Guide to Diagnosing Temperature Measurement Inaccuracies: Sensor Selection, Thermal Lag, Wire Resistance and Transmitter Setup

1. Description of the problem and scope of application

This guide is intended for diagnosing temperature measurement inaccuracies in industrial systems. Applies to equipment with various types of temperature sensors, including thermocouples, thermistors, thermocouples and electronic sensors. The problem is considered critical because incorrect temperature readings can lead to improper process control, personnel hazards, and reduced production efficiency.

2. Safety precautions

Requirements for personal protective equipment: Use protective gloves, headgear and protective equipment against electrical damage during measurement.
Stopping the power supply: Before starting the diagnostics, perform the lockout/tagout procedure to ensure safety when working with electric drives and electronic systems.
Residual Energy Requirements: Check for residual energy in actuators or electronic systems before disconnecting and removing items.

3. Necessary diagnostic tools

Name of the tool	Model/specification	Measuring range	Purpose of use
Multimeter	Fluke 87V	0–2000 Ω, 0–20 mA, 0–600 V	Measurement of wire resistance, current, voltage
Thermograph	FLIR T1020	-20°C to 650°C, resolution 0.08°C	Determination of thermal distribution on the surface
Optical distance meter	Leica DISTO D8	0.01–50 m	Measuring distances to the sensor
Diagnostic vibration system	Keysight 35670A	0–100,000 Hz	Determination of the influence of vibration on measurement accuracy

4. Check before starting diagnostics

Requirements	action
Working conditions	Record the ambient temperature, humidity, pressure
Recent changes	Check whether sensors, cables, transmitter settings have been changed
A history of anxiety	Check the alarm logs of the temperature measurement system

5. Systematic diagnostic scheme

1. Symptom: Inaccuracy of temperature readings on the system screen.
   1. Check whether the ambient temperature corresponds to the measuring range of the sensor.
   2. Using a thermograph, determine whether there is unevenness of temperature on the surface.
   3. Measure the resistance of the sensor wires using a multimeter.
2. Symptom: Increase in temperature reading.
   1. Check whether there is no thermal delay due to incorrect selection of the sensor.
   2. Measure the resistance of wires, determine the effect of electrical resistance on accuracy.
   3. Check transmitter settings for noise or amplitude distortion.
3. Symptom: Decrease in temperature reading.
   1. Check for electrical noise due to distance from the sensor.
   2. Measure the distance to the sensor, determine the effect of signal delay.
   3. Check that there is no thermal drift effect due to incorrect use of the sensor.

6. Matrix of causes of defects

Symptom	Reasons (in order of probability)	Diagnostic test	Expected result
Inaccuracy of readings	Wrong sensor selection Thermal delay High wire resistance Incorrect transmitter setting	Measurement of wire resistance Determining the distance to the sensor Measuring the temperature around the sensor Checking transmitter settings	Wire resistance > 50 Ω Distance to the sensor > 10 m The temperature around the sensor is out of range Incorrect transmitter setting

7. Analysis of root causes

7.1. Incorrect sensor selection

Critical cause. The selection of the sensor does not correspond to the operating conditions. For example, a thermocouple is not suitable for measuring temperatures above 1200°C, and a thermistor is not suitable for wet or high temperature conditions. This leads to inaccurate readings.

How to check: Measure the temperature around the sensor and compare with the sensor requirements. Use a thermograph to determine the temperature distribution.

Potential consequences: Improper management of processes, causing energy and production accidents.

7.2. Thermal delay

Causal defect. The sensor does not respond quickly to temperature changes due to the incorrect selection of the sensor type or its incorrect location. For example, a thermocouple sensor may not meet operating conditions, resulting in a signal delay.

How to check: Measure the resistance of the wires, determine the distance to the sensor. Use a thermograph to analyze the thermal distribution.

Potential consequences: Incorrect process management, incorrect temperature regulation, low production efficiency.

7.3. High wire resistance

Causal defect. The high resistance of the wires causes measurement inaccuracy due to the effect of electrical resistance on the signal. Wire resistance exceeds 50 Ω.

How to check: Measure the resistance of the wires using a multimeter. Check the distance to the sensor.

Potential consequences: Measurement inaccuracy, deviation from standards, improper process management.

7.4. Incorrect transmitter setting

Causal defect. Incorrect transmitter tuning results in noise, amplitude distortion, or deviation from the measurement range. For example, measurement noise can cause additional effects on the readings.

How to check: Use a multimeter to check current and voltage. Perform a system vibration analysis.

Potential consequences: Measurement inaccuracy, deviation from standards, improper process management.

8. Recovery sequence

8.1. Incorrect sensor selection

1. Choose a sensor that meets the operating conditions. For example, to measure temperatures above 1200°C, use a thermodigger.
2. Install a new sensor in place of the old one.
3. Check the conformity of the measurement of the measuring system.

8.2. Thermal delay

1. Measure the resistance of the sensor wires. If resistance exceeds 50 Ω, replace.
2. Determine the distance to the sensor. If the distance is >10 m, install a new wire.
3. Install the sensor in the place of the old one, choosing the optimal location.

8.3. High wire resistance

1. Measure the resistance of the sensor wires. If the resistance is >50 Ω, replace the wire.
2. Check the distance to the sensor. If >10 m, install a new wire.
3. Install the sensor in the place of the old one, choosing the optimal location.

8.4. Incorrect transmitter setting

1. Using a multimeter, check the current and voltage at the output of the transmitter.
2. Perform a vibration analysis of the system using a vibration analyzer.
3. Configure the transmitter according to the manufacturer's documentation.

9. Prevention methods

The root cause	Prevention strategy	Monitoring method	Recommended interval
Incorrect sensor selection	Selection of the sensor according to operating conditions	Thermography	Annual inspection
Thermal delay	High-speed sensor selection	Measuring the distance to the sensor	Annual inspection
High wire resistance	Using wires with low resistance	Measurement of wire resistance	Annual inspection
Incorrect transmitter setting	Setting the transmitter according to the measurement range	Measurement of voltage and current	Annual inspection

10. Spare parts and components

Description of the component	Specification	When to replace	UNITEC-D category
Thermocouple temperature sensor	Type K, range -20°C to 1350°C	Wire resistance >50 Ω	Category 12
Temperature sensor of the thermocouple	Type J, range -20°C to 1200°C	Wire resistance >50 Ω	Category 13
Sensor wire	Copper wire with resistance <50 Ω	Wire resistance >50 Ω	Category 11
Temperature transmitter	Range 0–1000°C	Incorrect setting	Category 14

For component replacement see: https://www.unitecd.com/e-catalog/

11. List of references

• DSTU 3030-95: Requirements for measuring devices
• EN 60584: Thermocouples
• ISO 9020: Temperature measurement
• DSTU 2968-95: Organization of production
• UNITEC-D catalog: Technical documentation