🔍 Key Concepts

• Pneumatic force-balance devices convert a sensed variable into a proportional air signal. • Look at what process variable is actually applied to the sensing element labeled P₄ and the spring in unit 4. • Notice that the output air signal Pₐ goes to a control system, while the supply and bleed form a standard pneumatic transmitter/controller arrangement.

💭 Think About

• Ask yourself: in this sketch, is the instrument directly moving a valve stem or only producing a pneumatic signal that could be sent somewhere else? • Which of the answer choices normally uses a pressure acting on a diaphragm against a spring to create a proportional pneumatic output signal? • Do level or temperature measuring devices usually look like this, or do they require additional elements (floats, bulbs, capillaries) that are not shown here?

✅ Before You Answer

• Identify what physical quantity is applied at P₄; is it a pressure, level, or temperature signal? • Check whether any mechanical linkage to a valve stem is drawn; if not, be cautious about choosing a "valve positioner" answer. • Remember that a force-balance transmitter typically has: a supply, a bleed/nozzle, and an output air pressure that is proportional to the sensed variable. Match that to the most appropriate choice.