Field instruments are also referred to as smart transmitters or process control instruments. Field instruments are used for sensing, transducing, monitoring, recording, and controlling physical variables, such as flow, temperature, pressure, level, gas concentration, density, and humidity. Some field instruments are capable of measuring variables in conjunction with each other, such as mass flow as a function of differential pressure. Field instruments are often powered by 4-20mA current loops, but may also use local power. Communication is commonly by HART for loop powered instruments or by industrial field-bus for locally powered instruments. Wireless standards, such as wireless HART, are beginning to emerge.