A HVAC differential pressure transmitter is a device used to measure the difference in pressure between two points in a system. It is used to monitor the pressure drop across a filter, a fan, a pump, or any other device in a HVAC system. The differential pressure transmitter measures the difference between two pressures and sends an electrical signal to a controller or display device. This signal is then used to adjust the system to maintain a desired pressure level. The differential pressure transmitter is an important part of any HVAC system, as it allows for accurate monitoring and control of the system. By measuring the pressure drop across a device, the HVAC system can be adjusted to maintain a desired pressure level, thus ensuring the system is running at its optimal efficiency.