FAQ - Frequently Asked Questions
What should be considered when selecting the tube for a peristaltic pump?
The choice of tubing to be used for a peristatic pump is critical to meet the needs of both the peristaltic pump itself and the application. 3 factors of the most important factors are discussed below:
1 Correctly Dimensioned
An obvious starting point is that the tube needs to be correctly dimensioned for a particular peristaltic pump. As the rollers move over the tube they compress and occlude the tube. The peristaltic pump must therefore be designed so that the gap between the rollers and pump body is less than twice the wall thickness of the tube – otherwise occlusion will not occur, and the pump will be very ineffective. Therefore, a peristaltic pump is designed for a particular wall thickness of a tube. Within the Boxer range, this is either 1.0 mm, 1.6 mm or 2.4 mm.
Secondly, a peristaltic pump is designed to operate within a range of internal diameters of a tube. This data is provided in the datasheets and it is not recommended to operate the peristaltic pump outside this range. For example, the 9QQ peristaltic pump is designed for internal diameter tube of 0.5 mm to 3.5 mm and always with a wall thickness of 1.0 mm.
2 Elasticity / Fatigue
As described in the introduction, a peristaltic pump operates by compressing a tube around a partial circumference of the pump’s body. As the rollers pass over the tube it is vital for the function of the peristaltic pump that the tube re-opens once the rollers move away. This is determined by the elasticity of the tube. Consider a peristaltic pump running at 300 rpm with 4 rollers for 6 weeks continuously. During this time, each part of the tube is compressed over 72 million times. If not designed for operation in a peristaltic pump, a tube will suffer fatigue as elasticity is lost, the tube will flatten and flow performance reduces. For this reason, special thermoplastic elastomers have been developed as peristaltic pump tubing which remain highly elastic over a long operation life.
3 Chemical Compatibility
The final critical element to the choice of tube to be used in a peristaltic pump is chemical compatibility. Guides are available for different peristaltic pump tubes against a long list of media; however, it is almost always recommended to perform a dynamic test with the tube installed in the peristaltic pump under the regular conditions of the application. Some general guidance is provided below.
Boxer generally offers 4 different tubes:
Silicone: generally inert giving excellent chemical compatibility, low durometer (reduces load on peristaltic pump), low cost, however very limited life.
Lagoprene: a good general-purpose thermoplastic elastomer, good chemical compatibility, long life
Pharm-a-line: similar to lagoprene offering with good general-purpose tube with good chemical compatibility and long life, however additionally with biocompatibility to USP Class VI and FDA to 21CFR177.2600.
ED-Plex: a thermoplastic elastomer with chemical compatibility advantages over Lagoprene and Pharm-a-line for aqueous fluids, acids, bases and some oils / hydrocarbons.
Why does the flow of a peristaltic pump pulsate?
The flow of a peristaltic pump is interrupted as the rollers engage and disengage from the tube. For example, considering the outlet flow, as the roller moves away from the tube it expands back from flat to its original shape. During this expansion the flow from behind the roller is partly used to fill the void. Hence the flow from the peristaltic pump decreases momentarily during this part of the rotation. The level of decrease depends primarily on the angle of which the rollers break away from the tube, roller diameter and outlet pressure. In some cases, the flow will momentarily reverse.
If the peristaltic pump is assembled with more rollers the frequency of the 'flow interruption' is increased giving the effect of reduced pulsation. In addition, since more rollers generally means less volume between rollers, the expansion of the tube as the rollers move away is reduced in proportion to the volume available for flow.
More rollers are however only recommended where needed since tube wear is significantly increased.
How to decide between a DC motor + gear or stepper motor driven peristaltic pump?
Generally, a peristaltic pump from Boxer is available driven with either a DC motor (+ gear) or stepper motor. A DC motor peristaltic pump has a planetary gear between the motor and peristaltic pump head to reduce the motor speed (typically above 6000 rpm) to between 33 and 520 rpm. A stepper motor driven peristaltic pump is driven directly from the motor and can be run from 0 to above 500 rpm. So which configuration of peristaltic pump is best suited to the application? This can be determined by considering the advantages of both drives:
Peristaltic pump with DC motor + gear:
+ simple control via DC supply (no external driver needed)
+ smaller size
+ lower power consumption
+ more economic
+ less prone to stalling
+ less prone to overheating
+ higher resolution encoder available to provide accurate speed feedback
Peristaltic pump with stepper motor:
+ full range of speed available, from 0 to max rpm, typically above 500 rpm (DC motor + gear is limited to approx 50% on nominal speed)
+ controllable acceleration and declaration
+ higher accuracy doing
+ reversible with a control signal (DC requires reversed polarity of motor supply)
+ lower sound level (see FAQ on mechanical resonance)
+ long motor life, typically above 10,000 running hours (DC is typically 1000 to 4000 hours)
By far, the most common criteria for selecting a stepper motor driven peristaltic pump is the flow range the application requires. If, for example, an analyser specifies to control the flow from 10 ml / min to 150 ml / min, a stepper motor driven peristaltic pump in the clear choice since a DC + gear driven peristaltic pump will only be able to reduce the flow to approx 50% of nominal flow, so for this example this is approx 75 ml / min to 150 ml /min.