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

The pump curve uses water; you'd convert this to "what head in WATER would this pump produce?" if you're going to overlay on a water curve. Most commercial pump curves account for this automatically.

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:

Brine pumps must be designed for the SG; pump rated for water at 60 ft head delivers proportionally less in brine.

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 (-0.3% 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)