Exam Cheat Sheet · Quick Reference
Mississippi Irrigation Systems
Mississippi · PSI Services Contractor
Verified, not estimated. Every figure below is drawn from the official exam structure we maintain — question counts, passing standard and topic weighting. Practice questions are grounded in the source law with statute citations. We omit any figure we can't verify rather than guess at it.
Total questions
40
Passing score
70%
Exam time
120 min
Administered by
PSI Services Contractor
Format
Reference materials allowed
Mississippi State Portion 40 questions
Piping and Valves
14 Q · 35%
PVC pipe pressure ratings and classification (Schedule 40, Class 160, Class 200, Class 315)Pipe friction loss calculation using friction loss chartsWater velocity limits and their relationship to pipe sizingSurge pressure and water hammer causes, calculation, and preventionBackflow prevention device types, selection criteria, and installation requirementsPVC pipe fittings types and their applications (elbows, tees, couplings, adapters, saddles)Solvent-cemented joint procedures and curing requirementsPipe routing methods including in-line and loop configurations
Layout, Plans and Specifications
10 Q · 25%
Dividing a site into geographical areas based on plant water needs, soil type, and slopeSprinkler head selection and placement for specific site conditionsSquare vs. triangular spacing of sprinkler headsHead-to-head coverage and precipitation rate calculationsDrip irrigation system layout including emitter placement and tubing configurationIrrigation design graphics, symbols, and legendsAs-built drawings and documenting field changesIrrigation specifications and contract documents
Sprinkler Heads
6 Q · 15%
Types of sprinkler heads: fixed spray vs. rotarySprinkler head patterns: circular arcs, rectilinear, and trajectory anglesHead-to-head coverage and spray pattern overlap principlesTriangular vs. square spacing methods and advantagesWind-adjusted head spacing calculations as percentage of throw diameterPrecipitation rate calculation and matched precipitationSelecting sprinkler heads based on site size and area shapeSoil infiltration rates and watering schedule determination
Controls and wiring
6 Q · 15%
Controller types: electromechanical, solid-state, and hybridController programming features: stations, programs, start times, and watering cyclesPump and master valve circuit wiring integrationElectrical terms and concepts: volts, amps, watts, and voltage dropControl wire and common wire sizing using wire-sizing chartsWire placement, installation techniques, and UF wire specificationsCentral and satellite control systemsWater conservation control features: seasonal adjustment, flow meters, weather stations, and rain sensors
Excavation, Compaction and Grading
2 Q · 5%
Trench excavation requirements (depth, width, and grading standards)Backfill material specifications and compaction proceduresOverexcavation requirements for rock or unyielding materialPipe grading and drainage slope for freeze protectionMinimum cover depths for mainlines, lateral lines, and sleeved crossings under paved areasSoil types and water-holding characteristics (clay, sandy, loam)Soil permeability rates and their effect on irrigation designTrench inspection checkpoints and quality control during installation
Pumps
2 Q · 5%
Types of irrigation pumps: horizontal centrifugal vs. vertical turbineHow centrifugal pumps work: impeller, volute, and pressure generationPump selection criteria: required flow rate, pressure, water source, and power phasePumping in series vs. pumping in parallelBooster pumps for pressure deficiency situationsMultistage pumps and stacked impellersPressurized (hydropneumatic) tanks and jockey pumpsDetermining available flow and working pressure from municipal water supply
Key Distinctions
Triangular head spacingvsSquare head spacing
For the same 4-mph wind condition triangular spacing uses 55% of the throw diameter while square spacing uses 50%, so the wider triangular layout needs fewer heads (the reference's rectilinear example required two more heads with square spacing).
Irrigation Design Manual, Chapter 18 — Triangular Spacing in Square and Rectilinear Spaces sectionAVB (number required)vsPVB / DCVA (number required)
An AVB must be installed on each circuit (six circuits = six AVBs), whereas PVB and DCVA devices typically require only one unit per project.
Simplified Irrigation Design, Backflow Prevention, Chapter 16AVB and PVBvsDCVA and RPP
AVBs and PVBs seal tightly only in back-siphonage situations and are not effective against back pressure; a DCVA (non-toxic water) or RPP is used where back pressure may occur, such as from a booster pump.
Simplified Irrigation Design, Chapter 16 (Backflow Prevention)DCVA installationvsPVB / AVB installation
A DCVA may be installed below ground in a valve box provided the assembly has good drainage, while AVBs and PVBs must be installed 12 in above the highest sprinkler head and are not described for below-ground installation.
Simplified Irrigation Design, Backflow Prevention, Chapter 16HCVB mounting heightvsAVB / PVB / RPP mounting height
An HCVB needs a minimum of 6 in (152 mm) above grade, whereas AVBs, PVBs and RPPs carry a 12-in requirement.
Simplified Irrigation Design, Backflow Prevention, Chapter 16Spray headsvsRotary heads
Spray heads apply about 1 to 2 in/hr while rotary heads apply 0.07 to 1.46 in/hr, so mixing the two families in one zone underwaters some areas and overwaters others.
Irrigation Reference, Chapter 5 — Matched PrecipitationSchedule 40 PVCvsSchedule 80 PVC
At the same nominal size only the outside diameter is identical between the two schedules; inside diameter and wall thickness change.
Simplified Irrigation Design, Using Plastic Pipe, Chapter 17ASTM D-2241vsASTM D-1785 and ASTM D-2239
D-2241 governs PVC pressure-rated pipe, while D-1785 covers PVC schedule-rated pipe and D-2239 covers polyethylene (PE) pipe.
Simplified Irrigation Design, Chapter 17 (Using Plastic Pipe)SDRvsSIDR
SDR (Standard Dimension Ratio, SDR-PR) is the constant wall-thickness-to-outside-diameter ratio of PVC pressure-rated pipe; SIDR applies to polyethylene pipe, not PVC pressure-rated pipe.
Simplified Irrigation Design, Chapter 17 (Using Plastic Pipe)1-in electric globe valvevs1-in electric angle valve
At 25 gpm the globe configuration loses 6 psi while the angle configuration loses 4.5 psi.
Simplified Irrigation Design, Selecting Valves, Chapter 11Solid-state controllervsElectromechanical controller
Solid-state controllers are accurate to within 1 minute (and hold their program on battery 3 to 9 hours during a power outage), while electromechanical controllers may vary several minutes from the programmed run time.
Simplified Irrigation Design, Controllers, Chapter 15 — Types of Controllers sectionTechnical specificationsvsWorking drawings
Where the two conflict, the courts have adjudged the technical specifications to have greater importance than the working drawings.
Irrigation Reference, Chapter 27 — Working Drawings and SpecificationsRectilinear sprinkler patternsvsCircular sprinkler patterns
Rectilinear patterns are defined by the dimensions of the area covered (e.g., 3 ft × 12 ft, 9 ft × 18 ft), while circular patterns are defined by radius of throw, arc and trajectory.
Irrigation Design Manual, Chapter 2 — Sprinkler Head Patterns sectionKey Terms
Standard Dimension Ratio (SDR) Simplified Irrigation Design, Chapter 17 (Using Plastic Pipe)
The ratio of wall thickness to outside pipe diameter that stays constant across all sizes in a given PVC pressure class (abbreviated SDR-PR); Class 160 pipe is SDR-26.
Class 200 PVC pipe Simplified Irrigation Design, Chapter 17 (Using Plastic Pipe)
One of the four pressure-rated PVC classes, guaranteed by the manufacturer to hold 200 psi (14.06 kg/cm²) of water pressure in every size of the class because of the SDR system.
PVC 1120 designation Simplified Irrigation Design, Using Plastic Pipe, Chapter 17
A quality-stamp code meaning Type 1, Grade 1 material whose '20' is the tensile design stress in 100s, i.e., the material will hold up to 2,000 psi in tensile (pulling) stress.
Slip fitting Simplified Irrigation Design, Chapter 21 (Fittings for the Irrigation System)
A fitting into whose socket PVC pipe is slipped and glued — the standard solvent-cemented connection type in PVC irrigation systems.
Barbed male elbow Simplified Irrigation Design, Chapter 21 (Fittings for the Irrigation System)
A PE fitting that screws into a PVC threaded tee while PE flexible swing pipe is pushed onto the barbs, which hold the pipe without leaking.
Rigid nipple Simplified Irrigation Design, Fittings for the Irrigation System, Chapter 21
A 'small protuberance or extension' used in swing joints to connect two threaded street elbows, and also used to raise a sprinkler head set too deep.
Flexible riser Simplified Irrigation Design, Fittings for the Irrigation System, Chapter 21
A PE (polyethylene) lateral-line fitting for bubblers and fixed spray heads that can be cut to the needed length and bends and pops back when stepped on.
Atmospheric Vacuum Breaker (AVB) Simplified Irrigation Design, Chapter 16 (Backflow Prevention)
A back-siphonage-only device installed one per circuit that must not be used under continuous pressure for more than 12 hours at a time, because the disc float assembly could stick or dent and become inoperable.
Pressure Vacuum Breaker (PVB) Simplified Irrigation Design, Backflow Prevention, Chapter 16
A back-siphonage device installed 12 in (305 mm) above the highest sprinkler head; when internal pressure drops to 1 psi (0.07 kg/cm²) or lower the disk float assembly drops and admits air to stop back-siphonage.
Double Check Valve Assembly (DCVA) Simplified Irrigation Design, Chapter 16 (Backflow Prevention)
A backflow preventer that stops both back-siphonage and back pressure, suitable for non-toxic water (no RPP required) and permitted below grade in a valve box with good drainage.
Hose Connection Vacuum Breaker (HCVB) Simplified Irrigation Design, Backflow Prevention, Chapter 16
A backflow device that must be installed a minimum of 6 in (152 mm) above grade.
Surge pressure (water hammer) Simplified Irrigation Design, Surge Pressure and Water Hammer, Chapter 13
The shock-wave pressure from sudden valve closure, which may reach up to four times the working water pressure and moves back and forth in the pipe until its energy is expended in friction loss.
Surge pressure nomograph Simplified Irrigation Design, Surge Pressure and Water Hammer, Chapter 13
A chart that determines pressure rise from three inputs: water velocity in fps, valve closure time in seconds and tenths, and the length in feet of straight pipe affected by water hammer (pipe diameter and static pressure are not inputs).
As-built drawing Irrigation Reference, Chapter 28 — As-Built Drawings
A drawing the contractor produces after installation recording all field changes from the original plan (e.g., a mainline rerouted around a retaining wall); it is an invaluable tool for future maintenance and development.
Control border Irrigation Reference, Chapter 18 — Triangular Spacing in Square and Rectilinear Spaces
A side of a space needing the most even spray control — such as along a building, busy sidewalk or street — where heads are laid out first in a triangular rectilinear layout.
UF wire Simplified Irrigation Design, Sizing Wire to Electric Valves, Chapter 12 — Wire Placement Techniques section
Underground feeder wire, the low-voltage wire specified for irrigation systems, with insulation thick enough to be buried without conduit and 'UF' printed on the insulation.
Geographical areas Irrigation Reference, Chapter 6 — Determining Geographical Areas
Design areas defined before head layout on the basis of plant water needs (plant type, sun/shade, tree-root proximity), soil permeability rate and slope; pressure, pipe material and controller capacity come later.
Sprinkler performance (effective radius) Irrigation Reference, Chapter 3 — Locating Sprinkler Heads, Sprinkler Performance section
A single head supplies enough water for good plant growth only over the inner two-thirds of its radius of throw, so patterns must overlap for even distribution.
Formulas to Know
Precipitation rate (in/hr)
PR = (96.3 × total GPM of all heads in the area) ÷ total square feet of area covered; 96.3 is the U.S.-customary conversion constant (60 is the metric constant for L/min and m² to mm/hr)Working water pressure at a valve
Working pressure = static pressure − friction loss; e.g., 20 gpm in 100 ft of 1¼-in Class 200 PVC loses 1.5 psi/100 ft: 51.66 − 1.5 = 50.16 psi, and 43 − 1.5 = 41.5 psiElevation pressure change / head conversion
1 ft of elevation rise or fall = 0.433 psi (0.03 kg/cm²) change, so 5 ft × 0.433 = 2.2 psi loss; conversely 1 psi = 2.31 ft of head (1.00 ÷ 0.433)Total surge pressure
Total surge pressure = pressure rise from nomograph + working water pressure; e.g., 35 psi + 50 psi = 85 psi (acceptable, since even Class 160 pipe holds 160 psi)Sizing a mainline loop
Size the entire loop for one-half the flow of the largest valve circuit (40 gpm ÷ 2 = 20 gpm) because flow splits along both legsFriction loss in an unbalanced loop (rule of thumb, uniform pipe size)
Use one-half the gpm and one-half the total loop length: 20 gpm/810 ft → 10 gpm through 405 ft; 30 gpm/600 ft → 15 gpm through 300 ft (built-in safety factor ≈ 10%)Flow in the first lateral section at the valve
First section flow = number of downstream heads × gpm per head (13 heads × 2 gpm = 26 gpm; 6 heads × 4 gpm = 24 gpm); pipe size decreases at each successive headTriangular head spacing from throw diameter
Spacing = throw diameter × wind factor (no wind 60%, 4-mph 55%, 8-mph 50%); 0.55 × 30 ft = 16.5 ft; 16-ft radius = 32-ft diameter × 0.50 = 16.0 ftMaximum watering cycle time and cycles per week
Max minutes per cycle = soil permeability (in/hr) ÷ (PR in/hr ÷ 60): 0.2 ÷ 0.006 ≈ 33.3 → 34 min; cycles per week = total weekly minutes ÷ max cycle minutes: 334 ÷ 34 ≈ 9.8 → 10Design flow available from a water meter
Do not exceed 75% of the meter's maximum safe flow: 75% × 30 gpm = 22.5 gpm (85.16 L/min) for a ¾-in meter