Extended Operational and Trade Vocabulary

Reefer Containers and Cold Chain Logistics glossary

The refrigerated-container and cold-chain vocabulary: A-frame stowage and reefer plug arrangements, active and absorption refrigeration cycles, controlled and modified atmosphere, pre-cooling, and the class guidance for refrigerated cargo. Grounds each term in the reefer machinery or the cold-chain logistics process it belongs to.

253 defined terms.

Showing 250 on this page (page 1 of 2).

A

A-frame stowage: Vessel arrangement on container ships with reefer plugs concentrated in specific bays for power access.
ABS Guide for Refrigerated Cargo: American Bureau of Shipping classification guidance for vessels carrying refrigerated cargo and reefer containers.
Absorption refrigeration: Heat-driven cycle using ammonia-water or lithium bromide-water; rare in marine reefer containers, used in some ship provision plants.
Active reefer: Integral reefer container with onboard refrigeration unit drawing power from ship, terminal, or genset.
Adiabatic cooling: Heat removal by evaporation; pre-cooling technique for some perishable cargoes before stuffing.
Aeration vent: Adjustable fresh-air opening on reefer container for respiring cargo gas exchange.
AHRI 1110: Air-Conditioning, Heating, and Refrigeration Institute standard for performance rating of marine refrigerated container refrigeration units.
Air change rate: Volume of outside air introduced per hour via fresh-air vent, typically 0 to 280 m3/hr on modern reefers.
Air circulation rate: Internal evaporator fan throughput; on a 40RH unit roughly 4,500 to 5,500 m3/hr at high speed.
Air-cooled condenser: Heat exchanger rejecting refrigerant heat to ambient air via condenser fan, standard on container reefer units.
Alarm code: Numeric or alphanumeric fault indication displayed by reefer controller signaling component failure or out-of-range condition.
Ambient temperature: Outside air temperature surrounding the reefer; influences pull-down time and compressor duty cycle.
Ammonia (R717): Natural refrigerant GWP 0, ODP 0; used in shore cold stores and reefer vessel hold plants, not in container units.
APHIS: Animal and Plant Health Inspection Service, US agency enforcing wood packaging and quarantine rules.
APHIS-PPQ Form 203: USDA Plant Protection and Quarantine notice of arrival and inspection record for treated commodities.
Apples carriage: Typically 0 to -1 C, 90 to 95% RH; long-haul under CA at 2% O2, 1 to 3% CO2.
ASHRAE 15: Safety standard for refrigeration systems, governs refrigerant charge limits and machinery room ventilation.
ASHRAE 34: Refrigerant designation and safety classification standard defining A1, A2L, A3, B1 etc.
Atlantic salmon carriage: Chilled 0 to -1 C, gel-pack or vacuum-pack, typical 14 to 18 day reefer transit.
ATP Agreement: 1970 UNECE Agreement on the International Carriage of Perishable Foodstuffs and on the Special Equipment to be Used for Such Carriage.
Avocado carriage: Typically 5 to 7 C, 85 to 90% RH; CA 3 to 5% O2 and 5 to 10% CO2 extends shelf life.

B

Banana carriage: 13.3 C pulp temperature, 90% RH, ethylene-sensitive, 4 to 5 week transits common from Latin America and West Africa to Europe and East Asia.
Banana ripening room: Shore-side controlled chamber that ethylene-treats green bananas at destination, 14.5 to 18 C.
Bay plan reefer: Stowage diagram identifying reefer slots and plug positions on container vessel.
Beef chilled carriage: -1.5 C deep chill, vacuum-packed primal cuts, 60 to 120 day shelf life.
Beef frozen carriage: -18 C or colder, blast-frozen, long-term storage stable.
Black box recorder: Reefer event memory storing setpoint, supply, return, and alarm history.
Blackberry carriage: 0 C, 90 to 95% RH, MA packaging with elevated CO2.
Bleed valve: Service port for refrigerant charging or recovery on reefer unit.
Blocked stowage: Reefer cargo loaded tight against bulkheads, restricting air circulation; cause of hot spots.
Blueberry carriage: 0 C, 90 to 95% RH, MA packaging 5 to 10% CO2 inhibits Botrytis.
Boil-off: Vaporization of cryogenic refrigerant (LN2, dry ice) used in expendable refrigeration units.
Bulkhead, return-air: Perforated wall at door end channeling air back to evaporator.
Bunker reefer: Older term for a vessel-supplied refrigerated cargo hold.

C

CA (Controlled Atmosphere): Active gas management of O2, CO2, and N2 in reefer to slow respiration; typical 1 to 5% O2, 1 to 10% CO2, balance N2.
Captain Peter: Maersk Line digital reefer monitoring service giving shippers real-time temperature, humidity, and CA data.
Carbon dioxide (R744): Natural refrigerant, GWP 1, used in NaturaLINE and other low-GWP container reefer units, transcritical cycle.
Carbon dioxide scrubber: Lime or amine bed inside CA unit removing excess CO2 from cargo respiration.
Carrier Transicold: US-headquartered manufacturer of container reefer units, including PrimeLINE, PrimeLINE Plus, and NaturaLINE.
Cellular respiration: Living-cargo metabolism producing CO2, water, and heat; basis for CA setpoint selection.
CFM: Cubic feet per minute, US measure of evaporator and fresh-air airflow.
Cherry carriage: 0 C, 90 to 95% RH, MA or CA prolongs firmness; SO2 not used (damages cherries).
Chilled cargo: Product carried above its freezing point, generally -2 to 12 C depending on commodity.
Citrus carriage: Oranges 4 C, lemons 12 C, grapefruit 13 C, all 85 to 90% RH.
Cleaning, reefer: Wash and disinfect after each cargo; required before food-grade reuse.
ClimaCare: Maersk Container Industry service brand for spare parts and OEM support of Star Cool units.
Climacteric fruit: Fruit ripening with autocatalytic ethylene burst and respiration peak, e.g., banana, avocado, mango, apple.
Clip-on genset: Diesel generator clamped to a container front for road or yard reefer power, typically 15 to 25 kVA, 460 V 3-phase.
CMA CGM REEFLEX: CMA CGM IoT reefer monitoring service for live data and exception alerts.
Cold chain: Continuous temperature-controlled supply chain from origin packing house to retail or end-use.
Cold treatment, in-transit: APHIS-approved temperature regime applied during the reefer voyage to disinfest fruit-fly host commodities.
Cold-treatment compliant logger: USDA-approved temperature recorder meeting APHIS placement and accuracy specifications for in-transit cold treatment.
Compressor, scroll: Most common compressor type in modern container reefer units; orbiting and fixed scroll volumes.
Conair: Legacy porthole-system reefer container relying on external clip-on or vessel-supplied refrigeration via two ports; largely retired.
Condenser fan: Axial fan moving ambient air across condenser coil.
Controlled atmosphere generator: Onboard N2 separator and CO2 scrubber maintaining set gas mix during voyage.
Controlled room temperature (CRT): USP definition 20 to 25 C with excursions 15 to 30 C, used for many pharmaceuticals.
Cool-down: Pre-trip lowering of empty container interior to set point before stuffing cargo.
CSC Plate: Container Safety Convention safety approval plate; verified on container surveys.

D

Daikin LXE: Daikin Industries container reefer unit family, including LXE10E.
Daikin Reefer Master: Daikin OEM telematics package for container reefer fleet monitoring.
Daikin Star Cooler: Marketing name used in some markets for high-efficiency Daikin reefer model.
Dairy carriage: 0 to 4 C for fresh milk, cream, yogurt; HACCP and EU 853/2004 hygiene compliance required.
Damper, fresh-air: Mechanical or motorized vent adjusting outside-air introduction.
Data logger: Standalone or embedded device recording temperature, humidity, and sometimes gas composition.
Deck stow reefer: On-deck reefer slot, often with better ambient air but more solar gain.
Deep-freeze container: Reefer rated to approximately -29 to -35 C minimum, suitable for ice cream and frozen seafood.
Defrost cycle: Periodic evaporator heating to clear frost; initiated by timer, demand sensor, or pressure differential.
Dehumidification: Removing moisture from cargo hold air using refrigerant or desiccant units.
Delta-T: Temperature difference between supply and return air; indicator of cargo heat load and airflow.
Dew Point: Temperature at which water vapor condenses; key to ventilation decisions.
Discharge pressure: High-side refrigerant pressure at compressor outlet.
Drain hole: T-floor opening allowing defrost water and condensate to exit container.
Drainage, condensate: Continuous removal of evaporator drip pan water through floor drains.
Dry-ice CO2 charge: Expendable refrigeration system using solid CO2 sublimation; rare in international reefer trade.
Dwell time: duration containers or cargo remain in the terminal.

E

EBI (Emerson Brand): Cargo-monitoring instruments historically branded EBI, now part of Emerson Cargo Solutions.
Economizer: Refrigeration circuit enhancement boosting capacity at low evaporating temperatures; standard on PrimeLINE and Star Cool.
Electronic expansion valve (EEV): Stepper-motor refrigerant metering device; replaces thermostatic TXV in modern reefers.
Emerson GO Real-Time: Emerson satellite/cellular reefer telematics tracker.
Equipment Interchange Receipt (EIR): Gate document recording reefer set point, machinery condition, and PTI status at handover.
Ethylene (C2H4): Plant ripening hormone; produced by climacteric fruit, harmful to ethylene-sensitive cargo at parts-per-million levels.
Ethylene Scrubber: Filter in reefer to remove ripening hormone for sensitive produce.
EU F-Gas Regulation 2024/573: Recast HFC phase-down framework replacing 517/2014, restricts high-GWP refrigerants in new equipment.
EU Regulation (EC) 1/2005: Council Regulation on protection of animals during transport, including live animals in containers.
EU Regulation (EC) 178/2002: General Food Law establishing food safety principles in the EU.
EU Regulation (EC) 853/2004: Specific hygiene rules for food of animal origin.
Evaporator: Fresh water generator using waste heat from jacket water at low vacuum.
Evaporator fan: Centrifugal or axial fan circulating air through coil and across cargo.
Excursion: Maximum motion amplitude of cargo permitted before lashing failure.

F

Fan curve: Performance map relating fan static pressure and airflow.
FDA FSMA: US Food and Drug Administration Food Safety Modernization Act of 2011.
FEU per plug: Forty-foot equivalent reefer capacity per shipboard or terminal reefer outlet.
Floor, T-bar aluminium: Extruded aluminum reefer floor with longitudinal T-section channels guiding supply air under cargo.
Forced-air pre-cooling: Tunnel cooling of palletized produce before stuffing into reefer.
Frequency drive (VFD): Variable-speed control for compressors and fans, key efficiency feature of PrimeLINE and Star Cool.
Fresh-air vent: Adjustable inlet allowing CO2 venting from respiring cargo; rated 0 to 280 m3/hr typically.
Frost protection: Setpoint management preventing chill injury on freeze-sensitive produce.
Frozen cargo: Product carried at -12 C or colder; common setpoints -18 C, -25 C, -29 C.

G

Genset, clip-on: Detachable diesel generator powering reefer during road or yard moves.
Genset, underslung: Chassis-mounted diesel generator for over-the-road reefer transport.
Globe Tracker: Vendor of satellite reefer telematics and CA monitoring devices.
Grape carriage: 0 C, 90 to 95% RH, sulfur dioxide (SO2) generator pads control Botrytis.
Grapefruit carriage: Typically 13 C, 85 to 90% RH; chilling injury below 10 C.
Greenhouse warming potential (GWP): 100-year IPCC metric comparing refrigerant global warming impact to CO2.
Ground (earth) check: Pre-trip electrical safety test on reefer chassis bonding.

H

HACCP: Hazard Analysis and Critical Control Points food safety management system, codified in Codex Alimentarius.
Hapag-Lloyd LIVE Trace: Hapag-Lloyd online reefer tracking and exception monitoring platform.
Heat exchanger, plate: Compact brazed-plate evaporator option on some modern reefer units.
Heat load: Sum of conduction, infiltration, respiration, and lighting heat the reefer must remove.
HFC-134a (R134a): Hydrofluorocarbon refrigerant, GWP 1430, being phased down in new container reefers under EU F-Gas.
HFC-404A (R404A): HFC blend, GWP 3922, historically used in low-temperature container reefers.
HFC-407F (R407F): HFC blend, GWP 1825, lower-GWP retrofit for R404A.
HFC-449A (R449A): HFC/HFO blend, GWP 1397, R404A replacement.
HFC-452A (R452A): HFC/HFO blend, GWP 2140, R404A replacement.
HFO-513A (R513A): HFO/HFC blend, GWP 631, R134a replacement.
High-pressure cut-out: Safety pressure switch shutting down compressor on overpressure.
Hot stowage: Reefer placed near vessel heat source (engine room bulkhead, funnel uptake); to be avoided.
Humidity setpoint: Target relative humidity, typical 85 to 95% for fresh produce, 70 to 85% for some commodities.
Hybrid container vessel: Ship designed for both container and traditional refrigerated bulk operations.

I

ICCC: Inter-Carrier Container Code, used by carriers for plug-out reefer monitoring.
Identec Solutions: Provider of RFID and RTLS reefer monitoring hardware.
IDS (Independent Data Service): Third-party temperature and event monitoring distinct from carrier or shipper systems.
In-transit cold treatment (ICT): APHIS-approved on-board cold treatment under USDA SET protocol.
Insulation, polyurethane foam: 50 to 100 mm CFC-free foam between inner aluminum and outer steel reefer skins.
Integral reefer: Container with built-in refrigeration unit and standardized power connection, dominant modern design.
IPPC ISPM 15: International Plant Protection Convention standard for treatment of wood packaging material; relevant to crates and pallets in reefer cargo.
ISO 1161: Corner fittings standard for series 1 containers.
ISO 14001: Environmental management standard often integrated with ISM.
ISO 1496-2: Specification for thermal containers, defines structural and insulation requirements.
ISO 22000: Food safety management system standard, applicable across the cold chain.
ISO 6346: ISO standard for container coding and marking.
ISO 668: ISO standard for container classifications and dimensions.
ISO 830: Container terminology standard.
ISO 9001: Quality management standard often integrated with ISM.

J

Job log: Reefer movement and PTI log retained at depot for QA traceability.
Juice concentrate carriage: Frozen or chilled fruit-juice concentrate, typically -18 C frozen or 4 C chilled.

K

Kiwi carriage: -0.5 to 0 C, 90 to 95% RH, ethylene-sensitive (high CO2 tolerance).
kVA, reefer plug: Apparent power capacity of vessel or terminal reefer socket, typically 15 to 18 kVA per plug.

L

Latent heat: Heat absorbed or released during phase change, dominant for ice/water and freeze-cycle cargoes.
Lemon carriage: 12 C, 85 to 90% RH; chilling injury below 10 C.
Liquefied nitrogen (LN2): Cryogenic expendable refrigerant for specialty reefer systems.
Liquid-line filter drier: Refrigerant moisture and contaminant filter on high-pressure liquid line.
Live animal container: Containers permanently fitted for livestock under EU 1/2005 and IMO MSC.1/Circ.1349.
Logger placement: Sensor position inside cargo or air stream specified by USDA cold-treatment and pharma protocols.
Low-pressure cut-out: Safety switch protecting compressor against loss of refrigerant charge.

M

MA (Modified Atmosphere): Passive gas environment created by produce respiration inside sealed liner; no active gas injection.
Maersk Container Industry (MCI): Manufacturer of Star Cool reefer units, sold to Chinese consortium in 2021; brand retained.
MAGNUM PLUS / Magnum 2.0: Thermo King deep-freeze and pharma-capable container reefer units.
Mango carriage: 12 to 13 C, 85 to 90% RH; chilling injury below 10 C; ethylene-sensitive.
MARPOL Annex IV: Prevention of pollution by sewage.
Mass airflow: Cargo-traversing air mass per unit time; specified in some reefer commodity guides.
Modified Atmosphere Packaging (MAP): Gas-flushed retail packaging carried in standard reefers.
Mother vessel reefer plug: Shipboard reefer socket, IEC 60309 type, 440 V 3-phase 60 Hz 32 A standard.
MSC MyMSC: Mediterranean Shipping Company digital reefer monitoring portal.

N

NaturaLINE: Carrier Transicold reefer unit using R744 (CO2) natural refrigerant.
Nitrogen flush: Pre-loading inerting of CA reefer atmosphere with bottled or generated N2.
No-frost coil: Evaporator design or control logic minimizing frost build-up.
Non-climacteric fruit: Fruit not exhibiting ripening respiration burst, e.g., citrus, grapes, strawberries.

O

O2 setpoint: Target oxygen concentration in CA reefer, typically 1 to 5% by volume for apple and avocado.
Operating compressor hours: Cumulative compressor runtime recorded by reefer controller.
Orange carriage: 4 C, 85 to 90% RH; cold-treatment-host for fruit flies in many trade lanes.

P

Partial PTI: Reduced PTI subset performed on time-pressed turnarounds.
Pear carriage: 1 C (Bartlett) to -1 C (some cultivars), 90 to 95% RH; CA extends storage.
PerishableCare: Maersk premium reefer service tier with stricter handling and monitoring.
Pharma reefer: Container with tighter temperature stability (typical 2 to 8 C) and validated mapping for pharmaceutical shipments.
Phytosanitary certificate: Plant health certificate.
Pineapple carriage: 8 C, 85 to 90% RH; chilling injury below 7 C.
Plenum: Air distribution space above cargo and below ceiling, fed by supply duct.
Plug-in time: Period reefer is connected and powered at terminal, depot, or vessel.
Plug-out: Reefer disconnected, often during gate moves or hinterland transport.
Polyurethane (PUR) foam: CFC- and HCFC-free insulation, typical 50 to 100 mm thickness in reefer panels.
Porthole container: Insulated container without internal machinery; cooled by clip-on or vessel hold plant via two port openings; legacy.
Pre-Cooling: General term covering tank cool-down with LNG spray nozzles before bulk transfer.
Pre-trip alarm log: PTI record of triggered alarms during functional test sequence.
PrimeLINE: Carrier Transicold high-efficiency container reefer unit, R134a in current configurations.
PrimeLINE Plus: Updated PrimeLINE variant with enhanced energy performance and connectivity.
PTI (Pre-Trip Inspection): Standardized depot-side functional test of reefer unit prior to release.
Pull-down time: Duration from start-up to reaching set point with cargo or empty.
Pulp temperature: Temperature measured inside fruit or muscle, critical compliance metric for cold treatment.

Q

QC sample: Cargo subset removed at gate or load for quality inspection.
Quick-freeze: Rapid freezing process; not normally performed inside reefer but described for cargo arrival condition.

R

R134a: HFC refrigerant common in AC plants, phased down under MARPOL Annex VI Regulation 12.
R290 (propane): Hydrocarbon natural refrigerant, GWP 3, ASHRAE A3 flammable; limited container use.
R404A, R407F, R449A, R452A, R513A: See HFC entries.
R600a (isobutane): Hydrocarbon refrigerant, GWP under 5, used in some small refrigeration appliances; not in container reefers.
R717: Ammonia refrigerant used on reefer carriers and some chillers.
R744: Carbon dioxide refrigerant used in modern reefer container and cruise installations.
Reefer Plug: Onboard or terminal 440 V socket for reefer power.
Reefer rack: Stack with power supply for reefer containers.
REEFLEX: CMA CGM reefer telematics platform.
Refrigerant charge: Mass of refrigerant in reefer circuit, typical 3 to 6 kg depending on unit.
Refrigerant leak test: Pre-trip electronic or dye-based leak survey.
Relative humidity (RH): Ratio of water vapor in air to saturation at the same temperature.
Respiration rate: Volume of O2 consumed and CO2 produced per kg cargo per hour, temperature dependent.
Return air: Air drawn back to evaporator after passing across cargo; setpoint reference for chilled cargo.
Reverse logistics, reefer: Movement of empty reefer back to load port, often requiring depot inspection and PTI.

S

Salmon carriage: See Atlantic salmon.
Sanitary Transportation Rule: FDA FSMA rule (21 CFR Part 1, Subpart O) governing food transport.
Sashimi tuna carriage: -60 C super-freezer for sashimi-grade bluefin and yellowfin.
Scroll compressor: Dominant compressor architecture in modern reefer units.
Sensitech TempTale 4: USDA cold-treatment-compliant temperature data logger family.
Set point: Operator-specified target temperature for return or supply air.
SET protocol: USDA Maritime Cold Treatment in-transit compliance specification.
Shelf life: Period cargo retains marketable quality at specified temperature.
Shore power: OPS or cold ironing infrastructure, IEC/IEEE 80005 series.
Sniffer test: Refrigerant leak inspection using electronic halogen detector.
SO2 generator pad: Sulfur dioxide-releasing sheet packed with grapes to control Botrytis cinerea.
Solar gain: Heat absorbed from sunlight by deck-stowed reefers; reflective paint reduces impact.
Star Cool Integrated: Maersk Container Industry integrated reefer unit with R134a, slim design.
Star Cool Integrated SAA: Star Cool variant with sensor-controlled airflow for energy savings.
Stratification: Vertical density layering of the water column resisting mixing.
Strawberry carriage: 0 C, 90 to 95% RH; MA with elevated CO2 controls decay.
Sub-zero treatment: Storage below 0 C for certain frozen and ultra-low-temperature cargoes.
Super-freezer: Reefer rated to -60 C or colder, used for sashimi tuna and some biologicals.
Supply air: Air discharged into cargo zone after evaporator; setpoint reference for frozen cargo.
Surface temperature: Cargo skin temperature, distinct from pulp temperature.

T

T-bar floor: Aluminum extruded floor with T-section air channels; standard reefer construction.
Temperature mapping: Validation exercise placing many loggers in empty container to characterize uniformity (pharma requirement).
Temperature uniformity: Maximum deviation between supply and return or among cargo positions.
TempTale: See Sensitech.
Terminal reefer monitoring: Yard service connecting, plugging, inspecting, and logging reefer events.
Thermal break: Insulating gasket reducing conductive heat transfer at door and corner posts.
Thermo King: US-headquartered reefer unit manufacturer (Trane Technologies), MAGNUM and MAGNUM PLUS lines.
Thermo King Magnum 2.0: Updated Magnum reefer with energy and pharma performance enhancements.
Thermostatic expansion valve (TXV): Mechanical refrigerant metering device, largely replaced by EEV.
Three-phase, 60 Hz: Standard reefer power, 380 to 460 V depending on terminal or vessel.
Tracking, real-time: Continuous satellite or cellular reefer telematics.
Trim air: Conditioned air introduced via fresh-air vent for gas management.
Tuna carriage: Sashimi-grade -60 C super-freezer; other tuna grades -18 to -25 C.

U

Ultra-low temperature (ULT) shipment: Cargo carried at -60 to -80 C, e.g., COVID-19 mRNA vaccines via specialized active containers and dry ice.
UN R134a, R404A etc.: ASHRAE refrigerant numbers; UN dangerous goods numbers differ (e.g., UN 3159 for R134a).
Underdeck stow: Reefer slot in vessel hold, generally lower ambient and shielded from solar gain.
UNECE Working Party on Perishable Foodstuffs: Maintains the ATP Agreement.
USDA APHIS PPQ Treatment Manual: Definitive guide for approved phytosanitary treatments including cold treatment regimes.
USDA cold treatment schedules: 14 days at 0 C, 16 days at 0.55 C, 18 days at 1.11 C, 20 days at 1.39 C, 22 days at 1.67 C for Mediterranean, oriental, and Caribbean fruit flies (T107-a).
USDA SET: Sea-borne in-transit cold treatment program.

V

Vaccine cold chain: 2 to 8 C standard; -20 C frozen; -70 C ultra-cold for Pfizer-BioNTech COVID-19 vaccine.
Vacuum-pack beef: Primal cuts in oxygen-barrier film; 60 to 120 day chilled shelf life.
Variable Frequency Drive (VFD): Speed control for cargo and send-out pumps reducing electrical consumption.
Ventilation setting: Fresh-air vent opening expressed in m3/hr or CFM.
Voltage, reefer: Common values 380 V (50 Hz), 440 V (60 Hz), 460 V (60 Hz).
VPD (Vapor Pressure Deficit): Difference between saturation and actual vapor pressure; controls cargo moisture loss.

W

Water-cooled condenser: Vessel-side water loop condenser option for hold-stowed reefers; rare today.
Wattage draw: Real-power consumption of reefer unit, typical 2 to 8 kW at steady state.
WCO Harmonized System: Customs classification, reefer-relevant chapters include 02 (meat), 03 (fish), 04 (dairy), 07 (vegetables), 08 (fruit), 30 (pharma).
WHO Good Distribution Practices (GDP): Pharma transport quality requirements applied to reefer movements.

X

X-Ray Inspection: Radiographic NDT method for weld root and internal defects.
XtendFresh: Carrier Transicold controlled-atmosphere kit retrofitted to integral reefers.

Y

Yard plug: Depot reefer power outlet, often 32 A, 440 V 3-phase 60 Hz.