MIL-PRF-87252 Specific Heat: PAO Coolant Calculator

MIL-PRF-87252 Specific Heat Capacity Chart

MIL-PRF-87252 PAO coolant specific heat capacity versus temperature chart in J/kg-K compared with 60/40 ethylene glycol and water

About the data: no government report or journal paper with measured MIL-PRF-87252 properties was found. All specific heat values on this page are manufacturer typical values from one table (ROYCO 602 data sheet, identical in the AeroShell Fluid 602 listing). They are not specification limits and carry no stated uncertainty. MIL-PRF-87252D itself only gives informational “typical” values.

Sheet values: −17.8 °C to 260 °C (0 °F to 500 °F), four points. The solid red line is a straight-line fit to those points. The dashed red line below −17.8 °C (0 °F) is extrapolated and is not measured data. Coolant loops at −40 °C therefore rely on the extrapolation (the specification’s typical value at −40 °C is shown for reference).

Circles are the four ROYCO 602 sheet points. Triangles are MIL-PRF-87252D Table III typical values (informational). The blue line is 60/40 ethylene glycol–water by mass from CoolProp, drawn from −50 °C to 100 °C only.

Data from: ROYCO 602 data sheet (ANDEROL, 2004) and AeroShell Fluid 602 (AeroShell Book, 2003); MIL-PRF-87252D Table III; comparison from CoolProp 7.2.0 (Melinder 2010).

Toggle data sets by clicking the legend.

MIL-PRF-87252 Specific Heat Capacity Calculator

Degrees (°F):

Degrees (°C):

J / (kg·K):

kJ / (kg·K):

Btu / (lbm·°F):

cal / (g·°C):

J / (g·K):

kWh / (kg·K):

Data basis:

Per litre, ρ·Cp (kJ/(L·K)):

60/40 EG-water (J/(kg·K)):

60/40 EG-water, ρ·Cp (kJ/(L·K)):

Calculator valid −54 °C to 260 °C (−65.2 °F to 500 °F), atmospheric pressure, using the linear fit to the sheet values. Below −17.8 °C (0 °F) the “Data basis” row says Extrapolated. The per-litre row uses the density fit (extrapolated below 0 °C). The glycol rows are shown only from −50 to 100 °C.

Specific Heat Capacity of MIL-PRF-87252 PAO Coolant

What Is Isobaric Specific Heat Capacity (Cp)?

Specific heat capacity (Cp) is the heat needed to raise the temperature of one unit of mass by one degree at constant pressure. For this coolant it is given in J/(kg·K) [SI] or Btu/(lbm·°F) [US]. For a coolant it is the most important property, because it sets how much fluid you must pump to carry away a heat load:

Q = ṁ · C_p · ΔT

where Q is the heat flow (W or Btu/h), ṁ is the mass flow (kg/s or lb/h) and ΔT is the temperature rise of the coolant (K or °F).

Worked Example: Heat Load and Flow

Example. An avionics rack puts 5 kW (17,060 Btu/h) into the coolant, and the loop is designed for a 10 K (18 °F) rise at a mean temperature of 40 °C (104 °F). At 40 °C the Cp is 2258 J/(kg·K) (0.539 Btu/(lb·°F)) and the density is 780 kg/m³. The mass flow is ṁ = 5000 / (2258 × 10) = 0.221 kg/s (797 kg/h, 1758 lb/h). The volume flow is 0.284 L/s, or 4.5 gal/min. 60/40 glycol–water needs only 0.155 kg/s and 2.3 gal/min for the same job. Use the same method with the LMTD calculator to size the heat exchanger that rejects the load.

About This Data

The specific heat values are the four entries of the ROYCO 602 data sheet, tested by ASTM D2766: 0.49, 0.54, 0.63 and 0.72 cal/(g·°C) at −17.8, 37.8, 149 and 260 °C (0, 100, 300 and 500 °F). The AeroShell Fluid 602 listing repeats the same table. They are manufacturer typical values for one product with no stated uncertainty. No government or journal measurement was found.

The curve is the straight-line fit Cp = 2119.89 + 3.4461·T J/(kg·K), solid from −17.8 to 260 °C and dashed (extrapolated) below. It departs from the sheet values by at most 9.2 J/(kg·K). Cp rises almost linearly with temperature, from about 1982 at −40 °C to 2464 at 100 °C.

MIL-PRF-87252D Table III gives its own typical values (0.48 at −40 °C, 0.53 at 25 °C, 0.55 at 75 °C, 0.65 at 149 °C). They are shown as separate markers and are not averaged with the sheet values. See also density, thermal conductivity, Prandtl number and the general properties page. For glycol–water see ethylene glycol heat capacity.

MIL-PRF-87252 PAO Coolant — Isobaric Specific Heat Capacity vs. Temperature

Temp (°C)	Temp (°F)	Sheet Cp (cal/(g·°C))	Sheet Cp (J/(kg·K))	Sheet Cp (Btu/(lb·°F))	Linear fit (J/(kg·K))
-17.8	-0.0	0.49	2050	0.49	2058.6
37.8	100.0	0.54	2259	0.54	2250.2
149	300.2	0.63	2636	0.63	2633.4
260	500.0	0.72	3012	0.72	3015.9

Sheet columns are the ROYCO 602 “Specific Heat, Cp, cal/g-°C” entries (ASTM D2766), repeated in the AeroShell Fluid 602 listing. 1 cal/(g·°C) = 4184 J/(kg·K) = 1 Btu/(lb·°F). Fit: Cp = 2119.89 + 3.4461·T J/(kg·K), T in °C. Typical values, no uncertainty stated.

Specification Typical Values

Temp (°C)	Temp (°F)	MIL-PRF-87252D Table III (cal/(g·°C))	Table III (J/(kg·K))	Sheet-fit Cp (J/(kg·K))	Difference
-40	-40.0	0.48	2008	1982*	-1.3 %
25	77.0	0.53	2218	2206	-0.5 %
75	167.0	0.55	2301	2378	+3.4 %
149	300.2	0.65	2720	2633	-3.2 %

Table III (printed p.8) lists “typical (inherent)” values for information. They are not a requirement. The −40 °C fit value is extrapolated (“*”). The two sources agree within about 3 %.

Temp (°C)	Temp (°F)	Cp (J/(kg·K))	Cp (kJ/(kg·K))	Cp (cal/(g·°C))	Cp (Btu/(lb·°F))	Basis
-54	-65.2	1933.8*	1.9338*	0.4622*	0.4622*	Extrapolated
-40	-40.0	1982.0*	1.9820*	0.4737*	0.4737*	Extrapolated
-20	-4.0	2051.0*	2.0510*	0.4902*	0.4902*	Extrapolated
0	32.0	2119.9	2.1199	0.5067	0.5067	Measured range (fit)
20	68.0	2188.8	2.1888	0.5231	0.5231	Measured range (fit)
40	104.0	2257.7	2.2577	0.5396	0.5396	Measured range (fit)
60	140.0	2326.7	2.3267	0.5561	0.5561	Measured range (fit)
80	176.0	2395.6	2.3956	0.5726	0.5726	Measured range (fit)
100	212.0	2464.5	2.4645	0.5890	0.5890	Measured range (fit)
120	248.0	2533.4	2.5334	0.6055	0.6055	Measured range (fit)
150	302.0	2636.8	2.6368	0.6302	0.6302	Measured range (fit)

“*” marks values extrapolated below the lowest sheet temperature (−17.8 °C, 0 °F). The fit is measured-range only from −17.8 to 260 °C.

MIL-PRF-87252 vs 60/40 Ethylene Glycol–Water

At 20 °C the PAO coolant carries about 29 % less heat per kilogram than 60/40 glycol–water (2189 vs 3098 J/(kg·K)) and about 48 % less per litre (1735 vs 3337 kJ/(m³·K)). For the same heat load and temperature rise you need roughly 1.4 times the mass flow and 1.9 times the volume flow. PAO is chosen for its dielectric strength and its low-temperature flow, not for its heat capacity.

Temp (°C)	Temp (°F)	PAO Cp (J/(kg·K))	MEG-60 Cp (J/(kg·K))	PAO lower by (per kg)	PAO ρ·Cp (kJ/(m³·K))	MEG-60 ρ·Cp (kJ/(m³·K))	PAO lower by (per litre)
-50	-58.0	1948*	2612	25.4 %	1632*	2883	43.4 %
-40	-40.0	1982*	2684	26.2 %	1648*	2958	44.3 %
-20	-4.0	2051*	2828	27.5 %	1679*	3100	45.8 %
0	32.0	2120	2967	28.5 %	1708	3228	47.1 %
20	68.0	2189	3098	29.3 %	1735	3337	48.0 %
40	104.0	2258	3218	29.8 %	1760	3426	48.6 %
60	140.0	2327	3323	30.0 %	1783	3492	48.9 %
80	176.0	2396	3410	29.8 %	1803	3533	49.0 %
100	212.0	2464	3476	29.1 %	1822	3549	48.7 %

“*” marks PAO values that use extrapolated Cp or density (below −17.8 °C for Cp, below 0 °C for density). ρ·Cp is the heat stored per cubic metre per kelvin. 1 kJ/(m³·K) = 0.0149 Btu/(ft³·°F). MEG-60 values are CoolProp 7.2.0 (Melinder 2010), valid here from −50 to 100 °C only.

Sources

ANDEROL Inc., ROYCO 602 Coolant for Aircraft Avionics Systems data sheet (edited 19 Oct 2004). Royco602.pdf. Manufacturer typical values: specific heat (ASTM D2766 as printed) at −17.8, 37.8, 149 and 260 °C.. Opened and read 25 Oct 2026.
Shell Aviation, AeroShell Fluid 602, The AeroShell Book, Edition 18 (2003), pp. 240–243. AeroShellfluid602.pdf. same specific heat table. Opened and read 25 Oct 2026.
US Department of Defense, MIL-PRF-87252D, Performance Specification: Coolant Fluid, Hydrolytically Stable, Dielectric, 23 November 2015. everyspec.com. Table III (p.8) typical specific heat at −40, 25, 75 and 149 °C; no requirement. Opened and read 25 Oct 2026.
CoolProp 7.2.0, incompressible fluid INCOMP::MEG[0.6] (60 % ethylene glycol in water by mass), 1 atm. coolprop.org/fluid_properties/Incompressibles.html. The CoolProp page cites Å. Melinder, Properties of Secondary Working Fluids for Indirect Systems, IIF-IIR Publishing, 2010, as the source of the MEG data (T_min −100 °C, T_max 100 °C, mass fraction 0 to 0.60). The Melinder book itself was not opened. Specific heat of the comparison fluid.

No measurement method detail or uncertainty is given on the sheets. Values below −17.8 °C are extrapolated.

MIL-PRF-87252 Specific Heat Capacity: PAO Coolant

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