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What metric unit conventions are used in fire protection?
We will now define the most commonly used units for each measurement worldwide, the Metric System. Note that sometimes, the unit in the Imperial System and the Metric System is the same. This is the case, for example, for the current or voltage of an electric drive, expressed in amperes and volts. There is a slight difference between the commonly used units in the metric system and the International System, as most of the time, commonly used units are derived from the SI units for clarity and playing with not too big or not too small numbers. For example, as the SI unit for distance is meter, a bit more than 3 ft, it is difficult to express the size of a sprinkler thread in meter, and then, millimeter, representing 1/1000 meter, is generally used. For most generic measurements - Temperature in degrees Celsius.
- Spacing and distances are in meters. This is used in pipe lengths, height of storage, separation distances between racks, etc.
- Area of application, generally in square meter
- Angle, to determine the slope of a ceiling, for example, generally expressed in degrees.
For fire pumps and water supplies - Flow is generally expressed in cubic meters per hour. This is to avoid too big or too small values when speaking of flow delivered by a fire pump of a city water supply. But the use of liters per minutes is often found, in particular for hydraulic calculation of sprinkler systems, as we start with a small flow at the first sprinkler to make the calculation, where liter per minute is more convenient.
- Some countries sometimes use liters per second, or even kilogram per second, to be more in correlation with the coherent SI units.
- This is based on the fact that one liter (volume) of water weight one kilogram (weight). This means that this unit for flow is only valid with water, and under normal temperature and pressure conditions.
- Pressure is generally expressed in bars. As the coherent SI unit is Pascal, and one bar corresponds to 100 000 Pa, pressure is sometimes expressed in kilo Pascal. It can also be expressed in terms of meters of water column, with 1 meter of water column corresponding to 0.98 bars (under normal temperature and pressure conditions).
- Rotational speed is expressed in rounds per minute. This is the number of rotations of the pump impeller during one minute.
- Power for diesel engine drives is generally expressed in kilo Watts.
- Current and Voltage for electric drives are expressed in Amperes and Volts.
For water supplies
- Volume for the water tanks is generally expressed in cubic meters.
- Duration for the water supply is generally expressed in minutes or hours. Hours are generally used for long duration.
For sprinkler systems- K-factor in liters per minute per square root of bars, as the K factor is defined by the formula Q = K √P, with Q a flow in liters per minutes, and P a pressure in bars.
- Thread in millimeters
- Design density in liters per minute per square meter (l/min/m2). As one liter is 1/1000 cubic meter, one liter per square meter is equivalent to 1/1000 meter or one millimeter. The terminology millimeter per minute (mm/min) is very often use to categorize sprinkler densities.
- Hydraulic demand in liters per minute for the flow and bars for the pressure.
- Pipe diameter size, generally in millimeters
- Speed for the velocity of water inside the pipes, in meter per second.
For fire alarm systems- Light intensity is in Candelas, which is used for the light intensity coming from a strobe. Common candela ratings at 15, 30, 75, 90, 110, 115, and 177.
- RTI, or the Response Time Index for measuring response of heat detectors, is in square of meter per square of seconds.
- Sound intensity, used for measuring horn and speaker volume, is measured in decibels. A common requirement is a sound rating for horns to be 15 decibels above the ambient sound condition. Of course plenty of other nuance requirements apply there.
- Illumination, which is a measurement of a light intensity over an area, is measured in lux. A lux is the same as a lumen per square meter. This is almost like a density of light on a surface. It’s used when doing performance-based calculations for light intensity, and also on the emergency lighting side of electrical lighting design.
For gas protection systems- Volume, for the application of the system or room size, expressed in cubic meter
- Weight, for the quantity of gas, expressed in kg
- Duration, for the application or delivery time, generally expressed in minutes
For smoke control and performance-based designs- Volumetric flow of air or gas is expressed in cubic meters per hour or Liters per second.
- Gas speed is usually measured in meters per second, but could also be meters per minute.
- Particle density is measured in parts per million.
- Pressure differential, such as between two adjacent spaces, is measured in millibar.
- Heat release rate, or the amount of heat that is projecting from a fire source, can be measured as Kilowatts, or Megawatts. It could also be in Joules per second, which is the same as a Watt.
- Energy delivered by a fire, generally expressed Joules or Calories.
- A joule is the heat produced where one ampere is passed through a resistance of one ohm for one second, or it is the work required to move over a distance of one meter against a force of one newton.
- A calorie is the amount of energy required to raise the temperature of 1 g of water by 1°C (e.g., from 14°C to 15°C); a calorie is equal to 4.184 J.
- Heat flux, generally expressed in KiloWatt per square meter, or Watt per square centimeter. One centimeter being 1/100 meter.
- Heat flux, generally expressed in kilowatts per square meter, or watts per square centimeter, which is the same unit.
An important note here is that in Europe, for example, the preference for the exact unit that is being used will differ from one country to another. For countries leaving the Imperial System to the metric system, such as Canada, they tend to be as close as possible to the coherent SI units, using liters per second for flow, for example. That is a quick introduction to the unit conventions used as part of the metric system worldwide. For Franck Orset, I’m Jeff Kelm, this is MeyerFire University.
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