IESL Part III key terms

The water content a soil retains after excess water has drained away by gravity, usually one to three days after rain or irrigation. It marks the upper limit of water readily available to crop roots.

The soil moisture level below which plants can no longer extract water and stay wilted even in a humid atmosphere. The gap between it and field capacity defines available water capacity.

Deciding when and how much water to apply to a crop, using soil moisture measurement, evapotranspiration estimates, or crop growth stage. Good scheduling avoids both yield-losing stress and wasteful deep percolation.

The combined loss of water from a cropped surface through evaporation from soil and transpiration from plant leaves. Reference values are adjusted by crop coefficients to estimate actual crop water demand.

Growing different crops in sequence on the same land over successive seasons. It interrupts pest and disease cycles, balances nutrient use, and helps maintain soil organic matter and structure.

The decomposed and decomposing plant and animal residues in soil, plus living microbial biomass. It holds nutrients and water, improves aggregation and aeration, and is a key indicator of soil fertility.

A decision framework that combines monitoring, cultural, biological, and chemical controls, using pesticides only when thresholds are exceeded. The aim is economically and environmentally sound suppression of pests rather than eradication.

The pest population density at which the cost of control equals the value of crop loss the pest would cause. Control is justified only when the population is expected to exceed this level.

For a heat pump or refrigeration cycle, the ratio of useful heating or cooling delivered to the work input required. Values above one are normal because the cycle moves heat rather than generating it.

The working fluid in a refrigeration or heat pump cycle that absorbs heat by evaporating at low pressure and releases it by condensing at high pressure. Selection considers ozone depletion and global warming potential.

A packaged machine that cools water or another secondary fluid for distribution to air handling units or process loads. Vapour-compression and absorption types are common in large building cooling plants.

The number of times the total air volume of a space is replaced in one hour by supply, exhaust, or infiltration. It is used to size ventilation, judge indoor air quality, and check envelope leakage.

Ventilation that modulates outdoor air intake in response to measured occupancy or pollutant levels, typically via carbon dioxide sensors. It cuts fan and cooling energy while maintaining acceptable indoor air quality.

Closure devices installed in ductwork where it penetrates a fire-rated wall or floor, designed to close on heat or smoke detection and stop fire spread through the duct. They require periodic inspection and testing.

The maximum pressure a foundation soil or rock can sustain without shear failure or excessive settlement. It is compared with applied foundation pressure, usually with a factor of safety applied.

The vertical downward movement of a structure's foundation caused by compression of the supporting soil. Total and differential settlement are both checked, since uneven movement damages structures more than uniform movement.

The time-dependent reduction in volume of a saturated soil as water is squeezed out of its pores under sustained loading. It governs long-term settlement of clay foundations and embankments.

The ratio of a structure's or material's capacity to the load or stress actually applied. It covers uncertainty in loads, material strength, and analysis methods; its required value depends on the design code.

A design philosophy that checks a structure against defined conditions of unacceptable performance: ultimate limit states for collapse or instability, and serviceability limit states for deflection, cracking, and vibration.

A deep foundation of slender columns, driven or bored into the ground, that carries load through skin friction and end bearing to stronger strata. Used where shallow bearing capacity or settlement limits are inadequate.

In distillation, the ratio of liquid returned to the top of the column to the distillate withdrawn. Raising it improves separation and purity but increases energy and capital cost.

An ideal contacting stage in which the leaving streams reach equilibrium with each other. The number of theoretical stages, adjusted by stage efficiency, sets the height of a distillation or absorption column.

An accounting of material entering and leaving a process or control volume, stating that accumulation equals input minus output plus generation. It is the basis for sizing equipment and checking process performance.

The ratio of actual heat transfer in an exchanger to the maximum thermodynamically possible transfer between the two streams. It falls as the number of transfer units decreases.

A protective device that opens automatically to release fluid when pressure exceeds a set point, preventing equipment rupture. Correct sizing and setting are essential safeguards in process plants.

The ratio of real power to apparent power in an alternating current circuit, equal to the cosine of the phase angle between voltage and current. A low value increases current and losses for the same delivered power.

Adding capacitors or active compensators to reduce reactive power drawn from the supply, raising power factor towards unity. It lowers line current, losses, and utility reactive power charges.

The product of root-mean-square voltage and current in an alternating current circuit, measured in volt-amperes. It combines real and reactive power and determines the rating of transformers and cables.

A conductor connecting exposed metalwork to earth so that a fault current flows and trips the protective device, while limiting touch voltage. Its continuity and impedance are verified during inspection and testing.

A device that trips when the vector sum of currents in the monitored conductors is non-zero, indicating leakage to earth. It protects people against electric shock and reduces fire risk from earth faults.

A stability test for a feedback system based on the open-loop frequency response plotted in the complex plane. It relates encirclements of the critical point to closed-loop pole locations.

The amount by which the open-loop phase can increase before a unity-gain feedback system becomes unstable, measured at the gain crossover frequency. It is a common robustness indicator in control design.

The ratio of a system's output to its input in the Laplace domain, assuming zero initial conditions. It captures the dynamic behaviour of linear time-invariant systems for analysis and design.

Varying a property of a carrier wave, such as amplitude, frequency, or phase, in proportion to an information signal. It shifts the signal to a band suited to the transmission medium.

The ratio of desired signal power to noise power at a point in a system, often expressed in decibels. It sets an upper bound on the reliable data rate of a communication link.

Dividing a single IP address block into smaller logical networks by borrowing bits from the host portion of the address. It improves routing efficiency, security segmentation, and address utilisation.

The router address a host sends traffic to when the destination lies outside its own local subnet. Misconfigured gateways are a common cause of partial network connectivity.

A service that automatically assigns IP addresses, subnet masks, gateways, and other parameters to hosts on a network. Centralised configuration reduces manual errors and address conflicts.

The ratio of a component's strength, such as yield or ultimate stress, to the stress it is expected to experience in service. It accounts for load uncertainty, material variability, and consequences of failure.

A local increase in stress near geometric discontinuities such as holes, notches, or fillets. It is quantified by a factor applied to nominal stress and is critical in fatigue-prone designs.

Failure of a material after repeated or fluctuating loading at stress levels below its static strength. Cracks initiate at stress raisers and propagate until the remaining section fractures.

Connecting parts of an electrical installation to the general mass of earth to fix a reference potential and provide a fault current path. It underpins protection against shock and equipment damage.