What should a ratiometer temperature indicator read when the electrical power is off?

What should a ratiometer temperature indicator read when the electrical power is off?

• A. The same as outside air temperature gauge
• B. Zero
• C. Ambient room temperature
• D. Pointer off scale on the low side

Answer: (D)

A ratiometer temperature indicator needs electrical power to drive the pointer. The sensing element and the bridge it’s part of depend on current to deflect the needle against a restoring force. When power is removed, there’s no current to hold the deflection, so the internal spring pulls the pointer to the low end of the scale, causing an off-scale low indication. This design clearly signals that the instrument isn’t energized, rather than showing a valid temperature. In other words, without power, you can’t read a real temperature—the pointer defaults to the low end to warn of power loss.

A ratiometer temperature indicator needs electrical power to drive the pointer. The sensing element and the bridge it’s part of depend on current to deflect the needle against a restoring force. When power is removed, there’s no current to hold the deflection, so the internal spring pulls the pointer to the low end of the scale, causing an off-scale low indication. This design clearly signals that the instrument isn’t energized, rather than showing a valid temperature. In other words, without power, you can’t read a real temperature—the pointer defaults to the low end to warn of power loss.