Gauge reference pressure measuring devices read pressure relative to atmospheric pressure. This ensures that the pressure reading is nominally zero when the pressure on the device is released.
A vented gauge reference is always open to atmospheric pressure.
The main advantage of this type of gauge reference is that over time, as atmospheric pressure changes, the zero reading will self compensate so that the device always reads zero when the pressure is released, no matter how much atmospheric pressure changes. This is particularly useful for low pressure ranges which would otherwise be significantly affected by changes in atmospheric pressure.
A sealed gauge reference means that the atmospheric pressure is sealed in at around 1 bar.
It is not always possible to provide a vent path, for example a depth sensor mounted inside a submersible vehicle which has no vent tube to the surface but the sensor has to measure depth relative to atmospheric pressure at the surface.
The other use for sealed gauge is where a manufacturer wants to provide secondary containment for high pressure safety in case the diaphragm (primary containment) bursts.
Since the introduction of more compact technologies such as silicon pressure sensors, rather than sealing in the atmospheric pressure, it is now more usual to seal in a vacuum and offset the output by 1 bar to mimic the way a sealed gauge device works.
It is mostly high pressure ranges that use a sealed gauge reference, but they can also be used for lower ranges if pressure testing over short periods of time, where the effect of atmospheric pressure changes is minimal.
Since it is very easy to create a sealed gauge from an absolute reference device, it is possible to provide a test tool such as the leo record pressure data logger which can be used for both absolute and sealed gauge reference measurements.
Is it possible to replace a pressure transmitter with a range of 0..350bar sg by one that has a range of 0..350 bar vg?
Yes it is possible. Both sensors are referenced to an atmospheric pressure. SG = Sealed Gauge, so it is a fixed to a 1 bar reference. VG = Vented Gauge, so the reference is vented to atmospheric pressure which is always changing. The changes in atmospheric pressure will be insignificant compared to the full scale range of 350 bar range and the error generated will be within the overall accuracy of most types of pressure transmitter.