Choosing, Installing, and Calibrating Industrial Thermometer

1. Right Choice for Pressure (Differential Pressure) Measuring Instruments

The correct selection of pressure measuring instruments involves defining the type, range, accuracy, and sensitivity, as well as considering additional functions such as remote transmission, indication, recording, regulation, and alarm.

By Process Measurement Requirement:

• Static, Steady State Conditions: The full-scale creep should not exceed 2/3 of the highest rule value at use pressure.

• Pulsating Pressures: The maximum measured pressure should be equivalent to 1/2 of the full scale.

• Accuracy Level: Choose the accuracy level required for the production process.

By the Type of Measured Medium:

• Considerations: The phase (gas, liquid), temperature, layering, corrosion potential, pollution level, and flammability.

• Specialized Instruments: Use oxygen gauges, acetylene gauges, corrosion-resistant Thermometer, high-temperature Thermometer, and diaphragm-type Thermometer.

According to the Environment:

• Environmental Factors: Consider factors such as temperature, corrosion, vibration, and humidity levels, which can affect the performance of the instrument.

• Vibration Resistance: For environments with extreme vibrations, select a Thermometer that can withstand such conditions.

By Instrument Location and Lighting:

• Dial Sizes: Choose instruments with dial sizes that are easily visible to personnel.

2. Correct Mounting of Pressure (Differential Pressure) Measuring Devices

Accurate pressure indication requires a properly installed measurement system. This ensures that the whole system, and not just the instrument, accurately represents the properties of the measured medium.

Key Aspects:

• Location of Pressure Ports: Properly locate the pressure ports to prevent mistakes.

• Routing of Connection Pipes: Position connecting pipes to avoid induced discrepancies.

• Proper Installation of Instruments: Place instruments in locations where they can provide accurate readings.

3. Assessment and Testing of Pressure (Differential) Measuring Devices

Calibration and proofing of instruments are necessary before use. Newly purchased pressure measuring instruments should be metrologically inspected before installation to ensure accuracy, considering potential damage during transport. Regularly used instruments should be periodically verified according to national standards. Calibration should include both the primary and secondary instruments if part of a remote system.

Special Note on Single-Coil Bourdon Tube Pressure Gauges:

Single-coil Bourdon tube pressure gauges, widely used in industrial applications, often exhibit substantial deviation during calibration. Adjustment methods include:

1. Uniform Deviation at Every Point:

   • If the reading is incorrect, place the pointer back to zero after pressurization and re-verify starting at the first verification point (excluding zero).

2. Linear Error:

   • If the error grows slowly, move the adjustment screw outward to increase the lever length. Conversely, move it inward to decrease the lever length.

3. Positive (Fast) / Negative (Slow) Deviation:

   • For positive deviation (fast), rotate the movement counterclockwise to increase the angle between the push rod and sector gear. For negative deviation (slow), rotate clockwise to decrease the angle.

4. Deviation Near Halfway Point:

   • Pressurize and refit the needle at the midpoint. If the error remains, check and adjust the angle between the pull rod and sector gear.

5. Deviation at a Single Point or Two Points:

   • Check the gear mesh for dirt or burrs (positive deviation) or wear and damage (negative deviation).

6. Continual Drift Despite Recalibration:

   • If the pointer does not return to zero after multiple adjustments, the Bourdon tube may be deformed and require replacement.

After adjustments, reassemble the instrument and perform verification according to the appropriate standards.

Proper selection, installation, and calibration of pressure gauges are vital for accurate readings and plant safety. Well-maintained pressure gauges help identify potential hazards, enabling safety management and ensuring proper functioning of processes.

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