# Tag Archive: temperature

## Hydraulic oil density and Specific Gravity

Hydraulic oil Density is the ratio of its mass to the volume of space it occupies:

## \rho=\frac{m}{V} ##

The accepted units of measurement for density according to ASTM are kilograms per cubic meter (kg/m3, SI unit) or grams per milliliter (g/mL).

Hydraulic oil Specific Gravity (or “Relative Density”) is the ratio of Hydraulic oil density to water density at the specific temperature:

## SG_{oil}=\frac{\rho_{oil}}{\rho_{water}} ##

Substance with SG <1 will float on water (like hydraulic oil); substance with SG >1 will sink in water (like honey).

Next, both hydraulic oil density and Specific Gravity vary with temperature and pressure.

This is why the ASTM D1298-12b “Standard Test Method for Density, Relative Density, or API Gravity of Crude Petroleum and Liquid Petroleum Products by Hydrometer Method” states that accurate determination of the API gravity, density or relative density (specific gravity) uses a standard temperature of 60 degrees F (15 degrees C).

Reference: [https://www.astm.org/d1298-12b.html]

You can find in the oil specification table of every hydraulic oil manufacture either Oil Specific Gravity or Oil Density at the specific temperature of 60°F or 15°C.

As a result, to calculate Specific Gravity or Hydraulic oil density at 15°C and, respectively, Hydraulic oil density or Specific Gravity at 15°C. By ASTM D1298-12b water density at 15°C is 0.999103 g/ml, therefore:

## SG_{oil}=\frac{\rho_{oil}}{0.999103} ##

## \rho_{oil}=\frac{SG_{oil}}{0.999103} ##

## SUN cartridges and low temperatures

Recently we got a request from the customer to check the system components to work in a low-temperature environment up to -40°C. Most components in the customer’s system were SUN cartridges.

And I was really surprised by the mess of info on the Sun Hydraulic website about seals that SUN uses in their cartridges.

For example, on the “Seals: Materials of Construction” page you can find the manufacturer’s temperature range: from -22° to 230° F (-30° to 110° C) for Buna N (Nitrile) and -22 to 260° F (-30 to 125°C) for Polyurethane O-rings.

But, if you download Technical Product Information PDF from their library, you can find different temperature range for Buna N: -50°F to 200°F which means -45°C..+93°C and no info about Polyurethane O-rings.

This is why I was confused and had a lot of questions to SUN:

1. Why the different info is provided on SUN website?
2. Why there isn’t a choice for Polyurethane O-rings in seal material when selecting cartridges in the SUN catalog?
3. Why SUN has so high temperatures for the low environment for both Buna N and Polyurethane seals?

Really, all competitors have better low-temperature limits for their cartridges, for example:

– Parker’s Nitrile: from -40°C to +93.3°C
– Danfoss/Comatrol, Buna-N: from -40°C
– Bucher, Buna “N”: from -40°C to +120°C
– Doering, Buna N: from -42.7°C to +93.3°C
– Eaton, Buna N: from -40°C to +120°C
– Hydraforce’ Buna “N”: -40°C to +100°C; polyurethane: -54°C to +107°C.

Because it was critical for my application, I have sent a request directly to SUN to get answers to the questions above and have gotten next response: the correct low work temperature for Buna N is -30°C. Сoncerning the polyurethane seals – this is not a standard option, but any cartridge can be assembled with polyurethane o-rings that provides low temperature rate to -40°C. Not a big difference in the price, but because this is going to be a custom modification the delivery time for cartridges (statement on November 2021) is 15..16 weeks (what actually is not bad for SUN).

## Temperature-Viscosity Chart

The interpolation calculator provided below is used to determine:

• the kinematic viscosity values, depends from temperature, calculated by the formulas specified in ASTM D341 – (‘Standard Practice for Viscosity-Temperature Charts for Liquid Petroleum Products’);
• the Viscosity Index (VI) by the formulas specified in ASTM D2270 (Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C 1) or in ISO 2909 (Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 and 100°C 1) or in ГОСТ 25371-2018 (Нефтепродукты. Расчёт индекса вязкости по кинематической вязкости)

#### Annotation.

Any mineral oil is supplied with the specification of kinematic viscosity values at two different temperatures, usually at 40°C and at 100°C (or at 100°F and at 210°F). These data is enough to determine kinematic viscosity at any other values of temperature and as well Viscosity Index.

You can calculate pre-filled values of oil classified by ISO 3448 with grades VG22, VG32, VG46 or VG68 (at viscosity Index = 100) or enter specific values for any other oils.

## Pressure change due to temperature

A change in temperature will cause hydraulic fluid to try to have a corresponding change in volume. If the fluid is trapped in a chamber and is unable to change volume, there will be a change in pressure.

The difference in pressure is based on the bulk modulus (stiffness) of the fluid. A mineral based oil may have a pressure difference of about 11 bar for each 1°C change in temperature (90 psi for each 1°F change in temperature):

## \triangle p=\triangle t\cdot k ##

where k= 90 (imperial units) or k= 11 (metrical units):

## \triangle p\;\lbrack PSI\rbrack\;=\triangle t\;{\lbrack^\circ F}\rbrack\cdot90 ##

what is equal to:

## \triangle p\;\lbrack bar\rbrack\;=\triangle t\;{\lbrack^\circ C}\rbrack\cdot90 ##