On some dial pressure gauges, you'll notice two units of measure. For example, you may have PSI units in a black scale while kPa units are in red. While this is useful, wouldn’t it be better if you could have even more units to refer to?
We aren’t suggesting that you muddy up a dial display with even more scales. Rather, we suggest that you take advantage of one of the many features offered by a digital pressure gauge.
Most digital pressure gauges allow you to select one of several units of measure – and change it at any time. For example, in addition to PSI and Pa, popular units of measure include Bar, Torr, inHg, and ftH2O. With so many different industries preferring one unit of measure over another, it would be nice to have a gauge that could speak the language for any given industry or discipline.
Let’s go ahead and take this opportunity to discuss what some of these units of measure actually mean.
Pounds per Square Inch (PSI)
This unit of measure is the most common pressure unit used in the United States. It is the measurement of pressure resulting from one pound of force being exerted on an area of one square inch. It is used as a unit of measure in a variety of industries including Oil & Gas, Automotive, and Aerospace.
Pascal is the official SI unit of pressure. This means that it is included in the world’s most widely used system of measurement, the “International System of Units,” which is the modern form of the metric system. One Pascal is equal to one Newton per square meter. The Pascal was named after the seventeenth-century philosopher and scientist Blaise Pascal. A pressure of 1 Pa is very small. 1 PSI is approximately 6900 Pa. Kilopascals, is therefore very popular. 100 kPa is roughly equal to atmospheric pressure at sea level.
One bar is about equal to the atmospheric pressure on Earth at sea level. Originally, British meteorologist William Napier Shaw introduced bar and millibar in 1909. However, in 1982, the International Union of Pure and Applied Chemistry defined bar as 100,000 Pa. Since then, the millibar has continued to be used in the USA for weather forecasting while other countries such as Canada have transitioned to kilopascals (kPa) and hectopascals (hPa).
So 1 mbar = 1 hPa and 1 kPa = 10 mbar. So you have different names for the same thing - one of the reasons why a digital pressure gauge is nice.
The Torr was named after Evangelista Torricelli, an Italian physicist who discovered the principle of the barometer in 1644. Originally, one Torr was supposed to equal one millimeter of mercury (mmHg). However, since then both units have undergone redefinitions and have resulted in a 0.000015% difference. One Torr is now defined as exactly 1/760 of a standard atmosphere or 101,325 Pascals. Today, the Torr is most often used in the medical industry to measure blood pressure (example: 120/80 Torr).
Inches of Mercury (inHg)
This is the first manometric unit we will discuss. Manometric units depend on an assumed density of a fluid and an assumed acceleration due to gravity. For example, inHg is equal to the pressure exerted by a 1 inch circular column of mercury of 1 inch in height at 32°F at the standard acceleration of gravity. The use of manometric units is often discouraged but continues in various applications. The inHg unit is used in the USA for weather reports, refrigeration and aviation.
Feet of Water (ftH2O)
Another manometric unit, ftH2O, is equal to the amount of pressure exerted by a 1 foot circular water column at a height of 1 foot. This unit of measure is most often used in the pump industry and is commonly referred to as head. For example, 80 feet of head is equal to 80 ftH2O. Scuba divers will often use a slightly different unit called FSW or feet of seawater, as seawater is a little denser than fresh water.
Now that you are familiar with some of the more popular units of measure, here is a unit conversion chart you can use as a reference.
Finally, if you are using an APG digital pressure gauge, and there is a unit of measure that is not included as a standard, you can enter a custom multiplier into the digital interface. This will certainly involve some conversion math, but you’ll only have to figure it out once. Once you have the multiplier figured it out, enter it into the gauge and label the new unit of measure. This is common with weight measurements
If you have any questions or would like to learn more about our digital pressure gauges, feel free to contact us.