Flow Rate: 'Speed' or 'Velocity'?

[syx]

Dear Chromatographers,

It may look ridiculous, but I have a difficulty when trying to translate “flow rateâ€

[Kostas Petritis]

Syx,

It is neither of what you mentioned. It is a technical term but I would guess that each language would have a term to express volume per unit time (maybe you can look at the specifications of commercial pumps of your country -not necessary HPLC pumps- as they might express their maximum flow rate. Alternatively, what is your language? There are people here from several countries, someone may speak your language...

[Victor]

Syx-first a word of caution.

Flow rate is often a term used to describe the volumetric flow through a column. In HPLC this might be measured in mls of mobile phase per minute.

Speed or velocity are terms which are used to describe the distance covered by the mobile phase in unit time. So typical units are cm/second.

So a small and large diameter column operated at the same flow velocity will not have the same flow rate.

So I believe both of your answers are technically incorrect.

The difference between speed and velocity is strictly that speed is a scalar quantity whereas velocity is a vector. Speed has magnitude only, but velocity has magnitude and direction. Velocity is generally used in chromatography; although the direction of flow is not usually in question!