## Offset

For a control system, this relates to the difference between the set point and the sustained value under proportional control. It is often termed 'sustained deviation'.

## Ohm

This is the electrical resistance between two points of a conductor when a constant potential difference of 1 volt is applied to these points, produces in the conductor a current of 1 ampere, the conductor not being the seat of any electromagnetic force.

## Oil

This is a substance that is liquid at ambient temperatures and does not mix with water. It may however mix with other oils. Oils can be grouped as follows; vegetable oils, volatile essential oils, petrochemical oils, and synthetic oils. Oil can quite often be used as a fuel for a boiler in a steam system.

## On-off control

This is a mode of control in which only two situations exist, either on or off with no intermediate situation.

## Orifice plate flowmeter

A type of flowmeter that is based on the principle of placing a constriction (plate, flow nozzle, venture sections) in a closed channel carrying a stream of fluid causing an increase in velocity and hence an increase in kinetic energy at the point of constriction. From the energy balance as given in Bernoulli's Theorem there must be a corresponding reduction in pressure. Rate of discharge from the constriction can be calculated, knowing this pressure reduction, the area available for flow at the constriction, density of the fluid, and coefficient of discharge. The latter is defined as the ratio of actual flow to the theoretical flow and makes allowance for steam constriction and frictional effects.

## Oxygen scavenging chemicals

These are chemicals such as catalysed sodium sulphite which are added to boiler feedwater to reduce the amount of oxygen.

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