Misting system glossary

Short, precise definitions of the terms used across MistGuide. Each entry links to the guide that covers it in depth.

Jump to: 1 A B C D E F G H I L M O P R S U V W

1

10-24 thread

The most common misting nozzle thread (No. 10 screw size, 24 threads per inch). It is not interchangeable with 12-24. More on 10-24 thread

A

Anti-drip nozzle

A misting nozzle with a small spring-loaded check valve that closes when line pressure falls below its opening pressure, so water left in the line does not drip out after the pump stops. More on anti-drip nozzle

B

Backflow preventer

A device that stops water in an irrigation or misting line from flowing back into the drinking water supply. Plumbing codes generally require one on potable connections; the approved type depends on local code. More on backflow preventer

Bypass valve

A valve that sends pump flow the nozzles cannot use back to the pump inlet or a drain, limiting pressure in a positive-displacement system. More on bypass valve

C

Cavitation

The formation and collapse of vapor bubbles when pump inlet pressure falls too low. It sounds like gravel in the pump, causes pressure fluctuation and erodes internal parts. More on cavitation

Centrifugal misting fan

A misting fan that atomizes water by flinging it off a spinning disc or impeller rather than forcing it through high-pressure nozzles. More on centrifugal misting fan

Check valve

A one-way valve that lets water flow in one direction only. In misting it keeps lines full or prevents drain-down, and is built into anti-drip nozzles. More on check valve

D

Descaling

Dissolving mineral deposits (mostly calcium carbonate) from nozzles and components, usually by soaking in vinegar or citric acid. More on descaling

Diaphragm pump

A pump that moves water with flexing diaphragms. Used as a booster in mid-pressure misting systems, typically up to a few hundred psi. More on diaphragm pump

Droplet size

The diameter of mist droplets, measured in microns. Smaller droplets have more surface area per unit of water and evaporate faster. More on droplet size

Dv50 (volume median diameter)

The droplet diameter at which half the sprayed water volume is in smaller droplets and half in larger ones. A common way to state spray fineness. More on dv50

E

Evaporative cooler (swamp cooler)

A unit that blows air through water-soaked pads. It cools air more predictably than open-air misting but uses a reservoir and pads that need maintenance. More on evaporative cooler

Evaporative cooling

Cooling produced when liquid water turns to vapor and absorbs heat from the surrounding air. It is the principle behind misting systems and swamp coolers. More on evaporative cooling

F

Flash evaporation

The near-instant evaporation of very fine droplets, typically from high-pressure nozzles, that lets mist cool the air without wetting surfaces. More on flash evaporation

Fog (fogging system)

Very fine mist, usually under about 10 to 20 microns, produced at high pressure. Used to raise humidity and cool greenhouses or large spaces without wetting foliage or surfaces. More on fog

G

GFCI

Ground-fault circuit interrupter: a device that cuts power when current leaks to ground. Outdoor pump and fan circuits should be GFCI-protected. More on GFCI

GPH and GPM

Gallons per hour and gallons per minute. Nozzles are usually rated in GPH or fractions of a GPM; pumps in GPM. 1 GPM equals 60 GPH. More on GPH and GPM

Grains per gallon (gpg)

A unit of water hardness. One grain per gallon equals about 17.1 mg/L as calcium carbonate. More on grains per gallon

H

Hard water

Water with high dissolved calcium and magnesium. It leaves scale in nozzles and white spots on surfaces as mist evaporates. More on hard water

Heat index

A feels-like temperature that combines air temperature and humidity, published by the National Weather Service. Misting lowers air temperature but raises humidity, so the heat index falls less than the thermometer. More on heat index

High-pressure misting

Misting at about 800 to 1,000 psi using a plunger pump. It produces droplets in roughly the 5 to 20 micron range that evaporate quickly. More on high-pressure misting

Humidistat

A control that switches a misting system on or off based on relative humidity, commonly used to stop misting when the air is too humid for it to help. More on humidistat

I

Impingement pin nozzle

A nozzle in which the water jet strikes a tiny pin just outside the orifice, shattering it into fine droplets. More on impingement pin nozzle

L

Legionella

Bacteria that grow in warm, stagnant water and cause Legionnaires' disease when inhaled in fine droplets. Misting systems must not let water sit in warm lines. More on Legionella

Low-pressure misting

Misting at household or hose pressure, about 40 to 80 psi, with no pump. Droplets are relatively large, so it tends to wet surfaces. More on low-pressure misting

M

Micron

One millionth of a meter (0.001 mm). A human hair is roughly 50 to 100 microns thick. More on micron

Mid-pressure misting

Misting at roughly 100 to 300 psi using a booster pump, a middle ground in cost and droplet size between hose kits and high-pressure systems. More on mid-pressure misting

Misting ring

A ring of tubing with nozzles fitted around the front of a fan so the airflow carries mist toward people. More on misting ring

O

Orifice

The tiny hole in a misting nozzle, sized in thousandths of an inch (for example 0.012 in) or millimeters. It sets flow and, with pressure, droplet size. More on orifice

P

Plunger pump

A positive-displacement pump with ceramic or steel plungers, the standard for high-pressure misting at 1,000 psi. More on plunger pump

Positive-displacement pump

A pump that delivers a nearly fixed volume per stroke regardless of pressure. It needs an unloader or bypass because it will keep building pressure if flow is blocked. More on positive-displacement pump

Propagation mist

Short, frequent bursts of mist over cuttings or seedlings that keep a water film on the leaves until roots form. More on propagation mist

psi

Pounds per square inch, the unit of pressure used for misting systems in the US. 1 bar is about 14.5 psi. More on psi

R

Relative humidity (RH)

The amount of water vapor in the air as a percentage of the maximum it could hold at that temperature. More on relative humidity

Reverse osmosis (RO)

A filtration process that removes most dissolved minerals from water. RO water largely eliminates nozzle scale and spotting. More on reverse osmosis

S

Sauter mean diameter (D32)

The droplet diameter with the same volume-to-surface-area ratio as the whole spray, a measure relevant to evaporation rate. More on sauter mean diameter

Sediment filter

A cartridge filter that removes particles before the pump; high-pressure systems typically use a 5 micron rating. More on sediment filter

Slip-lock fitting

A push-in fitting for misting tubing that grips the tube with a collet and seals with an O-ring. More on slip-lock fitting

U

Unloader valve

A valve on a plunger pump that diverts flow back to the inlet when outlet pressure exceeds its setting or flow is shut off. More on unloader valve

V

Vapor pressure deficit (VPD)

The difference between saturation vapor pressure and actual vapor pressure, in kPa. It measures how strongly air pulls water from leaves. More on vapor pressure deficit

W

Wet-bulb depression

Air temperature minus wet-bulb temperature. It is the maximum cooling evaporation can provide under current conditions. More on wet-bulb depression

Wet-bulb temperature

The lowest temperature air can reach by evaporating water into it. Misting can approach but never go below it. More on wet-bulb temperature

Winterizing

Draining and protecting a misting system before freezing weather so trapped water cannot crack tubing, nozzles or the pump. More on winterizing