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

Atmospheric Vacuum Breaker (AVB)vsPressure Vacuum Breaker (PVB)

Both protect only against back-siphonage (not back pressure), but an AVB must be installed on every circuit (one per circuit) while a PVB typically requires only one unit per project, and both must be at least 12 in above the highest sprinkler head.

Backflow Prevention, Chapter 16
AVB / PVBvsDouble Check Valve Assembly (DCVA)

AVB and PVB seal only against back-siphonage and cannot be installed below ground, whereas a DCVA prevents both back-siphonage and back pressure and may be installed below ground in a valve box with good drainage.

Chapter 16 (Backflow Prevention)
DCVA (Double Check Valve Assembly)vsRPP (Reduced Pressure Principle device)

Both handle back pressure, but the RPP is required when the water supply is toxic or hazardous, while a DCVA is sufficient for non-toxic water systems.

Chapter 16 (Backflow Prevention)
Triangular Head SpacingvsSquare Head Spacing

Triangular spacing uses a wider percentage of sprinkler throw diameter (e.g., 55% vs. 50% in 4-mph wind) and typically requires fewer heads to cover the same rectilinear area.

Irrigation Reference, Chapter 18 — Triangular Spacing of Sprinkler Heads
Spray HeadsvsRotary Heads

Spray heads have precipitation rates of approximately 1–2 in/hr, while rotary heads range from 0.07–1.46 in/hr; mixing them in the same zone causes uneven watering.

Irrigation Reference, Chapter 5 — Matched Precipitation
Schedule-Rated PVC Pipe (Schedule 40 / Schedule 80)vsPressure-Rated PVC Pipe (SDR-PR)

Schedule-rated pipe is governed by ASTM D-1785 with fixed wall thicknesses per schedule, while pressure-rated pipe is governed by ASTM D-2241 and uses a constant Standard Dimension Ratio (SDR) across all sizes within a pressure class.

Chapter 17 (Using Plastic Pipe)
Schedule 40 PVCvsSchedule 80 PVC

Both share the same outside diameter for a given nominal size, but Schedule 80 has a thicker wall and smaller inside diameter than Schedule 40.

Using Plastic Pipe, Chapter 17
PVC Pipe (Pressure-Rated)vsPE (Polyethylene) Pipe

PVC pressure-rated pipe uses an SDR-PR ratio system (ASTM D-2241), while PE pipe uses a SIDR system (ASTM D-2239) and has the key advantage of resisting cracking from frozen water in cold climates.

Chapter 17 (Using Plastic Pipe)
In-Line Mainline LayoutvsLoop (Pipe Loop) Layout

An in-line mainline carries the full circuit demand through a single path, while a loop splits flow along two legs so the entire loop is sized at only half the largest circuit's flow, allowing smaller pipe sizes and less friction loss.

Chapter 20 (Pipe Loops)
Slip FittingvsThreaded (Street) Fitting

A slip fitting accepts PVC pipe inserted into a socket and bonded with solvent cement, while a threaded fitting screws onto a male thread and is used in connections such as barbed male elbows into PVC threaded tees.

Chapter 21 (Fittings for the Irrigation System)
Working Water PressurevsTotal Surge Pressure

Working water pressure is the normal operating pressure at a given point in the system, while total surge pressure equals working pressure plus the pressure rise from a closing valve and can reach up to four times the working pressure.

Chapter 13 (Surge Pressure and Water Hammer)
As-Built DrawingvsWorking Drawing

Working drawings show the original planned layout, while as-built drawings record all field changes made during actual installation; if the two documents conflict, technical specifications take legal precedence.

Irrigation Reference, Chapter 28 — As-Built Drawings

Key Terms

Maximum Allowable Pressure Difference Across a Circuit Chapter 9 (Basic Hydraulics)
Per Chapter 9 (Basic Hydraulics), the psi difference between the first and farthest sprinkler head on a circuit must be less than 20%; exceeding this causes mismatched output and throw distance.
Maximum Recommended Water Velocity Chapter 9 (Basic Hydraulics)
Per Chapter 9 (Basic Hydraulics), water in irrigation pipe must not exceed 5 ft/sec (1.52 m/sec); higher velocities increase turbulence and create water hammer when flow is stopped.
AVB Continuous-Pressure Limit Chapter 16 (Backflow Prevention)
Per Chapter 16 (Backflow Prevention), an Atmospheric Vacuum Breaker must not remain under continuous pressure for more than 12 hours at a time, as the disc float assembly may stick or deform and become inoperable.
PVB Installation Height Requirement Backflow Prevention, Chapter 16
Per Chapter 16 (Backflow Prevention), a Pressure Vacuum Breaker must be installed at least 12 in (305 mm) above the highest sprinkler head to function correctly.
HCVB Minimum Height Above Grade Backflow Prevention, Chapter 16
Per Chapter 16 (Backflow Prevention), a Hose Connection Vacuum Breaker must be installed a minimum of 6 in (152 mm) above grade.
PVB Activation Threshold Backflow Prevention, Chapter 16
Per Chapter 16 (Backflow Prevention), when internal pressure drops to 1 psi (0.07 kg/cm²) or lower, the PVB's disk float assembly drops and admits air to stop back-siphonage.
Class 200 PVC Pipe Pressure Rating Chapter 17 (Using Plastic Pipe)
Per Chapter 17 (Using Plastic Pipe), Class 200 PVC pipe is manufacturer-guaranteed to hold 200 psi of water pressure regardless of pipe size, due to the SDR system.
ASTM D-2241 Chapter 17 (Using Plastic Pipe)
Per Chapter 17 (Using Plastic Pipe), ASTM D-2241 is the specification governing production of PVC pressure-rated (SDR-PR) irrigation pipe, distinct from D-1785 (schedule-rated PVC) and D-2239 (PE pipe).
Trench Backfill Layer Thickness Pipe Routing, Chapter 7
Per Chapter 7 (Pipe Routing), soil must be returned to the trench in 4 in (102 mm) layers and tamped between each layer to prevent settling and slumping.
Elevation-Pressure Relationship Pipe Routing, Chapter 7
Per Chapter 7 (Pipe Routing), every 1 ft of elevation change produces a 0.433 psi change in water pressure, which is why designers route pipe with the land contours to maintain uniform pressure.
Surge Pressure (Water Hammer) Maximum Multiplier Surge Pressure and Water Hammer, Chapter 13
Per Chapter 13 (Surge Pressure and Water Hammer), the shock wave from a suddenly closing valve can produce a total surge pressure up to four times the system's working water pressure.
Precipitation Rate Formula Constant (96.3) Irrigation Reference, Chapter 5 — Precipitation Rates
Per Chapter 5 (Precipitation Rates), the constant 96.3 is used in the formula PR = (96.3 × total GPM) ÷ area (sq ft) to convert gallons-per-minute and square feet into inches per hour.
Sprinkler Effective Radius of Coverage Irrigation Reference, Chapter 3 — Locating Sprinkler Heads, Sprinkler Performance section
Per Chapter 3 (Locating Sprinkler Heads), a single sprinkler head can sustain good plant growth only within the inner two-thirds of its throw radius; the outer one-third receives insufficient water, requiring overlapping patterns.
Technical Specifications vs. Working Drawings Precedence Irrigation Reference, Chapter 27 — Working Drawings and Specifications
Per Chapter 27 (Working Drawings and Specifications), when working drawings and technical specifications conflict, courts have established that technical specifications take legal precedence.

Formulas to Know

Working Water Pressure at a PointWorking Pressure = Static Pressure − Friction Loss
Total Surge PressureTotal Surge Pressure = Working Water Pressure + Pressure Rise (from nomograph)
Precipitation Rate (PR)PR (in/hr) = (96.3 × Total GPM of all heads) ÷ Total Area (sq ft)
Total Circuit Flow (Pipe Sizing)Total Flow = Number of Heads × GPM per Head
Loop Pipe Sizing FlowLoop Design Flow = Largest Circuit GPM ÷ 2
Loop Friction Loss (Rule-of-Thumb)Use ½ Total GPM through ½ Total Loop Length to find friction loss (includes ~10% safety factor)
Elevation-Pressure ChangePressure Change (psi) = Elevation Change (ft) × 0.433
Triangular Head Spacing DistanceHead Spacing = Sprinkler Throw Diameter × Spacing Percentage (60% no wind / 55% at 4 mph / 50% at 8 mph)
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