Laboratory Pumps – Choosing your Laboratory Pumps or HPLC Pump

Flow rate

This is normally the first citeria when considering a laboratory pump. We have flow rates from as little as 0.001mls up to 300ml per min. The low flow rates are achievable by either using a small 5ml head pump such as the 0-5ml LS class pump or by using the twin head LD pumps. Twin head laboratory pumps generally offer better control and will have lower pulsation as the heads pump “out of synch” to ensure that fluid is always pumping (single head pumps have a short backstroke to fill the piston chamber where there is no outflow). Single head pumps can typically pump up to 40ml/min. For flow rates above this we recommend the twin head pumps.

Flow Precison and Reproducibility

The laboratory pumps do not measure the actual flow rate of the liquid. Instead the flow is calculated by multiplying the piston chamber volume by the amount of strokes per second. For regulated labs it is always worth measuring the actual volume either “at the end of the line” or by using a flowmeter. This would account for any post pump leaks etc. The precision is the accuracy of the pump. So if you set the flow rate for 10ml/min and the measured flow rate is 10.1ml then the accuracy is 1%  (you measure the flow rate over a period of time). The reproducibility is the variation in the flow rate –  So in the above example if the flow rate varied from 10.0 to 10.1ml/min the reproducibility would be 1%. Larger laboratory pumps use bigger pistons and this often reduces the maximum pressure they can pump ( for example – a 10ml/min Series II pump can pump at 6000psi, the 40ml/min model can reach 1500psi) and  the absolute accuracy of the flow rate will similarly be effected. The new HF laboratory pumps are specifically designed to pump at high flow rates with high pressures if the application demands this. Very low flow rates are sometimes more accurate with a 5ml head or a twin head pump if precision is required

Pressure rating  and pressure reading

The next parameter considered after flow rate is the pressure rating. Long or thin columns/tubes require higher pressures for a given flow rate than shorter broader columns.  HPLC columns that use smaller particles also require higher pressures.  The maximum pressure of the laboratory pump is normally rated using Bar or psi (1bar = 14.5psi). Not all pumps measure pressure. This requires a small device called a pressure transducer to be fitted inside the pump (typically this is normally on the pulse damper but can be separate). The pressure measurement provides a useful diagnostic tool – if the pressure rises or falls it may be showing a blockage or a leak, and if it fluctuates it could indicate there is air in the system. Also for HPLC the pressure can give an indication of how the column is ageing. To help with this most manufacturers supply a test certificate showing the pressure at a given flow rate when the column is new. Typical pressures for a 25×0.46cm HPLC column are around 80-120bar. UHPLC columns require extra high pressures – and for these we recommend using the Ultra high pressure laboratory pumps.

Pulsation

Pulsation normally refers to changes in the pressure when the laboratory pump is running.  For some systems this isn’t a problem, but it can affect HPLC performance (particularly if using a Refractive Index detector). Pulsation occurs when the piston inside the pump is making a fill stroke (backstroke). The flow temporarily stops (backflow is prevented by a check valve) and this causes a pulse in the pressure. Pulsation is much more noticeable in a single piston pump compared to pumps that have two or more heads. In these systems the heads counter each others strokes so that one is always pumping whilst the other is making the backstroke. Pulsation can also be reduced by using a pulse damper. These require pressures of more than 500psi to become effective and work by having a flexible membrane over a small solvent reservoir (this acts in a similar way to a spring and smooths any pulses).  The pulse dampers have an upper pressure limit of around 5000 psi.  

Gradient

Many applications require the use of a solvent gradient. This is where two solvents are mixed in a changing proportion over a period of time. For example a solvent mix may be required to begin at 20% A and 80%B and to finish at 80%A and 20%B (typically the proportion of the stronger solvent is increased). There are two common ways to create a gradient – use a single laboratory pump with a proportioning valve that switches between the different solvent inlets to mix the solvents before they enter the pump giving a “low pressure” gradient – or to connect two more pumps together that mix the solvents together after they have left the pump (this is a high pressure gradient). The lowest cost of forming a gradient is to use the B series pumps as these can be used on a stand-alone basis. High pressure gradients require two pumps and software control (PC or control unit) .

Constant pressure

Almost all laboratory pumps are constant flow pumps – this means that the piston will move at a constant speed to ensure the flow rate remains the same. This also means that the pressure in the system will vary to keep the flow rate constant.  Constant pressure pumps however will vary the flow rate to keep the pressure constant (these are not recommended for constant flow applications). The most common use for constant pressure pumps is to pack HPLC columns.  

Maintenance

All laboratory pumps are made using well used technology and do not require any difficult annual maintenance. The piston seals are considered to be a consumable part and will need replacing every 4-18months depending on use and application. This a simple procedure that takes a few minutes. It is highly recommended to use only HPLC grade solvents and ensure that these are adequately filtered before entering the laboratory pump. laboratory pumps