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P5515 & P5514 , What fluid to use?

What fluid to use in a P5514 or P5515 hydraulic comparison test pump (comparator)?

All comparators are shipped dry (no fluid in them), although they have been operationally tested during manufacturing with a quick drying liquid like alcohol. 

These are the available models:

P5514-70M, PN 4185672, HYDRAULIC TEST PUMP, 70 MPA (10K PSI)
Hydraulic pressure generation and adjustment to 10k psi (70 MPa).
Screw press and reservoir with valve to manually generate and adjust pressure.

P5514-70M-EP, PN 4185685, HYDRAULIC TEST PUMP, 70 MPA (10K PSI)
Hydraulic pressure to 10k psi (70 MPa), EP seals.



P5515-140M, PN 4185697, HYDRAULIC TEST PUMP, 140 MPA (20K PSI)
Hydraulic pressure generation and adjustment to 20k psi (140 MPa).
Screw press with priming pump for fast or high volume applications.

P5515-140M-EP, PN 4185703, HYDRAULIC TEST PUMP, 140 MPA (20K PSI)
Hydraulic pressure to 20k psi (140 MPa), EP seals.

The datasheet (brochure) shows that the non-EP models have Viton o-rings wherever possible and the EP models have EP o-rings wherever possible. It states that the Viton model is operational with a wide range of fluids and that the EP model is for use with certain aggressive fluids.

To generalize, any fluid that is compatible with the noted o-rings, stainless steel, aluminum and paint can typically be used with the comparators.

Typical fluids include:
distilled water (good with Viton and EP)
isopropyl alcohol, IPA (good with Viton and EP - may make acrylics more likely to crack)
mineral oil (good with Viton, marginal with EP)
glycols, including propylene glycol - antifreeze (good with Viton and EP)
Sebacate oil (good with Viton and EP)
ST-55 oil (good with Viton, marginal with EP)
distilled water / alcohol mix
automatic transmission fluid (good with Viton, poor with EP)
low viscosity motor oil (5W, 10W - good with Viton, marginal with EP)
hydraulic equipment oils (consult manufacturer)
machine oils (consult manufacturer)
mineral oil (good with Viton, marginal with EP)

Note that if the comparator has an acrylic reservoir that alcohol will weaken and crack acrylic. An aluminum reservoir is available as PN 3921726 if desired. Brake fluids are not recommended (automotive or aircraft, poor with Viton, good with EP). Any fluid that will degrade paint will degrade the painted finish of the comparator.

Sebacate oil and ST-55 oil are pressure calibration fluids used in Fluke Calibration piston gauges and deadweight testers and have proven to be very good hydraulic calibration fluids, especially for close tolerance piston-cylinders. However, they are oils and are more difficult to clean from a gauge after calibration. See this article about Cleaning Pressure Gauges,

Here are part numbers. Contact the Fluke Calibration Orders Group at or 1.877.355.3225 for pricing, quotation or to order. SDS and/or MSDS sheets are on the website. Search for MSDS.


Distilled water and a distilled water/alcohol mix are quite easy to clean from gauges after calibration but don't lubricate as well as oils. Distilled water with perhaps a 10% to 30% mix with isopropyl alcohol will lubricate better and will also evaporate faster from gauges, but you might consider the aluminum reservoir. These fluids are almost universally available at low cost.

Fluid compatibility can be checked by doing an internet search for "o-ring compatibility". One such site is here, The only motor oil found on this site is City Service Motor Oil and shows good compatibility with Viton but poor compatibility with EPDM (EP).

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