Knowing which style of pressure gauge is right for your fluid power system is critical in ensuring consistent, efficient operation.
Contributed by Carl Dyke, LunchBox Sessions
Pressure gauges are unsung heroes in many hydraulic systems. They are neither directing flow nor controlling pressure. If the pressure gauge is damaged or goes missing, the machine may continue to operate as though the pressure gauge was never even required.
Even though a good pressure gauge should have no effect on its hydraulic system, it should not be ignored. It’s a first line of diagnosis and detection, and invaluable in a well-executed preventative maintenance program. A pressure gauge is your glimpse inside of an otherwise opaque system. It provides quantitative information about how the system is operating, and whether that operation is within normal limits.
The best gauge
Like nearly everything else in hydraulics, asking “what’s the best pressure gauge” is almost guaranteed to get you the unsatisfying answer, “it depends.” And indeed, this is a case of one size never fitting all. Gauges that excel at repeatable and accurate high-pressure measurements almost universally will be inaccurate in low pressure systems. A bourdon tube type of gauge might be perfect for your needs, until you take into account the coarse and corrosive media you need it to measure. And a diaphragm gauge might seem like the right answer, until you learn that pressure spikes are not well tolerated, and temperature swings make the readings less accurate. To make a good choice for your system, you’ll need to know a bit about the different kinds of gauges available and understand their specific mechanics and limitations. Then you’ll be in a much better position to make a smart decision.
A typical pressure gauge
The typical analog pressure gauge features an indicator needle that rests at approximately a 7 o’clock position and sweeps around to a 5 o’clock position. Throughout the range of the sweep the needle points to the current pressure value on a printed scale with units as desired. This is all that is visible and evident from the outside.
Behind the dial and inside the case is where the clever action takes place. Like an analog wristwatch the typical pressure gauge features a movement. This precision mechanism of gears and springs amplifies the small total travel of the primary sensing element and transfers this motion to the large sweep of the gauge needle.
Typical sensing range: 600 mbar up to 6,000 bar (10 to 87,000 psi)
The primary sensing element for so many fluid power pressure gauges is a Bourdon tube. Named after Eugene Bourdon’s 19th century invention, a Bourdon tube is a ‘C’ shaped or helical shaped tube that tends to straighten as the fluid is pressurized. While this straightening may be minor and in fact very subtle if viewed by the eye, the motion can be linked to a geared movement to create the full sweep of the needle, around the dial. The Bourdon tube tends to be a simple ‘C’ shape and made of a copper alloy for pressures below 60 bar (870 psi) where a stronger stainless-steel helical tube with a few complete wraps is necessary for the higher pressure ranges. The Bourdon tube is typically crimped, soldered or brazed to the socket which is the entry point for the pressurized fluid and also the fitting where the gauge is physically connected to the pneumatic or hydraulic system. While the Bourdon tube is not the only type of sensing element for pressure gauges, it is economical and effective for many applications.
Typical sensing range: 0 to 40 bar (0 to 580 psi)
From the outside, a diaphragm pressure gauge looks the same as a bourdon tube gauge, with the typical circular housing and an indicator needle that sweeps along a dial. The difference is found in the sensing element.
Instead of a Bourdon tube deforming according to pressure, the membrane in a diaphragm gauge flexes against spring resistance. This deflection drives the indicator needle around the gauge face, via mechanical linkage.
The large surface area of the diaphragm lends itself well to low pressure measurements. Small pressure changes are picked up easily. However, the diaphragm is vulnerable to high pressure. Exposure to a pressure spike beyond its rating may rupture the membrane. You’ll know if this has happened — the indicator needle will sit at the bottom of the scale and never move again.
Diaphragm gauges are also adaptable to different media mixtures. The diaphragm can be coated with one of several materials to resist abrasion or corrosion from unusual media. And unlike a winding Bourdon tube, the diaphragm has a large surface area, which provides lots of clearance. Challenging media full of particulate is less likely to clog this gauge. You can even purchase flange-fit gauges, which locate the diaphragm right at the interface point, making it nearly impossible to clog.
Typical sensing range: 0.03 to 5 bar (0.5 to 75 psi).
A bellows-type gauge is typically used in low pressure applications, but it can be specially constructed to tolerate higher pressure instead.
Some bellows gauges measure through the expansion of the bellows. The measured liquid is channeled into the bellows element, and as pressure increases, it pushes the bellows to expand. Alternately, a bellows gauge might direct the measurement liquid into the cavern around the bellows element to compress it. In either arrangement, it is the movement of the bellows that slides the mechanical linkage driving the indicator needle. Like the diaphragm and bourdon tube gauges, the needle is mechanically linked to the sensing element.
ABS plastic is generally the standard housing material for all three of these gauge types. This plastic is light and inexpensive, but it won’t survive much rough treatment. Consider the installation conditions. Will this gauge be permanently mounted in a relatively protected location? Or will it be carried around by millwrights, or located where an object may fall on it? The housing can generally be upgraded to stainless steel, which will fare better in adversity than the ABS plastic.
Gauges might be referred to as “dry,” or they can be liquid-filled. The liquid fill option provides dampening against environmental vibration. It can also provide lubrication for the movement mechanism. Many liquid gauges are hermetically sealed, but some have a small valve at the top.
Glycerin and silicon are two popular liquid-fill options for hydraulic pressure gauges.
Glycerin is typically used for indoor installations. It offers good dampening within a temperature range of –4° and 140°F (-20° and 60°C).
Silicone maintains a low viscosity even at colder temperatures and is often preferred for colder climate applications. Look for good dampening between -40° to 140°F (-40° to 60°C).
Maintenance and care
There aren’t really any serviceable parts in a typical gauge, but there are some requirements for long life and reliable accuracy. Often a pressure gauge is installed in a system with an on/off valve in place at the entry fitting. This valve allows the gauge to rest on the pin at zero until a reading is needed. The gauge stays calibrated longer when left in this state. The viewer of the gauge opens the valve when a reading is needed. Later when the system is shut down and the needle rests at zero, the valve is closed once again.
Some liquid filled gauges used as portable diagnostic instruments may include a small on/off venting valve at the top of the case. This valve may need to be open to the atmosphere for an accurate reading, but it will allow the liquid to leak out if not closed once again before the valve is tilted. If the fluid leaks out, the life of the gauge may be shortened.
All three analog gauge types we’ve looked at — Bourdon tube, diaphragm and bellows — will demonstrate some inaccuracy with temperature changes, whether they are changes within the measured fluid, or the ambient temperature around the gauge. This is due to fluid expansion, as well as the properties of the metals and materials used in the gauges and sensing elements themselves. To limit inaccuracy, try to take readings at roughly the same system temperature and ambient temperature. When that’s not practical, at least note the temperatures so that they can be considered when evaluating the reading.
If your gauge is not going to be permanently mounted to one location, consider adding a protective boot to cushion against accidental impact. In our own classroom environment, we’ve found that unprotected bourdon tube gauges can lose calibration after only one or two drops. Needless to say, all of our gauges are now covered with protective boots.
Pressure spikes can ruin calibration, or even destroy the delicate sensing mechanism of bourdon tube, diaphragm, and bellows gauges. If your system suffers an overpressure event, make sure to examine the gauges for signs that they have lost calibration. If your gauge is the type with an on/off valve, this is another reason to leave the gauge shut off from the system unless you intend to take a reading. When the valve is off, it should isolate the gauge from pressure spikes.
If you are convinced that the hydraulic system is completely shut down and depressurized, but the indicator needle does not rest on the zero pin, the gauge has gone out of calibration. It may be able to be recalibrated if it has not been damaged, but it’s more likely that the gauge will need replacement.
Differential piston gauge
Typical sensing range: Up to 680 bar (10,000 psi)
Let’s finish up with a look at a slightly different style of gauge. A pressure gauge with two connection points is often required to determine the level of clogging in a full pressure filter element, or to measure the margin pressure in a load sense hydraulic system. Differential pressure gauges compare the pressure between two different fluid zones. All of the gauge styles we’ve seen so far — bourdon tube, diaphragm and bellows — can be adapted for differential measurements, but the piston gauge departs from these in a few significant ways.
Piston gauges can have a small amount of bypass flow from the high-pressure side to the low-pressure side. (If your application cannot tolerate even a minimal amount of mixing between the two ports, a different style of differential gauge might be your best choice, or you may select a piston-type gauge with a separating diaphragm to prevent bypass flow.) The pressurized fluid on both sides of the gauge simply pushes on either side of the piston, with a spring working to hold the piston centered.
Instead of a mechanical linkage, a magnet on the piston carries the needle indicator along as it shifts. There is no delicate mechanism and no spring to become over-flexed or deformed by an extreme pressure spike, so expect a long and reliable life from your differential piston gauge.
Accurate gauges take the guesswork out of your hydraulic system performance. Keep your readings accurate and reliable by
selecting the best gauge for your system pressure and operating environment.
Protect your gauges by installing them in safe locations, choosing the appropriate casing and fill options, and even outfitting them with a protective boot if needed.
Understand the behavior and limitations of each gauge type, so that you are not fooled by a gauge that has lost calibration.
Filed Under: Fluid Power Basics, Fluid Power World Magazine Articles, Sensors & Gauges