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Magnesium: A light, ductile, silver-white, metallic element. Its presence in high non-chelated concentrations can lead to stains & scale when conditions are right.

Magnetic drive: A type of seal less pump that is most often limited to pumping clean lubricating liquids. Similar in concept to a canned pump.

Magnetic Field: A region of space surrounding a magnetized body or current-carrying circuit in which the resulting magnetic force can be detected.

Magnetic Polarity: It is a fundamental principle of a winding that adjacent poles must be wound to give opposite magnetic polarity. This does not mean that the coils actually have to be wound in this direction before being placed into the stator. It does mean that the winding must be connected so that, if the current proceeds through one pole in a clockwise direction, it must proceed through the next pole in a counterclockwise direction. This principle is used to determine the correctness of connection diagrams.

Magnetic seal: Uses magnetic materials rather than springs to keep the lapped seal faces together. Limited to non- corrosive fluids because of the magnets.

Magnetic Starters: A system or device required with three phase motors for motor control.

Magnetite: Iron oxide ( Fe3O4). Protective coating that forms on iron pipe to prevent further rapid corrosion. Usually black or maroon in color. Can be very destructive to mechanical seal components until the oxide stabilizes. We often find these iron pipes in new hot water systems.

Main Circulating Pump: The pump which pulls water from the pool and pumps it back to the pool through the filter, heater, chlorinator, etc. When the pump is working properly, there will be a strong suction at the suction outlet. Anything sealing off the suction outlet (on a system with only one suction outlet) will be held there by pressure of the water trying to flow into the pump.

Main Drain: The fitting on the bottom of the pool which leads to the main circulating pump’s inlet pipe. All pools and spas should have more than one main drain (except some above-ground pools which use the skimmer as a suction outlet). Since there can be a strong suction pull at the main drain, for swimmer protection it must be equipped with a correctly installed, anti-entrapment and anti-hair-entanglement certified cover, fastened with screws.

Main Outlet: The outlet fitting at the bottom of a swimming pool through which water passes to the recirculating pump (often erroneously referred to as the “main drain”.

Make-Up Water: Fresh water used to fill or refill the pool, spa, hot tub, or potable water system.

Manifold: A group of sprinkler valves that converts a single incoming water line into several outgoing line, each going to a separate zone. A sprinkler valve controls each outgoing line.

Manual Reset Overload: This line-interrupting protector has an external button that must be pushed to restore power to the motor. Use where unexpected restarting would be hazardous, as on saws, conveyors, compressors and other machinery.

Mating ring: Another name for the hard face in a mechanical seal. It can be either rotating or stationary.

Maximum contaminant level (MCL): The designation given by the U.S. Environmental Protection Agency (EPA) to water-quality standards promulgated under the Safe Drinking Water Act. The MCL is the greatest amount of a contaminant that can be present in drinking water without causing a risk to human health.

Maximum Contaminant Level (MCL)-1-: The highest level of a contaminant that EPA allows in drinking water. MCLs ensure that drinking water does not pose either a short-term or long-term health risk. EPA sets MCLs at levels that are economically and technologically feasible. Some states set MCLs which are more strict than EPA's.

Maximum Contaminant Level Goal (MCLG): The level of a contaminant at which there would be no risk to human health. This goal is not always economically or technologically feasible, and the goal is not legally enforceable.

Maximum Pull Out Torque: Same as Breakdown Torque (BDT) or Pull Out Torque. Usually is the maximum value of torque that a motor will develop without a sudden decrease in speed(breakdown).

Maximum Run Torque: See Breakdown Torque.

Maximum Suction Lift: The height (approx. 25') that water can be lifted by a centrifugal pump in actual conditions, taking into consideration altitude, friction loss, temperature, suspended particles, and the inability to create a perfect vacuum. The 25' suction lift is attainable for cold water (60°F) at sea level. Suction lift diminishes as elevation increases, due to the reduction in atmospheric pressure. In addition, suction lift decreases as the water temperature increases since warm water contains more entrained air.

Mechanical seal: A name for the joint that seals the fluid in the pump stopping it from coming out at the joint between the casing and the pump shaft. The following image (source: the Pump Handbook by McGraw-Hill) shows a typical mechanical seal. A mechanical seal is a sealing device which forms a running seal between rotating and stationary parts. They were developed to overcome the disadvantages of compression packing. Leakage can be reduced to a level meeting environmental standards of government regulating agencies and maintenance costs can be lower.

Mechanical seal-1-: A positive sealing device used to seal all fluids (liquids and gases). The primary seal is a set of lapped seal faces that are installed perpendicular to the shaft.

Mechanical Seal-2-: A seal behind the impeller which prevents water from running out along the shaft of a motor. aka; pump seal.

Mechanical Seal-3-: A two-part seal with one rotating part and one stationary. Touching surfaces on the parts are highly polished and provide excellent sealing capability and life. The surfaces can be damaged by dirt or grit in the liquid.

Mechanical Seal-4-: This is a spring-loaded device consisting of several parts that seals the rotating impeller in the pump case and prevents water from leaking into and damaging the engine. Mechanical seals are subject to wear when pumping water containing abrasives and will quickly overheat if the pump is run without filling the pump chamber with water before starting the engine or with a closed discharge.

Mechanical Seal-5-:  A spring-loaded pump component that forms a seal between the pump and the engine or motor. Pumps designed for working in harsh environments require a more abrasive resistant seal.

Mechanical Seal-6-:  Has a rotating part and stationary part with highly polished touching surfaces. Has excellent sealing capability and life, but can be damaged by dirt or grit in the liquid.

Mechanical Shaft Seal: A type of shaft seal used to prevent leakage of fluid, such as water or steam, between sliding or turning parts of machine elements and is able to be replaced in the field.

Mechanically Operated Centrifugal Switch: An electric switch that operates using the centrifugal force created from a rotating shaft. The switch is designed to activate or de-activate as a function of the rotational speed of the shaft. In single-phase, split-phase induction motors, the switch is used to disconnect the starting winding of the motor once the motor approaches its normal operating speed. The centrifugal switch consists of weights mounted to the shaft of the motor and held near the shaft by spring force. At rest, levers attached to the weights press a low-friction, non-conductive plate against a set of electrical contacts mounted to the motor housing, closing the contacts and connecting the starting winding to the power source. When the motor approaches its normal operating speed, centrifugal force overcomes the spring force and the weights swing out, raising the plate away from the electrical contacts. This allows the contacts to open and disconnects the starting winding from the power source; the motor then continues operating solely using its running winding. Motors using such a centrifugal switch make a distinct clicking noise when starting and stopping as the centrifugal switch opens and closes.

Meggar Test: A measure of an insulation system's resistance. This is usually measured in megohms and tested by passing a high voltage at low current through the motor windings and measuring the resistance of the various insulation systems.

Membranes: Thin films constructed of cellulosic or synthetic materials, which are designed to provide selective transport of solutes. Widely used for hemodialysis, micro filtration, ultrafiltration and reverse osmosis, membranes may be either homogeneous or asymmetric. Homogeneous membranes have a uniform structure or cross-section while asymmetric membranes have a dense "skin" which overlays a porous substructure.

Memory Effect: If NiCad cells or batteries are subjected to repeated shallow cycling, their internal structure changes and they lose storage capacity — known as the memory effect.

Mercuric Oxide Cell: Commonly called the ‘mercury cell’, a type of alkaline primary cell with a positive electrode of mercuric oxide (often with manganese dioxide), a negative electrode of metallic zinc and either potassium or sodium hydroxide as electrolyte.

Mercury (Hg): a metal which remains liquid at room temperature. This property makes it useful when used in a thin vertical glass tube since small changes in pressure can be measured as changes in the mercury column height. The inch of mercury is often used as a unit for negative pressure.

Metal bellows: Used in mechanical seal designs to eliminate the need for a dynamic elastomer.

Metal fatigue: A breakage of the metal caused by the bending and flexing of a metal part beyond its endurance limit.

Mg/l: Milligrams per liter: A measure of concentration of a dissolved substance in a liquid. A concentration of one mg/l means that one milligram of a substance is dissolved in each liter of water. For practical purposes, this unit is equal to parts per million (ppm) since one liter of water is equal in weight to one million milligrams. Thus, a liter of water containing 10 milligrams of calcium has 10 parts of calcium per one million parts of water or 10 parts per million (10 ppm).

Micro Filtration: The separation or removal from a liquid of particulates and micro-organisms in the size range of 0.1 to 2 microns in diameter.

Micro Porous: In the context of water purification, membranes having an average pore size, which is between 0.1 and 1.0 micron in diameter.

Microfarad: A measurement of capacitance (electrical storage capability). Capacitors or condensers have a range listed on the casing (15-22 MFD, example) showing the capacity it can handle. They will also show the maximum voltage they can handle (370V, example).

Microhm: A measurement of electrical resistance equal to one millionth of an ohm. The unit of measurement for testing the electrical resistance of water to determine its purity. The closer water comes to absolute purity, the greater its resistance to conduction of an electrical current. Absolute pure water has a specific resistance over 18 million ohms (megohms) across one centimeter of water at a temperature of 78 degrees Fahrenheit.

Micron Rating: A measurement applied to filters or filter media to indicate the particle size at which a substantial percentage of suspended solids above that size will be removed. As used in the water treatment industry standards, this may be an absolute rating or a nominal rating.

Micron: A unit of linear measure. It is one millionth of a meter, or one thousandth of a millimeter. The smallest particle than can be distinguished by the naked eye would be about 40 microns across.

Microorganism: A living, breathing creature in your pool. The purpose of disinfectants are to remove such "infectants."

Microorganisms-1-: Tiny living organisms such as bacteria, algae, diatoms, parasites, plankton, and fungi, which can be seen only with the aid of a microscope. Some microorganisms can cause acute health problems when consumed in drinking water. Also known as microbes.

Microwatt-Seconds Per Square Centimeter: A unit of measurement of intensity and retention, or contact time in the operation of ultraviolet (U/V) systems.

Milliamp-hours (mAh): The unit generally used to specify the storage capacity of smaller batteries. A battery with a capacity of 100mAh can supply 100mA for one hour, or 10mA for 10 hours etc. One mAh is the equivalent of 3.6 coulombs of charge.

Milligram (mg): One-thousandth of a gram.

Milligrams per liter (mg/l): A unit of the concentration of a constituent in water or wastewater. It represents 0.001 gram of a constituent in 1 liter of water. It is approximately equal to one part per million (PPM).

Milliliters/Minute: A common measurement for the flow rate of small RO systems. Usually measured with a graduate cylinder. One thousandth of a liter per minute. Milliliters/min x.38 = gal/day.

Milling machine:  An industrial machine that uses a rotary cutter to remove material from a part supported on its worktable.

Million gallons per day (Mgd): A rate of flow of water equal to 133,680.56 cubic feet per day, or 1.5472 cubic feet per second, or 3.0689 acre-feet per day. A flow of one million gallons per day for one year equals 1,120 acre-feet (365 million gallons).

Minerals: Such as Calcium, Manganese, Magnesium, Nickel, Copper, Silver, Iron, Cobalt or Aluminum. Their presence in high non-chelated concentrations can lead to stains & scale when conditions are right.

Minimum Basin Size: The minimum size for a sewage basin to prevent the pump from cycling on and off too frequently. Minimum pump run time should be between 15 seconds and 2 minutes. The longer the pump runs the better for the pump.

Minimum flow rate: Most centrifugal pumps should not be used at a flow rate less than 50% of the B.E.P. (best efficiency point) flow rate without a recirculation line. (What is the B.E.P.?) If your system requires a flow rate of 50% or less then use a recirculation line to increase the flow through the pump keeping the flow low in the system, or install a variable speed drive.See also the pump glossary BEP. The factors which determine minimum allowable rate of flow include the following:* Temperature rise of the liquid -- This is usually established as 15°F and results in a very low limit. However, if a pump operates at shut off, it could overheat badly. * Radial hydraulic thrust on impellers -- This is most serious with single volute pumps and, even at flow rates as high as 50% of BEP could cause reduced bearing life, excessive shaft deflection, seal failures, impeller rubbing and shaft breakage. * Flow re-circulation in the pump impeller -- This can also occur below 50% of BEP causing noise, vibration, cavitation and mechanical damage. * Total head characteristic curve - Some pump curves droop toward shut off, and some VTP curves show a dip in the curve. Operation in such regions should be avoided. There is no standard which establishes precise limits for minimum flow in pumps, but "ANSI/HI 9.6.3-1997 Centrifugal and Vertical Pumps - Allowable Operating Region" discusses all of the factors involved and provides recommendations for the "Preferred Operating Region".

Minimum flow: A condition that can cause excessive heat inside the pump volute. A temperature rise of 10°C (18°F) across the operating pump is considered excessive. Normally caused by throttling a pump discharge valve.

Minimum Flow-1-: The amount of flow required to keep solids in suspension in a piping system. When moving sewage or effluent water with solids, bigger piping is not better. Pipe that is too large will cause the flow velocity to slow down and the solids will settle out in the pipe.

Minimum NPSHA: The margin of safety or minimum NPSHA that should be available depends in part on the amount of suction energy of the pump. The suction energy level of the pump increases with: The casing suction diameter, The pump speed, The suction specific speed, The specific gravity of the fluid. Anything that increases the velocity of the pump impeller eye, the rate of flow of the pump, or the specific gravity, increases the suction energy of the pump.

Minimum run time: The minimum length of time that the pump should run while refilling the pressure tank. At least 1 minute run time is recommended.

Mining water use: Water use during quarrying rocks and extracting minerals from the land.

Mixed Bed: The intermixing of two or more filter or ion exchange products in the same vessel during a service run. The most common use is in ion exchange systems having a 40/60 percent cation to anion resin bed such as that for a deionization polisher system. In filtration, there may be an intermixing of two or more media in a single tank with each stratified into separate layers.

Mixed flow pump: Functions as a compromise between radial and axial flow pumps, the fluid experiences both radial acceleration and lift and exits the impeller somewhere between 0-90 degrees from the axial direction. As a consequence mixed flow pumps operate at higher pressures than axial flow pumps while delivering higher discharges than radial flow pumps. The exit angle of the flow dictates the pressure head-discharge characteristic in relation to radial and mixed flow.

MNPT: This refers to female pipe thread, in which the threads are on the inner side of the connection fitting.

Mod Media: Short for Modular Media, a type of filtration system. Consists of filters within filters: interchangeable and replaceable.

Model Number: Identification nomenclature for a product. Also called Model or Catalog Number or Product Number or Material Number.

Moment of inertia: Referring to rotation about an axis. In the pump business it refers to a formula that describes the shape of the shaft. A solid shaft would have a different moment of inertia than a hollow shaft.

MONEL®:  Is a nickel-copper alloy used primarily as interior trim on butterfly and ball valves. One of the most specified materials for corrosion resistance to sea and salt water. Monel is also very resistant to strong caustic solutions.  MONEL is a registered trademark of International Nickel.

Monitoring: Testing that water system operators must perform to detect and measure contaminants. A water system that does not follow EPA's monitoring methodology or schedule is in violation, and may be subject to legal action.

Monovalent Ion: A cation or anion having a single electrical charge.

Moody diagram: A graphical representation of the laminar and turbulent (Colebrook) flow equations.

Motor Canopy: The protective wrapper surrounding the electrical elements of a motor.

Motor Electrical Rating: In addition to horsepower, a motor's nameplate will specify the operating voltage and the current draw at rated horsepower. You have to operate the motor at close to its specified voltage, but the current (amps) will vary with the actual load. A lightly loaded motor will draw less current. However, since the power factor and efficiency (both described below) decrease with decreasing load, the current will not drop as much as expected. Since all types of power (mechanical, electrical, etc.) are equivalent, these electrical ratings can be related to the horsepower rating. Most generally, electrical power in watts is equal to the voltage in volts times the current in amps. 1 horsepower (mechanical power) is approximately equal to 746 watts (electrical power.) Thus a perfect motor (the type used in physics classes, which do not exist in the real world) rated at 120 volts and 1 HP would draw about 6.2 amps ([1 HP = 746 watts]/120 volts gives 6.2 amps.) However, in the real world things are not quite so simple, and corrections must be applied.

Motor frame: NEMA (National electrical Manufacturers Association) provides standards to which electric induction motors are built. Each frame size (for example frame 254T) is built to specified dimensions. The amount of room required for the pump assembly will depend on the size and construction of the motor. It is easy to find a chart that provides the motor dimensions vs. the frame size (See following chart).

Motor Pad: A rubber or other resilient material placed underneath an electric motor. The pad cushions the motor to reduce noise and vibration transmission to the mounting surface.

Motor: A machine for converting electrical energy into mechanical energy. Your motor is known as the dry end of the filter pump. It drives the impeller, which moves the water.

MPV: A Multiport valve. (See Below)

MSDS: Material Safety Data Sheet is a form containing data regarding the properties of a particular substance. An important component of product stewardship and workplace safety, it is intended to provide workers and emergency personnel with procedures for handling or working with that substance in a safe manner, and includes information such as physical data (melting point, boiling point, flash point, etc.), toxicity, health effects, first aid, reactivity, storage, disposal, protective equipment, and spill handling procedures. The exact format of an MSDS can vary from source to source.

MSDS-1-: Material Safety Data Sheet: Required by law for products with potential for pollution or health risks(examples = paint, petroleum products).

MTBE: Methyl Tertiary Butyl Ether. A volatile organic chemical (VOC) used as an octane-enhancing lead substitute and more recently as an oxygenating agent in gasoline to reduce carbon monoxide emissions from automobiles. MTBE is volatile, flammable and highly soluble in water. During refueling and gasoline production, MTBE is volatilized to the atmosphere where it dissolves into the atmospheric moisture and returns to earth as precipitation, polluting our water supplies. Since MTBE does not adsorb well with organic matter in soils it is easily washed away. In surface water, MTBE volatilizes into the air. While in ground water, MTBE persists and moves freely. MTBE occurrences in ground water above 40 ppb have so far been attributed to point source contamination such as underground gasoline tank leaks, overflows, etc. EPA has tentatively classified MTBE as a potential human carcinogen.

Multiple filter control valve: A multi-port valve with at least four positions for various filter operations, which combines in one unit the function of two or more single direct flow valves (Dial Selector Valve).

Multiport Valve: A 4 or 6 position valve combining the functionality of several valves into one unit, revolutionizing pool plumbing. The six common functions are: “FILTER”: Keep it here 99%, except when backwashing, rinsing or wasting “RINSE”: Use this setting for 20 seconds after backwashing to rinse tank “RECIRCULATE”: Use this if the filter's broken; at least you're circulating.

Multistage Pump: all the pumps or stages use a common shaft instead of individual motors (like a submersible turbine or a line shaft turbine).

Municipal water system: A water system that has at least five service connections or which regularly serves 25 individuals for 60 days; also called a public water system.

Muriatic Acid: A commercial name for hydrochloric acid. Used for lowering the pH and alkalinity of pool water.

Muriatic Acid-1-: The liquid dilution of Hydrochloric Acid used to lower pH and alkalinity, and to remove mineral stains and scale. Extremely caustic and corrosive.