Remote indication — choose the unsuitable thermometer: Which of the following is not suitable for distant (remote) reading over lengths up to about 60 metres in industrial practice?

Introduction / Context:
Many temperature instruments are installed at process equipment while indications must be read from a control room tens of metres away. Certain technologies are inherently suitable for remote indication via capillaries or electrical leads, while others require local visual reading. This question checks awareness of which is not appropriate for distant reading up to ~60 m.

Given Data / Assumptions:

• Distance to indicator is on the order of tens of metres.
• No special optical viewing system is assumed.
• Standard industrial mounting and cabling/capillary use cases apply.

Concept / Approach:
Mercury-in-glass thermometers require direct visual inspection of the mercury column at the bulb’s location, unless paired with special optical devices, which are not implied here. In contrast, filled-system (vapour pressure) thermometers can drive remote dials over long capillaries; RTDs transmit electrical signals over cable lengths of this scale; constant-volume gas systems can also be configured with remote manometry. Hence the traditional mercury-in-glass type is unsuitable for routine 60 m remote readout.

Step-by-Step Solution:

Verification / Alternative check:
Plant standards frequently prefer RTDs/thermocouples or filled systems when the operator panel is far from the measuring point, underscoring the limitation of simple glass thermometers.

Why Other Options Are Wrong:

Common Pitfalls:
Assuming that any thermometer can be “made remote” without additional mechanisms. The glass stem type is inherently local-read unless augmented with special optics.

Final Answer:
Mercury-in-glass thermometer