It can be hard to track the various measures used for hydrogen systems. Below are some conversions and definitions:

**STP = standard temperature/pressure = 0°C (32°f) and 1 bar (≈1 atmosphere).**But: Some references say STP is 25°C (77°f). We do not understand this discrepancy.

NTP = normal temperature/pressure = 20°C (68°f) and 1 atm (atmosphere). Functionally, NTP is almost the same as STP , and STP is more common.

**1 atm = 1 atmosphere = 14.7 psi.**

**scf = “scuff” = standard cubic foot = 1 c.f. (STP).**

ncf = normal cubic foot = 1 c.f. (NTP).

**1 gallon = 3.78 liters**

**1 cubic foot = 7.5 gallons = 28.25 liters.**

1 liter = 1000 cc (cubic centimeters).

1 kilogram = 1kg = 1000 grams.

**The Ideal Gas Law: Pressure x Volume = PV = constant.**

**no. of atoms x Temperature nT**In practice we have temperature and volume both constant. “No. of atoms” is equivalent to “amount of gas”.

**The Gas Law says that for fixed volume, if we double (or triple etc) the pressure we double (or triple etc) the amount of gas. This is an awesome result.**For example, a 60 cubic foot tank filled to 200 psi (14 atm) contains about 60 x 14 = 840 scf of gas.This equality is accurate for hydrogen (and most gases) to within 2% up to 30 atm (≈450 psi). Even at higher pressures, say 3000 psi, the Gas Law is accurate to within 15-20% for hydrogen (it predicts a bit low). Temperature is in degrees Kelvin, so it’s effectively constant: Because 0° C = 273° K, and 20° C = 293° K, the ratio of two real-life Kelvin temperatures is always close to 1, and thus does not affect the equation.

**A K-cylinder holds about 1 cubic foot. The scf it holds will vary with the gas and the company.**** A K-cylinder of hydrogen (at 2500 psi) holds roughly 200 scf. A K-cylinder of nitrogen (at 3000 psi) holds roughly 250 scf.** (Note that these figures are higher than the Gas Law predicts.)

A **mole** is a certain amount of stuff, defined as 6 x 10^{23} molecules of the compound (Avagadro’s number). The mass of 1 mole of a compound is equal to its atomic weight, in grams. For example, 1 mole of water (H_{2}O) has 2 x 1 + 16 = 18 grams mass.

**Avagadro’s Law:** **The volume of 1 mole of any gas (stp) = 22.4 liters. This is an amazing, useful result.**

**Mass of 1 mole hydrogen gas (H _{2}) = 2 grams. So the mass of 22.4 liters (stp) H_{2} is 2 g.**Mass of 1 mole nitrogen gas (N

_{2}) =28 g.Mass of 1 mole oxygen gas (O

_{2}) = 32 g.Mass of 1 mole air ≈ 29 g.Mass of 1 mole propane (C

_{3}H

_{8}) = 44 g.Mass of 1 mole natural gas (mostly methane, CH

_{4}) ≈ 16 g.Mass of 1 mole gasoline (C

_{8}H

_{18}) = 114 g.

Air ≈ 78% nitrogen, 21% oxygen, 1% other (argon, particulates,,water vapor), 0.03% CO_{2}(!).

Avagadro’s Law also implies that for gases at equal pressure and temperature, proportions of volume are the same as proportions of amount of gas. For example, 1 cc of H_{2} mixed with 1 liter of air gives a 0.1%, or 1000 ppm, concentration of H_{2}.

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**Amps x Volts = Watts. **For example, a 100 watt bulb draws 0.83 amps at 120 volts.

*Joule, Watt-hour, Btu* and *calorie* are all units of energy. Watt-hours are the most convenient unit for electric systems.

**1 Watt = 1 Joule/second. So 1 Watt-hour = 3.6 kJ.** This is the most relevant conversion.

1 kW = 1 kilowatt = 1000 watts.

1 Btu ≈ 1 kJ = 1 kiloJoule = 1000 J.

1 calorie = 4.184 J. (Aside: A “calorie” as used for food energy is actually a kilocalorie, or 1000 calories, so an english muffin contains 130,000 calories, = 130 “calories”.)

**Energy storage of a lead-acid L-16 battery: 350 amp-hours @ 6 volts =2100 Wh. nominal. Effective (usable) energy is half, ie about 1 kWh.** The usable storage is less because draining batteries beyond about half shortens their lifespan drastically.

**Energy Density of H _{2 }(STP) = 3.2 – 3.5 Watt-hours/liter.** The figure can be calculated different ways (in particular,assuming different temperatures), giving different results.

**Energy released by combustion of H**This is the energy released in the reaction H

_{2}= 242 kJ/mole._{2}+ ½ O

_{2}→ H

_{2}O (steam) + heat. In a fuel cell, part of this energy is electrical, part is heat. Compare natural gas (800 kJ/mole) and gasoline (5500 kJ/mole).Per kilogram, H

_{2}stores more energy because a mole of H

_{2}weighs so much less. But 242 kJ of H

_{2}takes up the same volume (= 22.4 liters, stp) as 800 kJ of natural gas. Gasoline, because it is a liquid, is much more dense- 22.4 liters of gasoline is roughly 160 moles, containing 900,000 kJ of energy! It is more energy-dense than dynamite. One can readily see why our civilization has become so dependent on oil- it is miraculous stuff. But hydrogen is miraculous too, in different ways.