Distinctions of Measuring Force Versus Pressure

Force and pressure are two distinct but related physical quantities. While both involve stress on the surface, they are fundamentally different in their definitions, units, and measurement methods. Understanding this difference is crucial for selecting the proper sensor in various applications.

Force is a vector quantity that describes a push or a pull on an object. According to Newton’s Second Law, it’s the product of an object’s mass and its acceleration (F=ma). In simpler terms, force is the total mechanical load applied to a body. It’s measured in units like Newtons (N) or pounds-force (lbf).

Pressure is a scalar quantity that describes the force applied perpendicular to a surface per unit area (P=F/A). It’s a measure of how concentrated the force is over a specific area. It’s measured in units like Pascals (Pa), pounds per square inch (psi), or bars.

The Science of Measurement

Measuring Force with a Load Cell

A load cell converts a force into an electrical signal. The most common type, and the one Interface specializes in, uses strain gages. A strain gage is a sensor whose electrical resistance changes in response to mechanical strain (deformation). Here’s how it works:

1. A metal spring element inside the load cell is designed to deform predictably when a force is applied.
2. Strain gages are precisely bonded to this spring element.
3. As the applied force causes the element to deform, the strain gages stretch or compress.
4. This change in length and cross-sectional area of the strain gage wires alters their electrical resistance.
5. The strain gages are typically arranged in a Wheatstone bridge circuit. This configuration is highly sensitive to small resistance changes and provides a stable, temperature-compensated output voltage that is directly proportional to the applied force. The output signal is a direct measure of the total mechanical load on the load cell.

This method is ideal for measuring force because the load cell’s design focuses on capturing the total deformation caused by the push or pull, regardless of how it’s distributed over the sensor’s surface.

Measuring Pressure with a Pressure Sensor

While some specialized Interface load cells, like the IPCD, are designed for pressure measurement by converting force on a diaphragm to an electrical signal, most pressure sensors are not load cells. Interface’s Interface Pressure Compensated Downhole (IPCD) Load Cell provides accurate measurement of downhole tension conditions in both vertical and horizontal wells. The oil and gas solution replaces wet load cells with a maintenance-free product.

A typical pressure sensor uses a diaphragm that deforms under the influence of pressure from a fluid.

1. The fluid’s pressure acts on the surface of the diaphragm.
2. This pressure causes the diaphragm to deflect.
3. The amount of deflection is proportional to the pressure.
4. A sensing element, such as a strain gage, a capacitive sensor, or a piezoresistive element, measures this deflection and converts it into an electrical signal.

The key difference here is that the sensor’s design is optimized to sense the uniform force distribution of a fluid over a specific, known area (the diaphragm). This allows the sensor to calculate pressure (P=F/A) directly from the measured deflection.

Network to Help in Selecting the Right Solution

Many Interface customers have projects and labs that measure force and pressure in various testing applications. To ensure you have the right solution, our team of experienced global representatives and distributors works closely together with our customers to provide the suitable Interface force measurement for your specific project. If a pressure sensor is needed, our sales network can also assist in guiding you to the best product that matches your requirements.

Interface offers a wide range of instrumentation solutions that are used in conjunction with pressure and force measurement sensors. Like our 9870 High-Speed High Performance TEDS Ready Indicator, it will measure and display load, pressure, and torque measurements accurately and graphically.

Illustrating Measuring Force Versus Pressure

Measuring Clamping Force on a Vise

Imagine a manufacturing process where a robotic arm uses a vise to clamp a part for machining. The quality of the final product depends on a consistent clamping force, not a consistent pressure.

Challenge – If you were to use a pressure sensor, you’d place it between the vise jaws and the part. The sensor would measure the force per unit area. However, if the part shifts or the contact area changes slightly (due to a non-uniform surface or a small piece of debris), the pressure reading would change, even if the total clamping force applied by the vise remains the same. A lower pressure reading might mistakenly lead the system to increase the clamping force, potentially damaging the part.

Solution – A load cell is mounted directly on the vise’s jaw, in line with the clamping action. The load cell measures the total force applied, regardless of the contact area. If the part shifts or the contact area changes, the load cell’s output remains stable because the total force is what it’s sensing. This ensures the part is clamped with the exact, consistent force required for a high-quality outcome, preventing damage and maintaining process integrity.

Measuring the Force of an Actuator

In automated assembly, a pneumatic or hydraulic actuator is often used to press two components together. The critical measurement is the total force the actuator applies to ensure a proper, secure fit. Read: Actuators and Sensors Combine Forces in Test and Measurement.

Challenge – Using a pressure sensor to measure the fluid pressure inside the actuator’s cylinder might seem like a good idea, as it’s directly related to the force it produces. However, this is an indirect measurement. The actual force delivered can be affected by friction within the cylinder, seal degradation, or varying temperatures—all factors that a pressure sensor inside the cylinder can’t account for. The pressure reading might be high, but the actual force delivered to the parts might be lower than required.

Solution – By placing a load cell at the end of the actuator’s ram, you can get a direct and highly accurate measurement of the total force being applied to the components. This load cell’s output reflects the actual force delivered, taking into account all the system inefficiencies. This ensures every press is performed with the required force, guaranteeing a reliable and consistent assembly, and allowing for precise quality control.

 

Pneumatic Actuator Seal Pressure Use Case: A company has to ensure the pneumatic actuator’s lip seal holds under different pressure loads. Interface suggests conducting a fatigue test using their 1200 Standard Precision LowProfile™ Load Cell. The 1200 is installed externally of the pneumatic actuator, where different pressure loads are measured. The test is conducted until the pneumatic actuator is dismantled. Precise force results are captured using the 9840 Calibration Grade Multi-Channel Load Cell Indicator. Read the use case.

These examples clearly demonstrate that while pressure is a measure of force over an area, the total force is what truly matters in many real-world applications. A load cell is the optimal tool for these jobs because it’s engineered to measure that total force directly and reliably, independent of distribution.

TIP: For a specific review of your application, be sure to reach out to our Application Engineers.

Why Load Cells Are Best for Force Measurement

While it is possible to design a load cell to measure pressure (as with the IPCD), load cells are commonly used because they are specifically engineered and calibrated to measure total force accurately.

Attempting to measure force with a standard pressure sensor is problematic because its output depends on both the force and the area over which it’s applied. If the force distribution changes, the sensor’s reading will change even if the total force remains constant. This makes a pressure sensor an unreliable tool for measuring the total force applied by a solid object.

Load cells are designed to handle force applied at a specific point or along a defined axis, like our multi-axis load cells. The strain gages are strategically placed to measure the deformation from this total load. A key advantage of a force-measuring load cell is that its output is not dependent on the precise point of force application (within its design limits) or the size of the contact area. It measures the total magnitude of the push or pull. Most importantly, load cells are calibrated by applying known weights or forces, ensuring a direct and accurate relationship between the output signal and the total applied force.

In summary, a load cell with strain gages is the most reliable and accurate sensor for measuring force, as it’s specifically designed to capture the total mechanical load. In contrast, a pressure sensor is designed to measure force distributed over a known area.