DP Transmitter

>a differential pressure instrument with the low leg vented to atmosphere will give a more consistent reading than a simple pressure instrument, owing to changes in atmospheric pressure due to changing weather conditions affecting the reported level in the tank otherwise.>

It won't. If the gauge and DP devices have the same range, each will produce exactly the same pressure measurement result.

Pressure measurement is like measuring voltage. A voltmeter has two leads, a positive lead and a negative lead. A voltage measurement is the difference between the voltage at the positive lead and the voltage at the negative lead. Voltage is inherently a differential measurement. You can't get a (viable) voltage reading with only one lead connected to a circuit.

All three pressure measurements, absolute, gauge and differential, are also inherently differential measurements. Each pressure measurement references a distinctly different reference point.

Absolute pressure measurement references (as close as one can get to) absolute zero pressure, a hard vacuum.

Gauge pressure measurement references atmospheric pressure.

Differential pressure measurement references some applied pressure which can be applied to a port connected to the measuring element.

An absolute pressure device has a chamber evacuated (as close as one can get) to absolute zero pressure, a hard vacuum. A mechanical bourdon tube absolute pressure gauge has its bourdon tube surrounded by a chamber that holds a hard vacuum.

A gauge pressure device has its element referenced to atmospheric pressure.
A mechanical bourdon tube 'gauge' pressure gauge has its bourdon tube surrounded by the atmosphere, subject to atmospheric pressure.

A differential gauge pressure device has some means to apply two different pressures to its sensing element, hence a high port and a low port.

A DP pressure device plumbed for head pressure on an open tank, with its low side open to atmosphere provides the same measurement as a gauge pressure device. Both reference atmospheric pressure.

Many manufacturers choose not to offer separate low range gauge pressure transmitters, for business reasons. DP's are routinely available in low ranges and it is easy to use them for low pressure gauge measurement by leaving the low port open to atmosphere.