Level Measurement · Vibrating Fork Level Switches
A vibrating fork level switch for point level in tanks and pipes, rated to 2 MPa with a fork temperature of -30 to 150 °C, in relay, transistor or CAN output and 24 V DC or 220 V AC supply.
QDY30A-SX ·
A level transmitter answers how much is in the tank. A level switch answers whether the level has reached a point, and for a pump cut-out, a high-level alarm or a dry-run protection that is the only question being asked.
The vibrating fork is the switch type that has no moving part to jam and no gap for a coating to bridge, which is why it is the usual choice for slurries, viscous liquids and solids that would defeat a float.
The switch is a stainless fork on a threaded or flanged body, with the electronics in the housing above it. The material gives a 2 MPa pressure rating, a fork temperature of -30 to 150 °C with higher temperatures to order, and an ambient limit of -20 to 70 °C at the housing.
Output is a relay, a PNP and NPN transistor pair, or CAN, depending on the form; supply is 24 V DC or 220 V AC. The process connection is G1/2A as standard, with flanged versions, and a PTFE-coated fork is offered for aggressive media.
The fork vibrates at its resonant frequency in air. When the medium covers it, the added mass changes that frequency, and the electronics detect the shift and change the state of the output.
That mechanism explains most of the practical behaviour. There is no float to stick and no gap for solids to bridge, so a slurry that would jam a float switch passes the fork without effect. The switch does not need calibration to a level, because it is not measuring level; it is detecting whether the fork is covered. And it needs a minimum density to work, because a medium too light to shift the frequency will not be detected — which is the one figure the material does not give.
Medium and density. The first question is what the fork will sit in and whether it is heavy enough to shift the frequency. Water, slurries and most process liquids are well inside the range; light powders, foams and aerated media are the cases to check.
Pressure and temperature at the nozzle. The 2 MPa rating and the -30 to 150 °C fork range cover most tanks and lines. Above 150 °C the material offers a higher-temperature construction to order, and the housing stays within its own ambient limit because of the heat-break section between the fork and the electronics.
Output. A relay drives a contactor or an alarm directly. A PNP/NPN pair suits a PLC input. CAN suits a plant that already carries a bus. The choice is decided by what the signal is connected to, not by the switch.
Connection and insertion length. G1/2A is the standard thread; a flange is used where the nozzle is larger or the vessel is classified. The insertion length follows the nozzle depth, and the nozzle drawing is more use than the pipe size.
The switch is screwed or bolted into the nozzle, with the fork at the level to be detected. The material requires the cable entry to face downwards and the gland to be tightened, which is the ordinary precaution against water running down the cable into the housing.
The fork should sit clear of the nozzle wall and clear of the inlet stream, so that a falling stream or a build-up on the wall does not cover it. The material gives no minimum clearance and no insertion depth, and neither is invented here.
The first limitation is density. A medium too light to shift the fork frequency will not be detected, and the material does not state the threshold, so a sample should be checked before the order.
The second is build-up. A coating that bridges the two tines damps the vibration and can hold the switch in one state; the PTFE-coated form is the answer for media that tend to adhere, but it is a mitigation rather than a cure.
The third is that the switch gives no warning of drift. It is a contact, not a measurement, so a failed switch is usually found when the pump runs dry or the tank overflows. Testing the output at commissioning, and recording the state with the fork covered and uncovered, is worth more than any figure on the datasheet.
Every value below is taken from source documentation. Where a parameter is not stated in the available material it is marked as information required rather than estimated.
| Function | Point level: the switch changes state when the fork is covered, and changes back when it is uncoveredStandard |
|---|---|
| Pressure rating | 2 MPaStandard |
| Fork temperature | -30 to 150 °C, with a higher temperature to orderStandard |
| Ambient temperature | -20 to 70 °C at the housingStandard |
| Repeatability | 0.2 % is quoted on one listing and 2 % on another; the two are not the same quantity and neither is resolved hereInformation required |
| Medium density | The material refers to a minimum density but does not give the figureInformation required |
| Viscosity limit | Not stated in the material, although high-viscosity and solids-bearing media are named as dutiesInformation required |
| Switching delay | Not stated in the materialInformation required |
| Relay output | SPDT or DPDT, depending on the formStandard |
|---|---|
| Transistor output | One PNP and one NPN output on the form that carries themStandard |
| Serial output | CAN on the form that carries itStandard |
| Supply | 24 V DC or 220 V AC, depending on the formStandard |
| Fail-safe direction | Not stated in the materialInformation required |
| Process connection | G1/2A thread as standard, with flanged versions; the cable entry is M12 x 1 on the form that uses a connectorStandard |
|---|---|
| Fork and body material | Stainless steel, with a PTFE coating for aggressive mediaStandard |
| Insertion length | Not stated in the materialInformation required |
| Ingress protection | Not stated in the materialInformation required |
| Weight | Not stated in the materialInformation required |
| Hazardous area | An explosion-proof form is listed; the marking and certificate are not statedInformation required |
|---|---|
| Cable entry orientation | The material requires the entry to face downwards and the gland to be tightenedStandard |
| Model | Configuration | Key ranges | Availability |
|---|---|---|---|
| QDY30A-SX, relay SPDT | Single changeover contact, 24 V DC or 220 V AC supply | 2 MPa; -30 to 150 °C fork; G1/2A thread | The general form for a pump cut-out or an alarm |
| QDY30A-SX, relay DPDT | Two changeover contacts for two independent circuits | Same pressure and temperature | Used where the switch has to drive an alarm and a pump starter separately |
| QDY30A-SX, PNP and NPN | One PNP and one NPN transistor output for a PLC input | Same pressure and temperature | For a control system rather than a contactor |
| QDY30A-SX, CAN | Serial output on a bus segment | Same pressure and temperature | For a plant that already carries a CAN network |
| High-temperature form | Fork and housing arranged for a higher medium temperature | Above 150 °C to order | Quoted against the duty rather than held as a stock range |
Certifications are only published once a certificate copy can be produced, so this section is deliberately empty rather than populated with assumed approvals. Tell us which approvals your application requires and we will confirm what is available for this instrument.
The following are required for a complete specification but are not stated in the documentation supplied to us. They are listed rather than estimated:
The medium and its minimum density, because a fork that vibrates freely in water may not shift enough in a light powder or a foam. Then the pressure and temperature at the nozzle, the process connection and its thread or flange standard, the output the control system needs, and the supply available on site. The insertion length follows from the nozzle depth, and it is worth giving the nozzle drawing rather than the pipe size alone.
A transmitter reads the level continuously and gives a number over its whole range; a switch changes state at one point and gives a contact. A pump cut-out, a high-level alarm and a dry-run protection are switch duties, and a switch is the simpler and more robust choice for them. A transmitter is the right instrument when the level itself has to be known, not only whether it has passed a point.
The material names high-viscosity liquids, liquids with suspended solids and slurries as duties, which is the usual reason a fork is chosen over a float or a capacitive probe: there is no moving part to jam and no gap for a coating to bridge. The viscosity limit and the minimum density are not given, though, so a sample of the medium and its density should be confirmed before the order rather than after.
It separates the fork, which sits at the medium temperature, from the housing, which carries the electronics and is rated to 70 °C ambient. On a hot line the fork can be at 150 °C, and without that separation the housing would run above its own limit. It is the reason the switch can be used on a hot process without a cooling extension.
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