Dealing with low flow measurements 

October 26, 2018

A low flow meter is a precision tool for measuring and monitoring very low fluid flow rates in the range of a fraction of a millimeter every hour. Such readings would otherwise not register on convection flow meters. Several industrial, chemical and medical applications demand accurate flow measurements at minute scales in such sensitive areas as dosing, chemical injection, precision painting and coating, and several other processes requiring low rate dispensing of fluid.

The challenge with low flow measurements

In typical mass flow cases, traditional meters utilized the physical fluid flow properties such as pressure differences, flow velocity, density, and displacement to measure the rate of flow to an acceptable accuracy of around 1% of full-scale flow. With low flow rates, a low flow meter cannot use the flow properties to measure the flow rate since it should not be exposed to the fluid. Most of the fluids dispensed at such low rates are often valuable or possessing chemical properties that are easily altered; this is the reason for the no contact rule of thumb. There must be zero contamination, and not even a drop of fluid should adhere to unwanted surfaces.

Solutions to low flow measurements 

Designers of low flow meters have had to come up with clever designs that satisfy the demands of the market. The trick is creating an accurate meter that can pick up the slightest changes in a flow channel without coming into contact withthe medium, which also means no moving parts.

  • Thermal flow meters

These are by far the most popular, accurate and low-cost of the low flow meters. Thermal meters have an accuracy of less than 0.5% of full-scale flow, and a turndown of 1000:1 – it can measure very low flows as well as moderately high flows. Thermal flow meters are used to measure the rate of flow of a variety gasses as well as liquids in petroleum and chemical industries.

They work by creating a temperature differential between two sensors by electrically heating one of them. As the fluid flows downstream, its molecules take away some of the heat which has to be compensated for by increasing the wattage on the heated sensor. The heating current required to maintain the differential in the two sensors is proportional to the rate of flow. By measuring this current, this low flow meter can measure the flow by referencing to an electrically calibrated scale.

  • Electromagnetic flow meters

Magnetic flow meters are another example of a no-contact low flow meter. The drawback with this design is that it can only measure conductive liquids – usually water-based fluids. Here is why. The meter has two electromagnets on either side of the flow pipe. They generate a constant electrical field across the pipe’s cross section. At right angles from these electromagnets are two electrical field sensors on either side.  Once a conductive fluid begins to flow, an induced electrical field is generated at both sensors; this is calculated to show the rate of flow.

  • Ultrasonic flow meters

This type of flow meter uses several pairs of ultrasound generators and sensors. The pairs are separated at a given distance along the length of the flow pipe, so that one sensor points towards the upstream direction and its counterpart in the downstream direction. Each member of a pair continually relays and receives sonic signals and the transmission time is measured. During flow, relays towards the direction of flow are accelerated while upstream transmission experience a latency caused by the mechanical drag of the fluid. These differences are calculated and translated to reveal the flow rate. These meters are only best at accurately measuring fluids with near consistent viscosity and flow properties.