In Grant water baths and circulators the pump is used only to circulate liquid to an external device, not for stirring within the tank. An independent stirrer is used for circulation within the tank, in order to achieve optimum temperature uniformity throughout the working area.
Pump performance is specified in terms of flow rate and pressure. The flow is normally quoted in litres/min. Pressure can be quoted in mbar, metres (of water) head and psi (pounds per square inch).
The relationship between pressures is:-
One atmosphere is 1010 mbar, 10.3 metres of water or 14.6 psi
In accordance with DIN58966 part 1, maximum flow is measured into an open vessel, through a horizontal pipe attached to the pump outlet; the pipe must be at least 0.5 m in length.
The maximum pressure is determined from the maximum height to which water can be pushed in a vertical tube connected to the pump output. The head is measured in metres of water.
The measurements described above are maximum flow, which is at zero head, and maximum pressure which is at zero flow. Neither of these conditions is likely to be met in practice. In practical terms, when a pump is connected to an external device, the actual flow rate is determined by the bore (diameter) of the pipe, and any friction in the pipes. Any kink or change in diameter in the pipe will cause a reduction in flow rate.
To achieve maximum flow rate:
The flow rate through the circuit can be adjusted by, for example, installing an in-line tap. If the flow rate is too low, check that: