This image shows two different pressure measurement devices.
The top symbol shows a pressure sensor with an analogue output signal.
The bottom symbol shows a traditional pressure measuring, manometer gauge.
Both symbols would have two connections if they measured differential pressure.
The symbol shows a fluid flow meter with digital display. For different displays or electrical communication systems, the flow meter symbol would remain the same but the symbol in the square connecting box would change.
The bottom symbol shows how to measure flow in both directions with a single flow meter. Four check valves are used in a Wheatstone bridge layout to direct the flow.
This symbol shows a fluid an inline particle counter or contamination monitor. This meter also has a third, tank drain line. The circle indicates the counter display. These units are commonly supplied by a pressure compensated flow control valve and this component would be shown with an additional symbol.
This symbol shows a thermometer or fluid temperature sensor with a single, adjustable switch or electrical break-contact.
This symbol shows a fluid level sensor with four switches or electrical break-contacts. The number is used in this case to indicate the number of contacts although it may be preferable to show each contact and write the electrical circuit number next to each individual switch.