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The purpose of the DH Instruments (DHI) Visible level Interface (VLI) P/N 100693 is to interface two
pressure media directly; for example, gas and oil or oil and water. A VLI is often used when the medium
used by a piston gauge is not the same as the medium used by the test. Max working pressure of VLI P/N
100693 is 100 MPa (15 000 psi)
1.2 OPERATING PRINCIPLE
This component interfaces two pressure media in a high pressure chamber. Using two media of different
densities allows a natural separation between the two fluids that can be referenced to an indicating
pointer in the VLI and viewed through a transparent lens.
1 ea O-Ring (spare) 101063
4 ea 4x12 mm Button Head Screw NPN
1 ea. Operation and Maintenance Manual 550039
OPTIONAL PART #
1 ea.
1 ea.
VLI Stand, High
OR
VLI Stand, Low
2. INSTALLATION
100703
100702
2.2 INITIAL SETUP
Place the VLI on the same surface as the piston gauge or other reference to which it is to be connected.
Ensure the VLI and the connecting hardware are clean and free of contamination.
Connect the VLI to the standard using the connecting hardware supplied.
If the piston gauge medium has the greater density, the connection should be made at the lower port
of the VLI. If this is not true, then the connection should be made at the middle port.
Fill the VLI through the connecting hardware to the tip of the interface level indicator with the fluid that
has the highest density.
Remove the upper pet-cock on the VLI. Carefully fill the rest of the VLI with the lighter medium until
the air is completely displaced form the VLI. Install the upper pet-cock and tighten hand tight.
When interfacing a gas medium with a liquid medium the control of pressure must be performed on
the gas side. A variable volume in the hydraulic side is necessary to adjust the change in the
interface level as pressures and temperatures change.
Pressures can be generated from either side, or on both sides. The interface reference level
can then be changed by adjusting the volume on either side. When interfacing two liquids it
is preferable if they are easily identifiable. If both liquids are the same color, identification of
the interface level may be difficult.
2.3.2 INTERFACING A GAS WITH A LIQUID
It would not be possible to generate pressure by decreasing the volume in the hydraulic
pressure circuit. Pressure is increased through the gas pressure circuit. The hydraulic circuit
volume can be adjusted to fine tune the interface reference level. Again, unless the fluid
temperature or the volumes change, the level will stay the same.
Decreasing pressure is achieved by releasing gas pressure through the gas pressure circuit.
Never open a hydraulic exhaust to release pressure when using a VLI with gas. Equipment
failure and personal injury may occur.
VLI™ OPERATION & MAINTENANCE MANUAL
An additional caution must be observed when interfacing a gas with a liquid in the VLI. This
is due to the fact that once a pressure is reached, there exists a possibility that some of the
gas will be dissolved into the liquid. This condition could lead to outgassing (rapid boiling)
when returning to zero.
To prevent boiling liquid into the gas or gas into the liquid, decrease pressures slowly to
approximately 200 psi (1.4 MPa). Hold that pressure until outgassing has apparently stopped
and then decrease to zero.