Xiaobei asked: What is the rationale behind using relative terms to refer to the positive and negative sides of a pressure transmitter?

When measuring the liquid level, since the pressure difference acting on the transmitter is not zero when the liquid level is at zero, zero-point adjustment is required to ensure that the transmitter’s output corresponds to the actual liquid level. Zero-point migration is divided into positive migration and negative migration.

What is the positive and negative sides of a transmitter relative to?

This post was last edited by jujiangliu on 2017-3-1 07:23. The output of the transmitter is “directly proportional” to the pressure on the positive pressure side, and “inversely proportional” to the pressure on the negative pressure side.

Generally, liquid level measurement is carried out under positive pressure. When there is no liquid present, a differential pressure may exist between the positive and negative sides; this pressure difference needs to be offsetted so that the display returns to zero. This differential pressure is what needs to be offsetted, and it can occur on either the positive or negative side

It is usually a double-flange level transmitter; should it be installed above or below the pressure tapping point?

The pressure transmitter does not have separate positive and negative ports; it has only one port. Differential pressure transmitters come in positive and negative pressure interfaces. Migration is only used during the liquid level measurement process. Things such as the drum liquid level, and whether the positive pressure side is connected to air or water, require distinguishing between positive migration and negative migration. Simply put, it is to *accustom people to observation or to eliminate large errors.

The pressure transmitter has only one interface, with positive pressure only and no negative pressure, unless suction is applied. For a differential pressure transmitter, the pressure difference is obtained by subtracting the pressure on the negative side from the pressure on the positive side. When only the high-voltage side is pressurized, the value you see is positive; otherwise it is negative. Negative migration means that, after installation but before operation, the pressure on the negative-pressure side is too high, resulting in an output of not 4 milliamps. Migration is used to adjust the output so that it becomes 4 milliamps

Poster: Pressure transmitters measure pressure; they have only one interface, with no distinction between positive and negative pressure interfaces. A differential pressure transmitter measures the difference between two pressures; therefore, it has two interfaces. The pressures connected to these two interfaces are different, which results in a distinction between high pressure and low pressure. That is, h represents the high-pressure side, and l represents the low-pressure side ； It is also appropriate to use the positive-pressure side to represent the high-pressure side, and the negative-pressure side to represent the low-pressure side. This is the purpose of having positive and negative pressure sides. As for negative transfer, someone has already spoken about it before. Migration actually occurs when, after the transmitter is installed, additional forces not required for measurement are applied to the pressure chamber, causing the transmitter’s output data to change and preventing it from accurately reflecting the actual value of the measured quantity. The process of correcting such changes is known as migration adjustment.