What's the difference between a Design Criteria and a Specification?

Question

This is a work-related question so if you have online references as well, that would be helpful.

Answer 1

Design criteria: Support cable must be able to safely hold the 1000 lb load.

Specification: Published tensile strength is 2000 lbs (1000 lbs with a safety factor of 2).

Design criteria: The unit must be able to fit through the laborotory door, which is 3 feet wide.

Specification: Maximum width is 3 feet.

The design criteria is the underlying requirement that determines the specification.  Design criteria is typically a verbal justification where a specification is typically numerical.

Answer 2

Design criteria is a set of conditions set forth that someone must follow in designing something.  It is basically the functional requirements for whatever it is that is being designed.  It is basically a guide line and focuses on either the inputs or the outputs of what is to be designed.  Sometimes they go into depth and give guide lines on where to start or type of equipment.

A specification is a document that is produced after the design is completed for a piece of material or equipment.  It goes into depth about they type of equipment, material, dimensions, components, etc..that will be used to build/develop/contruct what was specified.

Examples:

Design criteria for an HVAC system could be:

use ASHRAE 99.4% ambient conditions

indoor design temp. to be 78F/50% RH

use water cooled centrifugal chiller with forced draft cooling towers and 30% glycol.

A specification for the centrifugal chiller would define how much tonage, what the condensor section is to be made out of, the model number, type of refrigerant, maximum weight (for structural reasons), voltage and maximum amps (for electrical reasons), requirements for gasket material....basically any component used to build it would have some sort of description.  It also defines how it is to be tested, stored at the construction site and even installed in some cases.

Answer 3

Design Criteria should be boundries - not cost more than ..., not be larger than ..., not weigh more than ..., strength not lower than ...

Specification should be exact as to weight, hole spacing, strength, cost, etc.   Often specifications are written to match exactly one product.

Answer 4

Firstly your question should address either "a design criterion" (singular) or "design criteria" (plural).

Essentially the specification is the totality of the design and other criteria and requirements.  For example design criteria might be weather or seismic conditions to be withstood, response times not to be exceeded, noise levels to be met, lifetime and standards to be met.  The specification would include all of these plus (for example) the colour and appearance requirements, materials to be avoided (say asbestos), delivery conditions, personnel requirements, guarantees.

Answer 5

Let me try to clear this with basic dictionary definitions...

SPECIFICATION - a detailed description, especially one providing information needed to make, build, or produce something

CRITERION - standard for judging things by: an accepted standard used in making a decision or judgment about something

A specification identifies the needs, requirements or goals of a design.  The Design Criteria are the tools and basic processes used to determine if the specification has been met.  

Proper functional testing can be a criterion for determining if a specific design meets performance specifications.

Answer 6

design criteria is used when you are designing.E.g a cable can be calculated according to it's current carrying capacity but the technical specification is e.g the resistance of the cable which has the design current carrying capacity.

Answer 7

Well, here's my thoughts: Specifications establishes the absolute maximum and minimum operating conditions for a given component, device, or system.

Design Criteria: That to me, is designing the component, device, or system in such fashion that it operates "safely" within the range of its specifications.

For example, if you want to use a transistor in a circuit design and the transistor's specification says its maximum power dissipation is rated at 1 watt.  You would use a design criteria that limited the transistor's power dissipation to some safe factor less then 1 watt.  0.6 watts or 0.75 watts, something like that.

Now, that is what I believe is the fundamental difference between Design Criteria and a Specification.