Condition monitoring symbols are the lesser-known and lesser-used of the hydraulic library. It’s not that the components or the symbols representing them do not serve a useful purpose. It’s that the order of operations for designing and drawing a hydraulic system are actuators, pump(s), control valves and then conditioning components.
Monitoring symbols represent the nice to have components that may be optional in a hydraulic control package quote. As such, although a pressure gauge should be considered mandatory, perhaps they are omitted from the schematic under the assumption that the technician would install it wherever the plumbing allowed.
Of course, there is the other extreme where a customer adheres to a ten-page specification document requiring every monitoring component under the sun. These are my favorite customers, not just for the love of system design but also for the larger figure in their purchase order’s bottom right box.
If you recall from Hydraulic Symbology 101, many of the monitoring symbols start with the small circle. In Figure 1, you’ll see three such small circles, each different in their own way. The first symbol is the generic representation for any type of indicator. The X through the middle appears to bid for your attention by saying, “look over here, I’m important.” The circle is perched atop a solid line representing observed fluid. Depending on how you draw the circle, this line may stem from any of the circle’s 360 degrees.
The indicator may represent one of many hydraulic conditions, although aside from the pop-up differential pressure indicators used on filters, its use is lazy, in my opinion. It may represent pressure, flow, or fluid level, but each has specific symbols more useful on a drawing, especially during diagnostics or troubleshooting.
The more concise symbol for a pressure gauge resides next to the indicator, clearly showing an arrow to represent the needle and its diagonal orientation common to all variable symbols. The preference is for the symbol to point to the 1:30 position on the “clock,” but flipping and turning in one’s CAD software sometimes result in various orientations.
The symbol for a flow meter is easy to remember; it looks like a baseball. The opposed arcs inside the circle show a path of reduced flow, such as a flow restriction, but that restriction exists only to measure flow. The standalone symbol may not show the black lines sprouting from either side of the circle, but I included them here to show the flow path.
Monitoring symbols get more advanced as the component they represent does. The level/temperature gauge is as standard as are hydraulic reservoirs. The longer rectangle filled with a thermometer shown in Figure 2 is quite common, depicting the level/temp gauge seen most often. However, attached below is an electrical symbol for a switch. Three nodes represent the connections possible with the switch, with the common wire to the left and the two “switched” wires to the right.
Understanding this switch configuration separates hydraulics with electrics by differing terminology. Normally closed in fluid power means blocked fluid in the neutral position. Conversely, normally closed in electrical terms means a shut switch where electrons flow. A normally open electrical contact means no electron flow in its neutral state.
In this case, the switch symbol measures the fluid level and is shown currently as both normally open and normally closed. Many switches offer this flexibility using the common input to the left and then two contacts on the output side. In the state drawn here, wires could be connected to both of the right-side contacts; the upper node is normally open while the bottom node is normally closed. It’s typically wired just one way, often to the preference of the electrical designer. If wired normally open, the upper contact will close and signal a PLC or other device that the oil level is low.
The two square symbols in the middle of Figure 2 are general electrical symbols not necessarily specific to fluid power. The flow switch symbol looks the most part like a scene you find across the fairway at your local golf course, what with the flag perched on a sloped green down to a hole on the right. The flag represents flow, and it resides upon the open contact of a switch. This symbol is drawn normally open, meaning there is no electron flow that occurs in neutral. Flow acting upon the device’s mechanical portion pushes closed the contact, allowing the output to signal a PLC, warning light, or other downstream function.
The temperature switch symbol looks much like the flow switch save the zig-zag of the thermostat symbol atop the switch. This example is nearly identical to the electrical symbol of the same function. Temperature switches control the operation of temperature-related functions, such as to relay the start of a heat exchanger.
The last few symbols employ electrical symbols, which may beg the question of why these are monitoring symbols. Anywhere and anytime you measure a fluid property, whether to provide visual, mechanical, electrical or even hydraulic feedback, one can consider it a monitoring function.
The last symbol is a compound component able to perform many vital functions. Inside the top circle is a thermometer, keying in the temperature sensing nature of this device. The circle below shows a bobber floating atop a liquid, also signalling the device measures fluid level. The line dropping down shows two more bobbers, one at the bottom and another near the top. Finally, two squares pop out the right side, one with the variable arrow and the other the symbol for a transducer (as described in Hydraulic Symbology 301).
This digital level and temperature transducer perform many useful functions. Used in a hydraulic reservoir, the sensor measures the fluid level at two discreet points, one at the bottom to warn of critically low oil level, and the other at the upper limit to confirm fluid is full. The second function is to provide a temperature signal to the PLC to control a heat exchanger’s operation, which may either heat and cool.
This symbol is out of the catalog of a manufacturer that offers this product. The device also has a digital display showing temperature and to program the temperature switch functions. Atop the device is an M12 connector, making the interface between the device and a PLC quick and easy.
Filed Under: Sensors & Gauges