Best Practices for Measuring Water Temperature in a Tank at STP: Thermocouples, Ungrounded Options, and Thermowells

To measure water temperature in a tank at Standard Temperature and Pressure (STP), several temperature measurement devices can be used, with thermocouples being a common choice. This article explores the suitability of thermocouples, the advantages of ungrounded thermocouples and thermowells, as well as the use of platinum temperature sensors and digital sensors.

Thermocouple

Functionality: A thermocouple works by measuring the voltage generated at the junction of two different metals, which changes with temperature.

Use in Water: Yes, a thermocouple can be used to measure water temperature effectively. However, depending on the setup, electrical interference might occur, particularly with grounded thermocouples.

Grounded vs. Ungrounded Thermocouples:

Grounded Thermocouple: This type has its junction connected to the metal sheath, providing faster response times. However, if the sheath is in contact with the water, it can introduce electrical interference.

Ungrounded Thermocouple: This type does not have the junction connected to the sheath, reducing the risk of electrical interference. It may respond slightly slower, but this is generally negligible for most applications.

Risk of Interference

Risk of Interference: When a grounded thermocouple is submerged in water, particularly if the water is conductive, there is a potential for electrical interference. This can affect the accuracy of the readings.

Recommendation: If electrical interference is a concern, using an ungrounded thermocouple or isolating the thermocouple with a thermowell is advisable.

Thermowell

Purpose: A thermowell is a protective tube that houses the temperature sensor, allowing for accurate temperature measurements while protecting the sensor from direct contact with the fluid.

Advantages:

Using a thermowell can prevent electrical interference and physical damage to the thermocouple. It allows for easy replacement of the sensor without draining the tank.

Insulated Thermocouples and Additional Solutions

Even if a thermocouple is inside a metallic jacket, and the connecting wires are insulated, it can still be used to measure the water temperature. Keeping the wires short and connecting them to an analog-to-digital board can further process the information without affecting the accuracy of the measurement.

You could also use a platinum temperature sensor such as Pt100 (100 ohm at 0°C) or a digital temperature sensor such as DS18B20. The advantage of these sensors is that you don’t need to worry about analog circuitry, making the setup more straightforward and less prone to interference.

Conclusion

In summary, a thermocouple can measure water temperature, but if using a grounded type, be cautious about potential electrical interference. Using an ungrounded thermocouple or a thermowell would be a better approach to ensure accurate readings without interference.

By considering these factors and choosing the right temperature measurement device, you can accurately measure water temperature in a tank at STP while minimizing the risk of electrical interference and physical damage.