Question 1

Are "mil" and "mm" the same?

Accepted Answer

No. In the US, mil = 1/1000 inch = 0.0254 mm. 1 mm ≈ 39.37 mil, roughly a 39x difference. Mil is used in PCB design, coating thickness, etc. To avoid confusion, "thou" is sometimes used in the US.

Question 2

Should I use mm or inches in engineering drawings?

Accepted Answer

Internationally, mm is the standard for engineering drawings (ISO). In the US, inches are commonly used. Always specify the unit on the drawing. Also distinguish between decimal inches and fractional inches.

Question 3

What is the most common mistake in area conversions?

Accepted Answer

Using the length conversion factor directly. For example: 1 ft = 0.3048 m does NOT mean 1 ft² = 0.3048 m². You must square it: 1 ft² = 0.3048² = 0.0929 m².

Question 4

What is the relationship between hectare and acre?

Accepted Answer

1 hectare = 2.471 acres. Rounding to 2.5 can cause cumulative errors in land surveying.

Question 5

How much is a "barrel"?

Accepted Answer

It depends on the application. Oil barrel = 42 US gallons (159 L). US beer barrel = 31 US gallons. UK beer barrel = 36 Imperial gallons. Wine barrels vary by region (Bordeaux 225 L, Burgundy 230 L). Always verify which barrel is meant.

Question 6

What is the difference between US gallon and Imperial gallon?

Accepted Answer

1 US gallon = 3.785 L, 1 Imperial gallon = 4.546 L, about a 20% difference. Countries outside the US (UK, Canada, etc.) may use Imperial gallons, so be careful when comparing fuel efficiency (MPG).

Question 7

What are MT and MMT?

Accepted Answer

MT = Metric Ton = 1,000 kg. Don't confuse with SI prefix M (Mega). MMT = Million Metric Tons = 10⁹ kg, where "MM" follows the Roman numeral convention (thousand-thousands = million).

Question 8

Is a "ton" in HVAC a unit of weight?

Accepted Answer

No. In HVAC, 1 ton (= 1 RT, ton of refrigeration) = 12,000 BTU/h of cooling capacity. It's completely different from the mass unit ton. See the RT entry in the Power category.

Question 9

What is the difference between Troy ounce and regular ounce?

Accepted Answer

Regular (avoirdupois) oz = 28.35 g, Troy oz = 31.1 g. Precious metals (gold, silver) use Troy ounces. A Troy pound is 12 oz (regular pound is 16 oz).

Question 10

What is the difference between kgf and kg?

Accepted Answer

kg is mass (SI base unit), kgf (kilogram-force) is the force exerted by standard gravity on 1 kg. 1 kgf = 9.80665 N. The confusion arose historically because both use "kilogram" in their name.

Question 11

What is a slug?

Accepted Answer

A mass unit in the US customary system. 1 slug = 14.594 kg. It's the mass that accelerates at 1 ft/s² when 1 lbf is applied. Introduced to avoid lbm/lbf confusion, but rarely used today.

Question 12

What is a kip?

Accepted Answer

1 kip = 1,000 lbf (kilopound-force), used in US structural engineering for large loads. Don't confuse with kPa (kilopascal).

Question 13

What is the difference between psi, psig, psia, and psid?

Accepted Answer

psi generally means gauge pressure (psig). psig = relative pressure above atmospheric (a tire gauge reads 0 at atmospheric). psia = absolute pressure from perfect vacuum. psid = differential pressure between two points. Reading 100 psi as psia instead of psig can cause ~15% overpressure.

Question 14

What is the difference between bar, barg, and bara?

Accepted Answer

"bar" alone is ambiguous between absolute and gauge. barg = bar gauge (relative to atmosphere), bara = bar absolute (relative to vacuum). 0 barg ≈ 1.013 bara (at sea level). Design documents should always specify bara or barg.

Question 15

Are mmHg and torr the same?

Accepted Answer

Practically identical, but strictly different. After modern redefinition, torr is about 0.000015% smaller than mmHg. 1 atm = 760 torr = 760.000 mmHg (approximate). Medicine uses mmHg (blood pressure), vacuum engineering uses torr.

Question 16

Can I mix °C and K in thermodynamic calculations?

Accepted Answer

For temperature differences (ΔT), 1°C = 1 K, so they're interchangeable. But for formulas requiring absolute temperature (ideal gas law PV=nRT, radiation laws, etc.), you must use K.

Question 17

Where is Rankine (°R) used?

Accepted Answer

It's the absolute temperature scale based on Fahrenheit, where °R = °F + 459.67. Used in some US thermodynamics and aerospace formulas. It relates to Fahrenheit the same way Kelvin relates to Celsius.

Question 18

What is the difference between nautical mile and statute mile?

Accepted Answer

1 nautical mile = 1.852 km = 1.151 statute miles. A nautical mile corresponds to 1 minute of latitude, used in navigation and aviation. A "knot" is 1 nautical mile/hour = 1.852 km/h.

Question 19

Why does aviation use knots?

Accepted Answer

Aviation charts are based on latitude/longitude, so knots (where 1 minute of latitude = 1 nautical mile) are intuitive for distance calculations. It's specified as an ICAO international standard.

Question 20

Aren't N·m and J (joule) the same unit?

Accepted Answer

They have the same dimensions but different physical meanings. Torque is a vector (rotational force), energy is a scalar (work). SI uses only N·m for torque, not J.

Question 21

What is the difference between ft·lbf and lbf·ft?

Accepted Answer

Officially, lbf·ft = torque, ft·lbf = energy. But in practice, both are often called "foot-pounds." SAE standards use lbf·ft for torque.

Question 22

What does "MM" mean in MMBtu and MMkcal?

Accepted Answer

"MM" comes from Roman numeral convention: M(1,000) × M(1,000) = 1,000,000 (million). Same value as SI prefix M (Mega) but different origin. Widely used in US oil/gas/HVAC industries. MMBtu = 1,000,000 BTU, MMkcal = 1,000,000 kcal. Note: MBtu can mean either 1,000 BTU (Roman M) or 1,000,000 BTU (SI Mega) depending on context — use MMBtu to avoid confusion.

Question 23

What is the difference between kW and kWh?

Accepted Answer

kW (kilowatt) is power = rate of energy consumption. kWh (kilowatt-hour) is energy = power × time. Analogy: kW is car speed, kWh is distance traveled. Electricity bills are charged in kWh.

Question 24

Is there only one BTU?

Accepted Answer

Actually, there are several definitions: BTU(IT) = 1,055.06 J, BTU(thermochemical) = 1,054.35 J, etc. The difference is less than 0.5% but exists. Most engineering calculations use BTU(IT).

Question 25

What is a therm?

Accepted Answer

1 therm = 100,000 BTU ≈ 105.5 MJ. Used for natural gas billing in the US and UK. Relationship to MMBtu: 10 therms = 1 MMBtu.

Question 26

What is the difference between HP, BHP, and WHP?

Accepted Answer

HP = rated output. BHP (Brake HP) = output measured at the crankshaft with a dynamometer. WHP (Wheel HP) = output measured at the wheels (includes drivetrain losses, lowest value). The US has used SAE net HP as the standard since 1972.

Question 27

Are PS and HP the same?

Accepted Answer

No. PS (German Pferdesstärke) = CV (French) = pk (Dutch) = metric horsepower = 735.5 W. Anglo-American HP = 745.7 W. About 1.4% difference. European car specs use PS, US uses HP.

Question 28

What is a ton of refrigeration (RT)?

Accepted Answer

1 RT = 12,000 BTU/h = 3,516.85 W. Derived from the heat of fusion needed to melt 2,000 lb (1 short ton) of ice at 0°C to water at 0°C over 24 hours. Used in HVAC/refrigeration for cooling capacity.

Question 29

What is ksi?

Accepted Answer

1 ksi = 1,000 psi ≈ 6.895 MPa. Used in US structural engineering to express material strength. Example: A36 steel yield strength = 36 ksi ≈ 248 MPa.

Question 30

Stress and pressure have the same units — what's different?

Accepted Answer

Units are identical (Pa, psi), but stress is force/area inside an object, while pressure is force/area applied externally. Sign conventions and failure criteria differ.

Question 31

What is the difference between SCFM and ACFM?

Accepted Answer

SCFM = Standard CFM (volumetric flow at standard conditions), ACFM = Actual CFM (at actual operating conditions). At high pressure/temperature, ACFM and SCFM differ significantly. Note: "standard conditions" vary by organization — CAGI: 14.5 psia/68°F, ASME: 14.7 psia/68°F, ISO: 14.7 psia/59°F.

Question 32

Is GPM based on US gallon or Imperial gallon?

Accepted Answer

Unless otherwise specified, GPM generally refers to US gallons. 1 US GPM ≈ 3.785 L/min. Imperial GPM is about 20% larger. To avoid confusion, use L/min or m³/h.

Question 33

What is the difference between density and specific gravity?

Accepted Answer

Density has units (kg/m³), specific gravity (SG) is dimensionless. SG = substance density / water density (at 4°C). Water SG = 1.0, density = 1,000 kg/m³.

Question 34

Are specific gravity and specific weight the same?

Accepted Answer

Not at all. Specific gravity (SG) is a dimensionless density ratio, while specific weight is weight per unit volume (N/m³ or lbf/ft³). They're often confused because of similar names.

Question 35

What is the relationship between cP and Pa·s?

Accepted Answer

1 cP = 1 mPa·s = 0.001 Pa·s. Poise (P) is a CGS unit, Pa·s is SI. Note: 1 P = 0.1 Pa·s (10x, not 100x).

Question 36

How do I convert from cP to cSt?

Accepted Answer

cSt = cP / (density in g/cm³). Without knowing the density, conversion is impossible. Only for water (~1 g/cm³) does cP ≈ cSt hold.

Question 37

Which viscosity is used in lubricant grades?

Accepted Answer

Lubricants use kinematic viscosity (cSt) measured at 40°C and 100°C. ISO VG grade numbers (VG 32, VG 46, etc.) represent the midpoint cSt value at 40°C.

Question 38

Is R-19 insulation the same performance worldwide?

Accepted Answer

No. R-value numbers differ by unit system. US R-19 (ft²·°F·hr/BTU) is about R-3.35 in SI (m²·K/W). Always check units in international projects.

Question 39

Was the specific heat of water the basis for the calorie definition?

Accepted Answer

Yes. The calorie was originally defined as "the energy needed to raise 1 g of water by 1°C." That's why water's specific heat = 1 cal/(g·°C) = 4,186.8 J/(kg·K).

Question 40

Are "moment of inertia" and "second moment of area" the same?

Accepted Answer

In structural engineering, I = second moment of area (units: mm⁴ or in⁴). In dynamics, I = mass moment of inertia (units: kg·m²). Same name, completely different physical quantities.

Question 41

What is the difference between elastic and plastic section modulus?

Accepted Answer

Elastic section modulus = I/c (moment of inertia / distance to neutral axis). Plastic section modulus = sum of first moments of area above and below neutral axis. Plastic is always larger: ~10-15% for W-shapes, 50% for rectangles.

Question 42

Why are US and European notations reversed?

Accepted Answer

Historical convention differences. AISC (US): S = elastic section modulus, Z = plastic section modulus. Eurocode (Europe): W_el = elastic, W_pl = plastic. Since the symbols differ, always verify which design code is being followed in international structural design.

Question 43

Why are batteries rated in mAh instead of Wh?

Accepted Answer

mAh intuitively connects to current × time (2000 mAh battery, 500 mA consumption → ~4 hours). But Wh is more accurate for energy comparison. Laptop batteries are usually rated in Wh.

Question 44

How many Coulombs is 1 Ah?

Accepted Answer

1 Ah = 3,600 C. The Coulomb is too small for battery engineering, so Ah/mAh are used instead.

Question 45

What is the difference between 50 Hz and 60 Hz power systems?

Accepted Answer

50 Hz: Europe, most of Asia, Australia, etc. 60 Hz: US, Canada, South Korea, parts of Japan, etc. A 2-pole generator runs at 3000 rpm (50 Hz) or 3600 rpm (60 Hz). Connecting a 50 Hz motor to 60 Hz makes it spin 20% faster.

Question 46

Why distinguish between rad/s and Hz?

Accepted Answer

1 Hz = 2π rad/s ≈ 6.283 rad/s. In control engineering and vibration analysis, confusing angular frequency ω (rad/s) with frequency f (Hz) causes a 2πx (≈6.3x) error.

Question 47

What is the most common angle-related mistake in engineering calculations?

Accepted Answer

Entering a radian value when the calculator/code is in degree mode, or vice versa. sin(90) = 1 (degrees) but sin(90) ≈ 0.894 (radians). Small-angle approximation (sin θ ≈ θ) only works in radians.

Question 48

What unit does a Unix timestamp use?

Accepted Answer

Depends on the system: seconds (10 digits), milliseconds (13 digits), microseconds (16 digits). Misinterpreting the precision can give dates in the 1970s or tens of thousands of years in the future.

Frequently Asked Questions

Length

Area

Volume

Mass

Force

Pressure

Temperature

Velocity

Torque

Energy

Power

Stress

Flow Rate

Density

Viscosity

Kinematic Viscosity

Thermal Conductivity

Specific Heat

Moment of Inertia

Section Modulus

Electric Charge

Frequency

Angle

Time