# Glossary

A

### Absolute Pressure

The total of the indicated gage pressure plus the atmospheric pressure. Abbreviated “psia” for pounds per square inch absolute.

### Accuracy

Quantity defining the limit that errors will not exceed. When applied to flow meters, accuracy is specified in either % of full scale or % of rate.

### Atmospheric Pressure

The pressure exerted upon the earth’s surface by the air because of the gravitational attraction of the earth. Standard atmosphere pressure at sea level is 14.7 pounds per square inch (psi). Measured with a barometer.

B

### Barometer

An instrument for measuring atmospheric pressure.

C

### Coriolis Flow Meter

The Coriolis flow meter uses the Coriolis Effect to measure the mass flow of a fluid. The fluid travels through single or dual curved tubes. A vibration is applied to the tube(s). The Coriolis force acts on the fluid particles perpendicular to the vibration and the direction of the flow. While the tube is vibrating upward, the fluid flow in forces down on the tube. As the fluid flows out of the tube, it forces upward. This creates torque, twisting the tube. The inverse process occurs when the tube is vibrating downward. The amount of twist in the tube is directly related to mass flow of the fluid through the tube.

D

### Differential Pressure

The difference between two pressures. Differential Pressure Flow Meters use differential pressure to compute volumetric flow rates.

### Differential Pressure Flow Meter

Differential Pressure or Laminar flow meters use the pressure drop created within a laminar flow element to measure the mass flow rate of a fluid. A laminar flow element takes turbulent flow and separates it into thin channels. By reducing the diameter of the flow channel and affecting velocity, the flow becomes laminar through the channels. The decrease in pressure, or pressure drop, across the channel is measured using a differential pressure sensor. The Poiseuille Equation can then be used to relate the pressure drop to the volumetric flow rate. The volumetric rate can also be converted to a mass flow rate using density correction at a given temperature and pressure.

E

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G

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L

### Laminar Flow Meter

see Differential Pressure Flow Meter

M

### Mass Flow

Also called normal flow or standard flow. The mass flow rate can be thought of as what the volume flow rte would be if the gas flowing through the line were at standard conditions. Actual line pressure and temperature affect the density of the gas, which contracts (above atmospheric pressure and / or low temperature) or expands (under vacuum and / or high temperature), and thus affects the measured volume flow rate. This means that the exact same number of molecules of gas flow can be measured as radically different volume flows when the temperature or pressure is fluctuating. Some mass flow meters have an absolute pressure sensor, temperature sensor or other technique to determine and compensate for variable gas density on the fly. This means that a change in mass flow reading is known to mean an actual change in the number of gas molecules as opposed to a simple change in the gas density. In addition, when the volumetric flow rate is corrected to standard conditions, it is a simple matter to multiply the mass flow rate by the density of the gas at standard conditions (commonly published) to determine actual mass flow rate (e.g. grams / minute)

### Media

In reference to flow products or pressure products, media usually refers to the process gas or liquid to be used with the device.

### MFC

Commonly used acronym for Mass Flow Controller.

N

### NIST Traceability

All Alicat measurement and control instruments are furnished, at no extra cost, with a certificate of calibration. This certificate indicates the type of device, information about the customer who purchased the instrument and specific test data indicating the instrument’s performance in comparison to a known NIST traceable standard.

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P

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R

### Repeatability

Is the closeness of agreement between consecutive measurements of the same flow within a particular time frame. This can be specified as % of full scale or % of rate.

### Resolution

Smallest incremental change in a parameter that can be indicated on the display (Note: this is not the same as accuracy).

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T

### Thermal Flow Meter

As the name implies, thermal flow meters use heat to measure the flow rate of a fluid. Thermal flow meters traditionally work in one of two ways. The first type measures the current required to maintain a fixed temperature across a heated element. As the fluid flows, particles contact the element and dissipate or carry away heat. As the flow rate increases, more current is required to keep the element at a fixed temperature. The current requirement is proportional to the mass flow rate. The second thermal method involves measuring the temperature at two points on an element or “hot wire”. As the fluid flows over the element it dissipates heat. The upstream side of the element will be hotter than the downstream side. The change in temperature is related to the fluid’s mass flow.

### Turndown

Turndown conveys the same information as rangeability but in a slightly different way. Turndown is the ratio of maximum flow to minimum flow. For example, a 100LPM flow meter, with a 100:1 (100 to 1) turndown, the maximum flow rate would be 100LPM and the minimum flow rate where you could reasonably use the unit would be 1LPM. All Alicat flow meters and controllers will read all the way down to zero flow.

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V

### Volume Flow

Indicates the actual volume of a gas. Since gases are compressible, the actual mass of the gas will be constant with temperature & pressure changes, but the volume will vary.

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