【2026 Level Gauges】Selection of Output Methods for Level Switches

I. Relay output: The most \"tolerant\" solution for field applications. Relay output is the most widely used form among level switches in China at present, and it is also the output method that field engineers are most familiar with and most receptive to. Its core feature is that it drives the contacts to move through an internal detection circuit, thereby enabling control over the on/off state of external circuits, with good electrical isolation between the input and output. In engineering practice, the greatest advantage of relay outputs lies in their excellent load adaptability. Whether in AC or DC systems, and whether it is a control voltage of 24 V or power supplies of 220 V or 380 V, relay contacts can be directly used for switching. This allows the level switch to directly drive the contactor coil, intermediate relay, indicator lights, and audible and visual alarm devices, without the need for additional interface modules.

From a performance perspective, the response time of relay outputs is typically in the range of several dozen milliseconds, and this speed is more than sufficient for most level control and alarm applications. It should be noted, however, that relays are of electromechanical design, and their service life is inevitably affected by contact wear. The greater the load current and the higher the operating frequency, the faster the contact life decreases. Therefore, in applications where the operating frequency is not high and direct drive of high-power loads is required, such as high/low level alarms, safety interlocks, and start/stop control of pumps or valves, relay output remains the safest and most hassle-free choice.

II. Transistor output: Designed for high-frequency operation and long lifespan. The transistor output represents a typical contactless electronic output method, with conduction and shutdown being achieved through semiconductor devices, eliminating the need for mechanical contact points like those found in relays. Common transistor output types include NPN and PNP types, which are mainly used in DC control systems. Compared to relays, the most prominent feature of transistor outputs is their extremely fast response time. Its response time is typically in the range of milliseconds or even sub-millimeters, making it highly suitable for high-frequency operations or control applications with strict timing requirements. At the same time, since there is no mechanical wear, its theoretical service life is much longer than that of relays, giving it a clear advantage in systems that operate continuously for long periods.

However, in engineering applications, the transistor output also has clear boundary conditions. Firstly, it can only be used with DC loads, and it has strict requirements regarding voltage level and polarity ； Secondly, its output current capacity is limited, and it is generally only suitable for driving low-power loads. Therefore, the transistor output primarily serves a role in signal transmission rather than directly driving the actuator. In practical projects, the output of transistors is often connected directly to the digital input modules of PLCs, or used to drive devices such as solid-state relays and servo controllers. If it is necessary to control high-power loads in scenarios involving high-frequency operations, signal amplification is usually achieved by adding intermediate relays or solid-state relays.

III. Thyristor output: An electronic switching solution for AC systems. Thyristor output (also known as thyridium output) is a type of electronic switch designed specifically for AC circuits. Its conduction is triggered by a control signal, while its shutdown relies on the natural zero-crossing characteristic of the alternating current; therefore, it is not suitable for DC systems. 　　 In level switch applications, the performance of thyristor outputs lies between that of relays and transistors. On the one hand, it avoids the wear problems caused by mechanical contacts, resulting in a longer service life ； On the other hand, its response speed is significantly faster than that of relays, but generally slightly slower than transistor outputs. 　　 It should be noted that although thyristor outputs can withstand high AC voltages, their current-carrying capacity is usually limited; they are thus more commonly used for controlling signal-level or medium-power loads, such as directly driving contactor coils. In domestic level switch applications, this output method is relatively specialized and not widely used; it is more common in situations where there is a specific need for high-frequency AC switching.

IV. Two-wire output: a specific choice for intrinsically safe applications. The operation mode of the two-wire output differs from that of traditional digital outputs, as its principle is more similar to that of current-loop signals. The level switch performs both power supply and signal transmission on the same circuit, using changes in current status to indicate the level detection result. 　　 This type of output method is primarily used in hazardous areas where strict requirements for intrinsical safety apply, such as in the petroleum and chemical industries. In domestic level switch applications, the two-wire system is less commonly used, as it imposes high requirements on system design, power supply conditions, and load matching. However, in specific explosion-proof applications, its advantages remain irreplaceable.

V. NAMUR output: Standardized signals for hazardous and intrinsically safe applications. The NAMUR output is a standardized low-current signal that is commonly used in hazardous environments such as those in the chemical and pharmaceutical industries. NAMUR originates from the specifications for such sensors established by the German Industrial Association (NAMUR). Typically, NAMUR devices operate within an current range of 8–10 mA, using current changes to indicate high/low states, rather than directly driving the load. The main advantage of NAMUR outputs is that they meet intrinsically safe requirements. Due to its low and standardized signal current, it can be safely connected to intrinsically safe amplifiers or controllers, minimizing the risk of sparks or overheating in flammable and explosive environments. In practical applications, NAMUR-type level switches generally do not directly control the load, but rather provide reliable signals to control systems designed for hazardous areas.

NAMUR outputs are particularly suitable for direct integration with PLCs, safety controllers, or signal isolators in intrinsically safe circuits, making them an essential choice when complying with safety standards such as IECEx and ATEX. For applications that require safety and standardization, it serves as a valuable complement to relay, transistor, and thyristor outputs.

VI. Core principles for selecting engineering solutions Generally speaking, there is no universal \"optimal solution\" regarding the output method of level switches; the key lies in whether it suits the actual operating conditions and the control system in use. Relay outputs have strong versatility and high load capacity, making them suitable for traditional industrial environments ； Transistor outputs have advantages in high-frequency operation and long-life requirements ； Thyristors and two-wire outputs serve specific system conditions.

When selecting a solution for actual engineering projects, it is recommended to first clarify the following issues: Is the control system AC or DC? How much power is the load requiring drive? Is the operating frequency of the level signal high? Are there any explosion-proof or intrinsically safe requirements at the site?