In psychrometrics, the state of moist air is described by a set of properties used on the psychrometric chart to relate temperature, moisture, and energy. The seven commonly listed properties are dry-bulb temperature, wet-bulb temperature, dew point, relative humidity, humidity ratio (mass of water vapor per mass of dry air), enthalpy, and specific volume. Absolute humidity—mass of water per volume of air—looks related but isn’t treated as one of the standard independent psychrometric properties. It depends on pressure and temperature and isn’t used as a primary coordinate on the chart, so it isn’t included among the seven. The other properties cited (dew point, enthalpy, humidity ratio) are core elements of the psychrometric framework, describing condensation conditions, energy content, and moisture content per unit of dry air, respectively.

Back siphonage happens when the pressure in the water supply line falls below ambient pressure, creating suction that can pull non-potable water back into the potable system through a cross-connection. The condition that can cause this drop is a sudden decrease in system pressure. A sudden rise in pressure, stable pressure, or gradual warming of water do not create the negative pressure needed for back siphonage. This is precisely why backflow prevention devices are used—to prevent contaminants from being drawn back into the system when pressure dips occur.

Conductivity measures how well water conducts electricity, which rises with the concentration of dissolved ions. TDS is the total amount of dissolved solids, most of which become ions in solution. Because the amount of dissolved ions drives conductivity, you can estimate TDS from conductivity values rather than measuring solids directly. In practice, a conversion factor is applied: TDS (mg/L) ≈ conductivity (µS/cm) × factor, with the factor depending on the water’s ion composition (commonly around 0.5 to 0.7 for many freshwater samples). This is an approximation, since some nonionic substances don’t affect conductivity much and highly saline waters may require a different factor. pH, on the other hand, reflects acidity/alkalinity and does not indicate how much dissolved solid is present, which is why TDS is estimated from conductivity rather than measured by pH.

Boiling point is the temperature at which a liquid’s vapor pressure equals the surrounding pressure. If you lower the surrounding pressure, that balance is reached at a lower temperature, so the liquid boils at a lower temperature. This is why water boils at lower temperatures on high mountains, where air pressure is reduced. In a vacuum, a liquid can even boil at room temperature because the external pressure is so low. The key idea is that boiling begins when vapor pressure matches external pressure, and lowering that external pressure lowers the required temperature for boiling.