Information

3 Fluke Calibration How to choose a pressure comparator
Liquid pressure comparators
Liquid pressure comparators are normally used
for calibrating higher pressures. Since liquid
does not compress, a liquid pressure comparator
can generate higher pressures more easily than
a gas pressure comparator can. Liquid compara-
tors are also inherently less dangerous at higher
pressures, compared with the potential energy
stored in highly compressed gasses.
A liquid pressure comparator works by
changing the volume in the system. This is nor-
mally done with a screw-press variable volume
controller and a liquid reservoir. You fill the
system with liquid from the reservoir, and then
isolate the reservoir from the actual pressure
system. Then you turn the handle on the screw
press, causing a piston to move in or out of the
system. Depending upon the system’s design,
you might be able to reach pressures as great
as 140 MPa (20,000 psi).
Any gas bubbles trapped in a hydraulic
system need to be removed. Bubbles tend to
migrate upwards and collect at the highest
point in the system. To get the bubbles out, you
prime the system.
A priming pump draws liquid from the res-
ervoir and pushes it throughout the pressure
system. If the reservoir valve is open, then the
liquid is drawn in from the reservoir, pushed
through the system and then back into the res-
ervoir. If the reservoir is a relatively high point
in the system, then the air in the system will
also be pushed into the reservoir and bubbles
will appear.
If there are higher points on the system
than the reservoir, you might need to open the
system to atmosphere at a higher point by loos-
ening (“cracking”) a fitting. You would continue
to use the priming pump until liquid flowed
continuously through the opening in the fitting.
The ideal design for the priming pump includes
a displacement that allows for a large amount
of fluid to be pushed through the system at
one time and check valves to ensure that it is
always drawing fluid from the reservoir and
pushing it into the system.
With the check valve setup, the priming
pump can be easily used to also generate the
first bit of pressure. With the reservoir valve
closed, the priming pump draws liquid in from
the reservoir and pushes it into the closed
system, causing the pressure to go up. The
screw pump can then be used to generate large
pressures with no lag caused by air in the
system. You will see this type of setup on the
Fluke Calibration P5515 Pressure Comparator.
Media selection
Once you have chosen to use a liquid pressure
comparator, what liquid should you choose
to fill it? Your choice will determine what
materials are required for seals and wetted
components in the comparator and will poten-
tially impact the overall performance of the
comparator over its lifespan.
A basic mineral oil or silicon oil like Seba-
cate provides lubrication for moving parts and
is compatible with seal materials like Viton. If
your primary objective is good performance of
the comparator and extending its life span, this
type of fluid is ideal.
Unfortunately, oil is not acceptable in all
applications, especially those that need to be
hydrocarbon free. In those situations, you may
prefer to use distilled water or a water/alco-
hol mix. However, some materials may rust if
exposed to water, so be absolutely sure that all
wetted parts in the comparator are compatible
with water before using it as the fluid.
Overall, water is not as good of a lubricant as
oil, and oil is normally preferred when allowed.
The Fluke Calibration P5514 and P5515 stan-
dard units are compatible with oil (mineral oil
or Sebacate) and water.
Certain applications may require that the
calibration be performed in the exact same fluid
as what the device is normally operated in.
An example of this would be phosphate ester
based fluids used as fire resistant brake fluids
in the aerospace industry. These fluids can be
extremely aggressive, and many shops have a
segregated set of tools for use with these fluids.
The P5514-70M-EP and the P5515-140M-EP are
specifically designed for use with these types
of fluids.
Liquid (hydraulic)
Less than
10,000 psi?
Yes
P5514
No
P5515