Small turbine flowmeters are adaptable devices that Titan has successfully employed for a wide range of laboratory dispensing applications.
Turbine flowmeters offer the high level of repeatability and dependability required for accurate laboratory batch delivery systems. Assuring the linearity of these low cost small flow devices should however be carefully considered. Turbine flowmeters are intrinsically Reynolds number devices so as long as the operating characteristics do not change drastically even their non-linearity can be accommodated with a software correction. In most laboratory dispensing applications – the flow rate is constant so linearity is not normally an issue.
Care has to also be taken with the resolution of turbine flowmeters. For example – to deliver a pre-selected quantity of pure water at 1-litre per minute any one of our 800-Series flowmeters could be used. The problem comes with the volume size, accuracy requirements and delivery speed versus permissible pressure drop in the system. The chart below highlights the potential issues.
|Flow range l/min||0.05-0.5||0.12-1.5||0.0-4.5||0.25-6.5||0.3-10||0.5-15|
|Pulses per litre||17000||7000||3500||2100||1420||980|
|Desired flow rate l/min||0.5 max||1||1||1||1||1|
|Desired volume l||0.1||0.1||0.1||0.1||0.1||0.1|
|Number of pulses counted||700||350||210||142||98|
|Best Resulting resolution ±||0.07%||0.14%||0.24%||0.35%||0.51%|
|Dispense time seconds||6.00||6.00||6.00||6.00||6.00|
|Smaller volume measurement|
|Desired flow rate l/min||1||1||1||1||1||1|
|Desired volume l||0.05||0.05||0.05||0.05||0.05||0.05|
|Number of pulses counted||350||175||105||71||49|
|Best Resulting resolution ±||0.14%||0.29%||0.48%||0.70%||1.02%|
|Dispense time seconds||3.00||3.00||3.00||3.00||3.00|
As can be seen from the two examples above the relationship between best possible resolution, dispense volume and dispense time is completely linear. Unfortunately the system timings, start and stop response times and general system inertia can become significant. However, if these are known and constant – they can be calibrated out permitting quite small deliveries to be made relatively accurately.
In practice using turbine flowmeters it is difficult to achieve accuracies much better than ±0.5% . For laboratory dispensing applications requiring higher accuracy we have an ultrasonic system under test that may offer a more appropriate solution.
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