The HPG Prototype experiment (netID 9P) comprised two deployments.  See Webb & Barclay, JGR, 2023 for details.

HPG channel data are stored as pressure difference across the sensor in Pa:  10.6 counts/Pa and 105 counts/Pa for Pacific and Atlantic experiments, respectively.  The HPG tube ends were 2.57 m and 2.61 m apart for Y and X channels, respectively.  As an example, the gain for the Y HPG channel during the Pacific experiment would be 2.57 * 10.64 = 27.34 counts/(Pa/m).

Deployment 1:  Pacific Ocean, 2015, station PAC
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Two seismometers, one absolute pressure gauge, two horizontal pressure gauge channels:

9P.PAC.00.HHZ, HH1, HH2		Trillium Compact seismometer, deployed on seafloor
9P.PAC.00.HDH			HPG channel X
9P.PAC.01.HHZ, HH1, HH2		Trillium Compact seismometer, mounted inside frame
9P.PAC.01.HDH			HPG channel Y
9P.PAC.02.HDH			Absolute pressure gauge

Deployment 2:  Atlantic Ocean, 2018, station ATL
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Two seismometers, two horizontal pressure gauge channels:

9P.ATL.00.HHZ, HH1, HH2		Trillium Compact seismometer, deployed on seafloor
9P.ATL.00.HDH			HPG channel X
9P.ATL.01.HHZ, HH1, HH2		Trillium Compact seismometer, mounted inside frame
9P.ATL.00.HDH			HPG channel Y