Poiseuille’s law tells us that the rate of liquid flowing through pipe will fluctuate at the fourth power of the radius of the pipe. To be sure, other variables also greatly affect liquid flow rate. A liquid’s viscosity, the pipe’s length, the amount of pressure placed on the liquid and turbulence are also key factors in most practical applications.

Different applications will call for different levels of precision. But, the ability to reliably estimate or approximate liquid flow rate and ensure that variations can be properly controlled is a foundational requirement in virtually every situation.

Flow Rates and Liquid Flow Controllers

For those considering or who use liquid flow meters or controllers, estimating your flow rate is a needed first step for confirming that correct and precise measurements will be generated. Every flow product has a flow range, or a given scope of the minimum to maximum flow rate it can evaluate accurately. By defining the bounds of your flow range, you can ascertain that your flow meter or controller will provide correct measurements despite any fluctuations in conditions.

Flow range sizing is important when one considers that a single flow meter typically only measures 10:1. In other words, a unit rated for 10 L/minute will only measure as low as 1 L/minute accurately. If you find that your estimated flow range is too large, you might need to combine flow products to make sure you get exact measurements across the full range.

One simple approximation of flow rate is accomplished by evaluating line size. For reference, our products typically connect to tubing that are ½ inch in diameter or less. Using this information, you can plug your estimated metrics into the flow rate formula, Q= A x v, where A is the area of your line opening and v is the velocity of your liquid sample. You can also use one of several flow rate calculators to approximate your range more easily.

Flow Ranges and Practical Application

Once you’ve determined your flow range, a next step to assuring you’ve selected the right flow product is to assess and understand how the flow range relates to the product use. For example, our McMillan 101 and 106 FLO-SENSORS can measure extremely low liquid flow rates when you are gaging samples such as hydrocarbon fluids, fuels, light oils, coolant, solvents, mild acids, pesticides, alkalis and deionized water. Similarly, our Model 108 PTFE Liquid FLO-SENSORS have a flow range of 15 mL/min up to 10 L/min to precisely measure chemicals found in microelectronics manufacturing processes.

If you find that you need to measure and control micro to low flow rates, the IRIDIUM Series Liquid Flow Control System is applicable for flow ranges as low as 7 mL/min and as high as 10 L/min. With the option for such a low flow range, controllers are ideal for applications such as on-demand chemical dilution and blending, while also possessing the option to regulate sample flows at a higher range. Controllers are also capable of adapting to a variety of flow conditions and applications, which can give you flexibility if you approximate a wide flow range.

Estimation Assistance

As we’ve shared, estimating liquid flow rate is pivotal to supporting successful project applications and the selection of the right products to control for liquid flow variability. If you have additional questions about liquid flow estimation or need help in evaluating what liquid flow meter or controller is right for your situation, contact one of our knowledgeable team members today.