a shielding layer; Arrange sensors away from strong electromagnetic sources.
Pollution and corrosion:
The accumulation of dust, scaling, or corrosion by corrosive media on the thermal resistance probe can change its thermal conductivity and lead to temperature measurement deviation.
Solution: Regularly clean the surface of the probe; Choose corrosion-resistant materials (such as Hastelloy) for casing in corrosive environments, or install protective covers.
Cold end temperature compensation failure:
If a thermocouple compensation wire is used to connect the thermistor (incorrect connection), or if the compensation module fails, it will cause measurement errors in the cold end temperature.
Solution: Confirm that the measurement system is dedicated to thermal resistance (such as 4-20mA output), and avoid mixing thermocouple devices; Check if the compensation module parameter settings are correct.
4、 Calibration and Verification Methods
Comparative method calibration:
Place both the thermistor and a standard thermometer (such as a precision platinum resistance thermometer) in a constant temperature bath, compare the output values, calculate the error, and correct it.
Dry furnace testing:
Use a dry furnace to generate a stable temperature field and verify the accuracy of the thermistor over the full range, with particular attention to the linearity in the low-temperature range (-50~0 ℃) and high-temperature range (>300 ℃).
By optimizing hardware selection, standardizing installation processes, strengthening environmental protection, and regular calibration, the temperature measurement accuracy of thermal resistors can be stabilized within ± 0.5 ℃, meeting the requirements of industrial process control.