An instrument valve is used for isolation of pressure gauges, orifice plates, flush rings, and various measurement instruments.  The instrument valve normally “lives” in the open position and is closed only to isolate the instrument for service or replacement.

A Transmitter Isolation Valve is a valve that is used to isolate media in a tank from a pressure / level transmitter.  The pressure measurement allows calculation of the level of liquid in a tank.  The TIV lives in the open position thus creating a communication between the media in the tank and the transmitter.  The valve is only closed when the transmitter needs to be isolated for service or replacement.