Convert PSI to lbs Using a Digital Pressure Gauge

The PG7 digital pressure gauge lets you use custom multipliers for a psi to lbs conversion

In a hydraulic system, it is often beneficial to not only display the pressure in the system but to also display the force being exerted by a hydraulic ram. This could be simply to display the actual force being exerted by a hydraulic ram in a familiar format. APG gauges are able to display the pressure readings in a few industry-standard units as well as many other units for force by using a simple conversion factor or multiplier.

Basic Principle of Unit Conversion

The principle behind the process is a simple conversion from one unit to another; in this case the APG digital pressure gauge can convert PSI to lbs. The trick to understanding how this conversion works is to break it down into its base units. PSI is a unit of measure for pressure, and pressure (PSI) is the amount of force (lbf) applied per unit of area (in2). The equations below illustrate this. To convert PSI to lbs, simply multiply the pressure by the area over which the force is applied.

How to Convert PSI to lbs Using an APG Digital Pressure Gauge:

Years ago, a motorcycle design and manufacturing company approached us about using one of our 5000 psiPG7 Digital Pressure Gauges to measure lbs of force from a hydraulic ram. Lbs of force, however, is not a unit of pressure displayed by our gauge; therefore, the custom unit converter function that is built into all of our gauges needed to be used.

To convert PSI to lbs, APG Sensors’ digital pressure gauges should be set up to use the "Custom Units of Measure" function, which will allow the sensor to read in pounds. Then, the multiplier for converting psi to lbs needs to be determined, using the area where the force is applied. The manufacturer of the hydraulic ram will sometimes provide the area. When not provided, the area will need to be calculated.

How to Calculate the Area of the Hydraulic Ram

In order to get the PSI to lbs multiplier for our motorcycle manufacturing friends, we needed to calculate the piston area of the 5 inch diameter hydraulic ram, it is important not to confuse the area of the piston/bore side and the area of the rod side. In this case, the force is calculated by determining the area of the bore/piston side as this is where the hydraulic fluid pressure is concentrated by the hydraulic pump. The formula for this is: Area = pi X radius squared. So we multiply π (3.14) by 6.25 inches (radius of 2.5 inches squared) to get 19.625 square inches as illustrated below.

How To Calculate The Force Applied By The Hydraulic Ram

To set the gauge to display in lbs, simply use the area (19.25) in square inches calculated in the previous steps as your multiplier, and skip to the programming steps below. It is important to note, however, that depending on the maximum pressure of your gauge, the measurement might be too long to fit on the 4-digit digital display of the

pressure sensors. You will need to do some quick math to ensure the maximum measurement will fit on the pressure sensors display. To do this, we multiple the maximum PSI of the digital pressure gauge by the previously calculated area. In our example with the motorcycle company, we would multiply the maximum output of the PG7 (5000 PSI) by the area of the hydraulic ram (19.625) to get 98,125 lbf, illustration below.

98,125 lbf is a hard number to show on a four-digit display, so you (or, in this instance, our motorcycle manufacturing friends) will want to scale that reading to a larger unit of measure like tons for example.

Convert lbs to tons:

To determine the scale factor for tons, we convert the full-scale psi (pounds of force per square inch) to full-scale pounds. 19.625 square inches at 5000 psi (19.625 X 5000) equals 98,125 lbs. To convert this to tons, we will divide 98,125by 2000 lbs, or one ton, to get 49 tons. So, when our hydraulic ram is pressing down to full-scale measurement on our 5000 psi gauge, we want it to display 49 tons.

How To Calculate The Multiplier For Tons:

To get the multiplier that you'll enter into the gauge to display tons, divide the tons calculated above (49) by the psi full scale (5000). This equals 0.0098, which you enter into the custom multiplier screen on the digital pressure gauge. You can now move on to the programming steps.

Programming Steps for Setting the APG Digital Pressure Gauge

Once you have determined the correct multiplier for your gauge and application you can set up the gauge to display the desired units.

Here are steps to program the multiplier into the gauge:

Step 1: Press the On/Off button once to enter the main setup menu.

Step 2: Press the Zero button to cycle through the options until UNITS is displayed.

Step 3: Press the Select button to access the Units setting options.

Step 4: Press Zero to cycle through the Units options until CUSTOM is displayed.

Step 5: Press Select to access the Custom Units setting. A 5-digit conversion factor will appear with the first digit flashing.

Step 6: Press the Zero button to change the value of the flashing digit (options: 0-9).

Step 7: Press the Select button to accept the flashing digit and advance to the next digit. Repeat steps 6 and 7 as necessary.

Step 8 After the last digit is accepted by pressing Select, use the Zero button to scroll through the custom units of measure; LBF, KN, LBS, KG, and NEWTON.

Step 9: Press Select to accept the desired custom unit and return to the main setup menu.

Step 10: Press Zero to scroll until you see EXIT, and press Select to return to the main display.

As always, let our Measurement Experts know if you have any questions about how to calculate your custom multiplier. Once you know the multiplier, APG’s digital pressure gauges make converting PSI to LBS easy!

As always, let our Measurement Experts know if you have any questions about how to calculate your custom multiplier. Once you know the multiplier, APG's digital pressure gauges make converting PSI to LBS easy!

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