When a user needs to interact with a transmitter directly (i.e. not through a digital communication protocol like MODBUS or HART) it's important that the transmitter's interface technology is appropriate for its installation environment. These are the buttons or switches used to navigate the transmitter's menus and to modify settings. The selection of improper technology can lead to difficulty in completing the desired interaction, damage to the flowmeter, or worse: injuries to personnel and other equipment.
The key first point in selecting the proper switching technology is that the technician must be able to actually reach the transmitter. At times transmitters are installed in less than desirable locations, or orientations, because of lack of space or installation convenience. At times even in profoundly inconvenient, or potentially unsafe to reach (i.e. on a pipe off the edge of the mezzanine, or many feet above the ground, or even facing a wall!) While these examples present some maintenance hurdles, usually these issues can be avoided by combining some foresight with a remotely mounted transmitter with the appropriate switching technology.
Mechanical Transmitter Switch
The most common type of switch is a simple mechanical button. Pressing on the button physically connects two contacts. These switches have been around almost as long as electricity and could not be easier to operate. They are a solid, reliable choice for almost every application. The transmitter's front cover keeps the switches clean and dry, while protecting them from accidental operation and or damage.
The largest drawback of the mechanical switch is that the operator needs direct physical access to operate them. This means that the front cover of the transmitter must be removed so that the operator can touch the buttons. Generally, this is not an issue unless the transmitter is in an environment where it's unsafe to remove the cover, for example places where the electronics are likely to be splashed, or filled with dust.
Optical Transmitter Switch
When the environment requires that the transmitter buttons be operable without having to open the case, a popular solution is to use optical switches. These switches use two infrared electrodes; one is for releasing, another is for receiving infrared ray. If the infrared ray at the button is screened by a finger or the related, the receiving electrode will receive the reflected infrared ray, and the switch is triggered. Because these switches can be operated through the transmitter's glass there is no need to open the front cover to operate them. This is a great solution in cases where a mechanical interface is inappropriate, however; since optical switches are operated by differential light intensity they can be tricked by things like moving shadows or inconsistent light. This susceptibility makes optical switches somewhat finicky to operate..
Capacitive Transmitter Switch
Walsn's R&D department is working hard to roll-out this fantastic technology. These switches use a sensor like those that are found behind a smartphone's touchscreen.
The sensor generates, and simultaneously measures, a delicate distributed capacitance. When a conductor (i.e. human finger) approaches, it changes the amount of distributed capacitance the region can hold. The circuit detects this change and activates the corresponding switch. Anybody that has used a smartphone shouldn't be surprised that this switch can be operated through glass. This gives the technician the ability to operate the switch without having to open the cover, and without having to contend with transient lighting conditions. The capacitive sensor combines the advantages of the mechanical and optical switches.
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