Flow Meters in Wide Temperature Ranges

As some of you may be aware, in addition to temperature, humidity and vacuum chambers Bemco also designs and manufactures fluid conditioning systems for a variety of different applications, including radar / electronics cooling and leak checking. Often times our customers need these fluid conditioners or chillers to operate in wide temperature ranges, just last year we built a system that conditioned fluid from -100F to +300F. Additionally, in most cases the customer needs to closely monitor the fluid flow rate, and this is where things get interesting...

Temperature changes of several hundred degrees typically (depending on the fluid) have a drastic effect on the fluid viscosity. With the popular dielectric oil known as polyalphaolefin (PAO) this means the difference of honey (at cold temps) versus water (at hot temps). This massive change in viscosity has a major effect on many types of flow meters, most notably turbine style flow meters. The higher viscosity produces more drag through the turbine (at the same flow rate), which exerts a force on the bearing(s). This increased force on the bearing(s), increases friction, which slows the turbine down, indicating a slower flow rate. For this reason Bemco strongly prefers the use of gear type flow meters.

Gear type flow meters don't really "care" what the fluid viscosity is. Each gear cavity has a fixed volume, and it will require that volume of fluid to turn the gear a fixed amount regardless of what the fluid viscosity is. However, there is one limitation with gear type meters that can easily be overlooked...

In order for the gears to rotate freely, the gear meter is manufactured with clearances. Conveniently, when using most oils, the viscous / cohesive forces are enough to seal these gaps of just a few thousandths of an inch. If you've ever had trouble lifting a coaster off of a table top because of water in between the two surfaces, then you can understand how liquids can seal gaps. However if the fluid is too thin, it will "blow by" the gears through these clearances and cause lower flow rate readings than what's actually flowing through the meter.

For this reason all gear flow meters have a minimum allowable viscosity for accurate flow rate indication, but many manufacturers fail to publish this information. Above this minimum there is no viscosity affect on accuracy, but you need to be above the minimum. Be sure you call the manufacturer and find out what this minimum value is before you order it.

For more information about Bemco fluid conditioners (PCL Series) visit the Bemco website at www.bemcoinc.com