All About Manometers
Posted by Dr. David J. Silver, B.S., M.S. Ph.D., CIH on Jun 24th 2019
In the industrial hygiene and safety field, we have many tools available to us, none so versatile as the manometer. Manometers are used to measure pressure. There are three types of pressure measurement:
1.Positive pressure or gauge pressures are those greater than atmospheric
2.Negative pressures or vacuums are pressures less than atmospheric
3.Differential pressure is the difference between two pressures
Manometer applications include determining velocity and static pressures, leakage, fan and blower efficiency, filter resistance, and gas pressures. Ideal for wherever a portable, direct reading manometer is needed.
-U Tube Manometers
The simplest type of manometer is U-shaped and has a visibly colored liquid. When both sides of the open tubes are resting, no pressure applied, the heights of liquid are equal on both sides. Apply a negative or positive pressure to a side, and the other side will increase or decrease in height. The displacement will occur based on the gas type or air, pressure applied, and the density of the liquid which may be water, alcohol or mercury. The displacement may be in inches of water, millimeters of mercury, pounds per square inch (psi), or other.
-Well Type Manometer
A well-type manometer is similar to a U-tube manometer, however, one side of the manometer is increased many times in area to that of the other, and the volume of fluid displaced will represent very little change of height in the smaller area side (tube). Measurements are taken on one scale instead of reading two as in a U-tube manometer.
Inclined manometers used in conjunction with a pitot tube for air velocity measurements require low differential pressure readings and very small divisions. For this reason, the manometer is arranged at an incline so that the scale is stretched out compared to a vertical height of a U-tube manometer. For example, it would be difficult to read 0.01 inch divisions in a scale of two inches if vertical, however, when inclined, the divisions can be marked and the observations are much easier. Often, pitot tube traverses are made on duct work to assure the correct flow and velocity for general and local exhaust ventilation systems.
The simplicity of liquid and glass tube manometers provide a level of confidence based on knowing that the fixed volume of the tubes and the liquid density will not change. The drawbacks with liquid manometers are the potentially messy liquids, subjective readings, and leveling the manometer. The advantages of using a digital manometer are quick visual measurements and ease of use. As long as they are properly calibrated, zeroed and are within stated tolerances, you can rely on digital manometers.
-Dwyer Manometer Series 477B
The Series 477B Handheld Digital Manometers are versatile, hand-held, battery operated manometers available in several ranges from 0-20 in w.c. up to 100 psi. All models measure positive, negative or differential pressures with ±0.10% of full scale accuracy. You can select from up to seven common English and metric pressure units so conversions are not necessary. A memory function allows storage of up to 40 readings for later recall and a backlight provides auxiliary lighting for hard-to-see locations. Also standard are a hold feature plus both visual and audible overpressure alarms. Applications include lab calibration of other pressure instruments and air velocity/air flow measurements in commercial buildings.
A Magnehelic gauge is a differential pressure gauge for whether the pressure is positive, negative, or a pressure difference. The Magnehelic gauge is different from other pressure gauges in that they incorporate the magnehelic principle developed by Dwyer. Pressure on a thin diaphragm moves an indicator needle using magnetic linkage. The advantage over other pressure gauges is the elimination of problems like excessive wear, backlash, and toxicity. The magnehelic gauge can measure pressure difference at an accuracy of just plus or minus 2% at full scale.
Durable and Shock Resistant
Before the magnehelic gauge came along, gauges were notorious for having a high degree of error in measuring the amount of pressure. They are all highly fragile instruments that need a very controlled environment to function properly. In comparison, a magnehelic gauge is highly durable, shock resistant, and is not affected by vibrations and fluctuations in pressure and temperature. In these environments, the magnehelic gauge can still give pressure readings that are accurate within 2%.
Dwyer magnehelic gauges are typically used for HVAC applications and come in a variety of pressure ranges and configurations. The gauge works effectively in very cold and very hot weather; the gauge can be installed in any position and does not need to be level. Calibration is easy by disconnecting hoses and zeroing in ambient weather.
Count on EGas Depot for our assortment of manometers by Dwyer Instruments. We also sell calibration gases, gas meters, gas meters with PIDs, sound level meters, noise dosimeters, air sampling equipment, air sampling media, heat stress monitors, and dust monitors. We offer Gas Clip, MPower, Casella, Dwyer, Calibration gas, Elk River, and Allegro products. A full line from Casella and Dwyer Industries are available at EGas Depot www.egasdepot.com or call toll free at 833-264-0730.