Summer 2017

With multiple gas flow calibrator technologies on the market, knowing which is best for calibrating your air sampling system is not easy. To truly understand the difference between these products and their technology, we must review the basics of gas flow and what it means for a technology to truly be “primary”. When a calibrator uses the International System of Units (or SI base units, described below) of time and length to derive gas flow measurements, it can be considered a “primary technology”. Secondary technologies use a proxy method to assume gas flow, using the correlation between air flow and drops in pressure, thermal fluctuations, or other methods.

Accurate gas flow measurements are important. Personal sampling pumps calibrated with trustworthy, primary gas flow calibrators allow workers to confidently determine whether they are working within permissible exposure limits, and ultimately ensure worker safety. Primary calibrators can only be considered primary when using the International System of Units (SI Base Units) to derive gas flow measurements. Other technologies, such as meters with sensors that measure a pressure drop across a fixed orifice, cannot be considered primary technologies since they do not use SI Base Units to derive gas flow rates.

Some things are easier to measure than others. Measuring distances or dimensions is easy with a ruler or measuring tape. Time can be captured with a stopwatch. Air or gas flow is not as easy to quantify since you are measuring two metrics at once. Flow itself is defined as a quantity of a material transported across a defined boundary. Thus, gas flow is just a volume of gas transported across a defined boundary.

The key to understanding gas flow measurement starts with the knowledge of the seven SI base units (International System of Units) that have been quantified and refined worldwide, each of which have exact values that are unlikely to change. These SI base units are also called the “primary units of measure” and include length, time, mass, electric current, thermodynamic temperature, amount of substance and luminous intensity.

Since there’s no SI base unit for the direct measurement of gas flow, the base units of time and length are used to calculate flow rates. Remember that a flow rate is defined as the time it takes a quantity of something to cross a defined boundary. In this case the quantity is a volume, such as cubic centimeters or cubic inches, over a unit of time, such as seconds or minutes. To illustrate the operating principal of DryCal gas flow calibrators, imagine you have a bucket with a known volume and a stop watch. Imagine starting the stop watch when water begins to flow into the bucket, and stopping the watch when the bucket is full. Combining the volume of the bucket with how long it took to fill the bucket gives you a volume over a measured time, or a volumetric flow rate.

DryCal technology uses a glass tube with a calibrated volume instead of a bucket, and a moving piston that triggers light sensors instead of a stopwatch. DryCal instruments measure gas flow in a surprisingly simple way: a valve closes, diverting gas into the measurement cell. The gas pushes the piston upward to the timing start point where measurement timing begins. The timing stops after the piston reaches a location of known volume. The gas is then released from the flow cell, allowing the piston to drop to the bottom. The system instantly and automatically resets in time for the next reading.

However, gases do not always behave in simple ways. Remember that gases are compressible, and that changing the pressure or temperature of a gas changes the volume of that gas. In fact, a pressure change is needed to make gas flow in the first place. Thus, as gas flows, it inherently changes volume. If we measure gas pressure and temperature during measurements we can calculate the volume of the gas at a defined pressure and temperature. This is known as the process of standardizing gas flow. Standardized gas flow is defined as the volume of gas transported per unit time across a boundary with the measured gas volume converted to the volume the gas will occupy at a defined pressure and temperature.

When considering gas flow calibrators, remember to look for an instrument that provides accurate, reliable, primary gas flow readings, and also one that that offers temperature and pressure readings if standardized flow measurements are needed. Finally, consider a name that you can trust, such as DryCal from Mesa Labs, with over a quarter century of experience in the flow calibration industry.

Author: Kera Tucker Associate Product Manager for MesaLabs, (303) 987-8000