How do hydraulic diagnostic tools measure pressure, flow, and temperature?

Hydraulic technology is the driving force of many different industrial applications, including large machines and complex manufacturing processes. Like any other powerful source of energy, hydraulic systems require prudent assessment of performance to sustain their optimal capacity, prevent unexpected failure of your hydraulic equipment, and achieve maximum efficiency. Hydraulic diagnostic equipment is the key to achieving all of the above.

Each device listed here will measure three very important factors: pressure, flow, and temperature. Knowing how hydraulic diagnostic instruments work will give you a better understanding of how these devices help ensure that your hydraulic systems operate efficiently and without stress.

The Pulse of the System: Measuring Pressure

Among all parameters in hydraulic systems, the most critical is hydraulic pressure. The hydraulic pressure determines how much force can be applied by the system and is an important indicator of the hydraulics’ overall condition.

How hydraulic pressure is measured:

• Pressure Gauges: The most common method of measuring hydraulic pressure is using a direct-reading device called a pressure gauge. Most of these gauges use either a Bourdon tube or a diaphragm to measure pressure.
• Bourdon Tube: The Bourdon tube uses a "C"-shaped or "helical" shaped tube that is connected to the hydraulic line. As the hydraulic pressure inside the tube increases, the tube begins to straighten. The small amount of elongation of the tube is translated into a larger movement of a needle on a dial, indicating the amount of pressure in the system.
• Diaphragm: A diaphragm allows hydraulic pressure to act upon the diaphragm, causing it to move away from the hydraulic circuit. This movement can be converted into an electrical signal using strain gauges or can be mechanically attached to a needle.
• Pressure Transducer: To accurately measure hydraulic pressure continuously and with the highest degree of precision possible, pressure transducers are often used. A pressure transducer converts hydraulic pressure into an electrical signal.
• The Strain Gauge Transducer is the most widely used kind of transducer. Strain gauges have been bonded to the diaphragm, which consists of two plates. When the diaphragm is deformed by the application of pressure, the resistance of the strain gauges increases/decreases based on how much the diaphragm has been deformed.
• Piezoresistive Transducers are similar to strain gauges in that they utilize semiconductor materials that will have their resistance altered by mechanical loads placed upon them.
• Capacitance Transducers measure the change in capacitance between two charged plates, which results as the pressure causes one of the plates to move.

The electrical output from these transducers can then be displayed on a digital readout and/or logged into a computer database, or fed into a data acquisition system for further analysis.

The Lifeblood's Journey: Measuring Flow

The flow of fluid through a hydraulic system indicates how much work is being done within it. Determining the amount of flow through the system will indicate if there are leaks or restrictions in the system.

There are three basic ways that fluid flow can be measured.

• Turbine Flow Meters: These types of meters are used frequently in hydraulic diagnostics. A small 'paddle wheel' type of turbine is placed in line with the fluid. When the fluid flows past, it causes the paddle wheel to rotate based on the speed of the fluid flow, and the rotor speed indicates the rate of flow. The rotation of the paddle wheel can be detected by either a magnetic pickup or an optical sensor, which converts it to an electrical pulse signal. The electrical pulse is then converted into a flow reading.
• Gear Flow Meters: Gear flow meters are designed using exactly machined gears that mesh together. As the fluid moves past, it pushes the gear, causing it to rotate. Each rotation of the gear displaces a known volume of fluid. By counting the number of times the gear rotates, a flow rate can be calculated. Gear flow meters are typically used for more viscous fluids and provide higher accuracy than turbine flow meters.
• Variable Area Flow Meters (Rotameters): Rotameters, while not common in portable hydraulic diagnostic equipment, are simple and effective at measuring fluid flow in an open tube. A rotameter operates by having a smoothly tapered tube with a float inside. As the fluid flows upward through the tube, the float is raised higher until the force of the flow on the float and the weight of the floating object are in equilibrium. The position of the float indicates the flow rate.

The System's Temperature: Measuring Temperature

Heat is the most important indicator of how your hydraulic systems will run efficiently and where trouble may occur in the system. Too much heat will deteriorate hydraulic fluid, hurt seals, and shorten the life of components.

How Heat is Measured:

• Thermocouples - These are the most commonly used temperature measurement devices in hydraulic systems due to their ruggedness and broad temperature measurement capability. Thermocouples consist of two dissimilar metals joined at a point. When the junction is heated, a small voltage is produced based on the Seebeck effect, and that voltage output correlates with temperature. The thermocouple output is then converted to an actual temperature reading.
• Resistance Temperature Detectors (RTD's) - These temperature measurement devices are based on the predictable increase/decrease in the electrical resistance of certain metals (platinum typically) as the temperature increases/decreases.
• Thermistors - Like RTD's, thermistors are resistances; however, they have a much greater range of resistance change due to temperature changes, but are not as accurate as RTD's. They may be more sensitive but tend to have narrower measuring ranges and less linearity than RTDs.
• Infrared Thermometers (NLT) - Although they are rarely used with a hydraulic flow or pressure diagnostic block, they are considered one of the best and fastest methods for checking surface temperatures of components, hoses, and reservoirs without direct contact.

Integrated Diagnostic Approach
Older hydraulic diagnostic tools traditionally measured surface temperature only with infrared thermometers, as well as the other traditional methods described above. Today, many hydraulic diagnostic tools integrate the capabilities of all the measuring devices described above into one small, portable package.

Most hydraulic diagnostic products come equipped with the following features:

• Test Points - these are dedicated ports on the device where a pressure transducer and flow meter can be connected to the hydraulic circuit.
• Data Logging - the ability of the device to log pressure and flow measurements over a period of time, which will provide the technician with the data necessary to analyze trends and detect intermittent faults; and
• Digital Display - a graphical display of all measurements instantaneously, as they are taken.
• Some hydraulic diagnostic products also provide software integration, which allows users (technicians) to connect their devices via Bluetooth or similar wireless technology to their computers and/or mobile devices for better analysis, reporting, and predictive maintenance capabilities.

When technicians accurately measure hydraulic pressure and flow as well as the temperature of hydraulic components, they will be able to take advantage of the following benefits:

• Identify faults - the ability to identify the source of hydraulic malfunction quickly and efficiently;
• Perform preventative maintenance - through early detection of potential problems, we can prevent costly failures of hydraulic systems; and
• Maximize efficiency - now that we have accurately measured hydraulic pressure and flow as well as the temperature of hydraulic components, we can utilize this information to make sure that components are operating within the specified parameters of maximum efficiency.

Finally, since technicians will have verified that maintenance has corrected the problem, hydraulic diagnostic tools are the ultimate safeguard of hydraulic health by converting internal fluid dynamic data into workable knowledge. They enable technicians to not just see hydraulic systems, but understand how they operate (and what forces are operating) in order to ensure reliable and efficient operation of hydraulic systems.