Specific Gravity and Density for Trades Reference

Why this matters

Specific gravity (SG) and density show up in trade work whenever you're handling fluids that aren't pure water - pool chemicals, antifreeze, fuel oil, glycol-water mixes in HVAC, brine in commercial refrigeration, sediment loads in drainage systems. The math is straightforward, but knowing when SG matters - pressure calculations, pump sizing, fluid stratification - separates a tech who follows the procedure correctly from one who's confused by why the formula isn't working.

Definitions

Density = mass per unit volume.

• SI units: g/cm³ or kg/m³
• US units: lb/ft³ or lb/gal

Specific gravity (SG) = density of the substance / density of water (at 4 °C / 39 °F).

• Dimensionless ratio
• Water = 1.0 by definition
• Substance denser than water: SG > 1.0
• Substance less dense than water: SG < 1.0

Why both? Density tells you absolute weight per volume; SG tells you how a fluid compares to water in calculations.

Common SG values

Why this affects pressure calculations

Pressure from a column of fluid: P = ρ × g × h (in physics units) OR

P (psi) = h (ft) × SG / 2.31

So a 100-ft column of water at SG 1.0 produces 100 / 2.31 = 43.3 psi at the base. The same 100-ft column of brine at SG 1.2 produces 100 × 1.2 / 2.31 = 51.9 psi.

This matters for:

• Pump head calculations: pump pumping brine sees more pressure load per foot of head than pumping water
• Standpipe pressure: a hot water heater plumbing system has slightly lower pressure than cold due to density change at the same elevation
• Pressurized tanks: when filled with non-water fluids

Calculating head from pressure

Inverse: head (ft) = P (psi) × 2.31 / SG

Pump curve uses head in feet of WATER unless specified otherwise. For non-water fluids, you must convert.

Example: pump must produce 50 psi pressure boost in a chilled-water loop running 30% propylene glycol (SG 1.025): Head required = 50 × 2.31 / 1.025 = 112.7 ft of brine

You read that 112.7 ft straight off the water curve. Head in feet does not need converting between fluids; a pump that makes 112.7 ft on water makes 112.7 ft on the glycol. The conversion was from PRESSURE to head, which is where SG entered. What you do have to revisit is motor power, which scales with SG, and the viscosity correction if the fluid is thick or cold.

Mixing fluids (glycol antifreeze)

HVAC hydronic loops often use water + glycol antifreeze for freeze protection. Common mixtures:

Tradeoff: more glycol = lower freeze point BUT less heat capacity. A 50% glycol system carries 20% less heat per pound of fluid; pumps must move more fluid to deliver the same Btu, AND the pump motor sees a higher head due to the higher SG.

Use the MINIMUM glycol concentration needed for freeze protection. Most residential systems in temperate climates work at 20-30%; cold-climate systems at 40-50%.

Brine systems

Industrial refrigeration uses brine (calcium chloride or sodium chloride solution) for secondary cooling. Brine concentrations:

Size brine pumps for the SG, but be precise about what SG changes. A centrifugal pump develops the same HEAD IN FEET whatever the fluid density, so a pump rated 60 ft on water still makes 60 ft on brine. What rises with SG is the PRESSURE at that head and the brake horsepower the motor has to supply, both directly proportional to SG. A 1.23 SG brine costs you 23 percent more shaft power at the same duty point. Viscosity is a separate correction and it DOES cut head and flow, which is why cold brine and heavy glycol need the viscosity correction on top.

Pool chemistry SG

Pool chemicals vary in SG:

• Pool water (well-balanced): 1.000-1.005 (slight from dissolved chemistry)
• Pool salt (sodium chloride saturated): SG ~1.2; pool with 3500 ppm salt is SG 1.002 - basically water
• Liquid chlorine (sodium hypochlorite 12-15% strength): SG ~1.15
• Muriatic acid (HCl 31-32%): SG ~1.15
• Dichlor / Trichlor (granular): solid, doesn't affect water SG significantly

When adding liquid acid or chlorine: the dense chemical sinks to the bottom. Always pour into the deep end with pump running to circulate; otherwise it pools at the bottom and can damage liner / plaster.

Fuel oil

Residential fuel oil (heating oil #2): SG 0.83-0.85

• Density: 6.95 lb/gal (vs water 8.34 lb/gal)
• This is why a 275-gal oil tank weighs about 1,900 lb full, not 2,295 lb that pure-water would
• Tank base / floor must support this; concrete pads typically OK

Diesel / fuel oil:

• Floats on water; if water enters an oil tank, water sinks to the bottom (called "sludge layer")
• "Water in the tank" is a common service item - tank requires periodic bottom-water pumping out
• Filters at the burner catch sludge before it reaches the burner orifice

Density of air (HVAC context)

Standard air density at sea level, 70 °F: 0.075 lb/ft³

Air density drops with:

• Higher temperature (about -2% per 10 °F)
• Higher altitude (-3% per 1,000 ft elevation; significant at Denver and above)
• Higher humidity (slight)

HVAC equipment rated at sea-level conditions delivers proportionally less BTU at high altitude. Manufacturer derating tables apply.

Practical applications

Pool service:

• Adding a quart of muriatic acid is adding about 2.5 lb of chemical (vs 2 lb for a quart of water)
• Sinks to bottom; circulation critical

HVAC hydronic install:

• Calculate pump head requirements at the system SG (water-glycol mix), not just water
• Account for heat capacity reduction at higher glycol %; oversize pumps if needed

Plumbing pressure tank sizing:

• Pressure tank stores air over water; SG of water doesn't change but pressure-to-volume relationship is air-physics, not water-physics
• For unusual fluids (rare in plumbing), pressure differential changes with SG

Refrigeration brine:

• Always size pumps for the brine concentration in use
• High SG brine = higher pump load; oversize motor on the pump

Common mistakes

• Using a water pump curve for a glycol system without conversion → undersized pump
• Pouring liquid pool chemicals near the skimmer with pump off → chemical settles, damages plaster
• Sizing pressure tank for water in a fuel system → fuel SG different, pressure relationships shift slightly
• Forgetting altitude derating on HVAC equipment going from sea level installer to mountain climate

References

• ASHRAE Handbook - Fundamentals (fluid properties tables)
• ASTM D1429 (specific gravity of fluids)
• Manufacturer technical data for the specific glycol / brine product
• "Crane TP-410" (pipe friction loss in non-water fluids)