The KOBOLD KDG/KDF-9 is a compact variable-area (rotameter) flow meter and switch designed for measuring very low liquid and gas flows. The KDF-9 covers water flows from 0.005 GPH (0.02 l/h) to 26.4 GPH (100 l/h), and the KDG-9 covers air flows from 2 Nl/h to 3000 Nl/h. A precision needle valve is fitted as standard, and an adjustable inductive proximity switch is optional on most ranges. Built from stainless steel and borosilicate glass, it installs in a length of 3.54 in (90 mm).
How the KDG/KDF-9 micro flow meter works
The KDG/KDF-9 operates on the suspended float principle. The media flows vertically from bottom to top through a conical borosilicate glass measuring tube and lifts a ball-shaped float until the buoyant force of the flow balances the weight of the float. The flow rate is read directly on the tube scale at the upper edge of the ball.
Scales are direct reading for water on the KDF-9 and for air on the KDG-9. The flow rate is set within the measuring range using the integral precision metering valve.
Measuring ranges for liquids (KDF-9) and gases (KDG-9)
The KDF-9 for liquids is available in water-calibrated ranges from 0.02 to 0.25 l/h up to 10 to 100 l/h. The KDG-9 for gases is available in air-calibrated ranges from 2 to 20 Nl/h up to 300 to 3000 Nl/h, referenced to 17.4 psia (1.2 bar absolute) and 68°F (20°C).
Pressure, temperature, and accuracy ratings
The KDG/KDF-9 is rated PN16, 232 psi (16 bar) at 68°F (20°C). Process and ambient temperature limits are -4°F to 212°F (-20°C to +100°C) without a switch and -4°F to 158°F (-20°C to +70°C) with a switch fitted. Protection class is IP65 (EN 60529). Accuracy is ±3% at qG = 50% per VDE/VDI 3513.
Wetted materials and process connections
Wetted parts are stainless steel 1.4404 (316L) for the fitting and valve stem, borosilicate glass for the measuring tube, stainless steel 1.4401 or glass for the float, PTFE for the float stop, PTFE with 25% carbon fibre for the valve seat, and FPM or optional FFKM for the gasket.
Process connections are ¼" NPT female or G¼ female; the G¼ connection is not available with the differential pressure controller. An angled (90°) or straight hose connector for 5/16 in (8 mm) hose and a wall-installation option are also available. Weight is 0.88 lb (0.4 kg), or 1.76 lb (0.8 kg) with the controller.
Optional flow switch: monostable or bistable
The KDG/KDF-9 accepts one adjustable inductive ring-type proximity switch on all ranges except 0.02 to 0.25, 0.08 to 0.7, and 10 to 100 l/h for water and 2 to 20, 4 to 40, 12 to 120, and 300 to 3000 Nl/h for air. The 6 to 63 l/h and 200 to 2000 Nl/h ranges have a limited switching range.
A monostable switch activates momentarily as the stainless steel float passes it on rising or falling flow and is typically used as a minimum or maximum contact. A bistable switch activates as the float rises past it and remains active until the float falls back past it, so it can serve as a limit contact at any position on the tube. Electrical values follow DIN 19234 (NAMUR), and a switch-equipped unit must be operated through a NAMUR isolation amplifier or relay.
Optional differential pressure controller for constant flow
Versions E, F, A, and B add a stainless steel differential pressure controller with an FPM or PTFE diaphragm that holds the set flow rate constant while pressure changes on one side of the meter. Versions E and F compensate a variable upstream pressure; versions A and B compensate a variable downstream pressure for gases.
The controller regulates inlet or outlet fluctuations only; pressure on the other side must remain stable. A minimum pressure difference of 5.1 psi (350 mbar) is required, and the diaphragm withstands a one-sided load of up to 101.5 psi (7 bar). Documented uses include constant dosing, level measurement in open and closed vessels, and nitrogen (N2) superposition of combustible media.
Installation and operating requirements
Install the KDG/KDF-9 vertically with flow from bottom to top, free from pipe strain, in a line the same size as the device connection. Open the shut-off valves upstream and downstream slowly, vent the pipes carefully when measuring liquids, increase pressure slowly when measuring gases, and avoid float impact caused by solenoid valves.
The meter is intended for liquids that are sufficiently free-flowing, free of solids, and do not bond or settle, and for gases with linear flow behavior and adequate inlet pressure. For aggressive media, confirm the durability of all wetted parts.
Common Questions about KDG / KDF-9 - Micro Flow Meter and Switch
The KDF-9 is the liquid version with a water-calibrated scale, and the KDG-9 is the gas version with an air-calibrated scale. Both share the same stainless steel fitting, glass measuring tube, needle valve, wetted materials, connection options, limit switch option, and differential pressure controller option. The model is chosen by the media and the required range.
The lowest standard water range is 0.02 to 0.25 l/h (about 0.005 to 0.066 GPH) on the KDF-9, and the lowest air range is 2 to 20 Nl/h on the KDG-9. Both of these ranges use a glass float and cannot be fitted with a limit switch. Special ranges outside the standard list are available on request.
Yes. The inductive proximity switch is a NAMUR sensor with electrical values to DIN 19234, so it must be operated through an external isolation switching amplifier or NAMUR relay. The switch is supplied with a 6.6 ft (2 m) cable, or with a junction box as an option.
Yes. The ring-type limit switch sits alongside the measuring tube and is held by two fixing screws at the back wall of the instrument. To change the switch point, unlock the screws, slide the contact up or down, and lock the screws again. On two ranges the contact cannot be moved all the way to maximum flow.
Without a limit switch, the KDG/KDF-9 is rated for process and ambient temperatures from -4°F to 212°F (-20°C to +100°C). With a limit switch fitted, both limits drop to -4°F to 158°F (-20°C to +70°C). Storage temperature is -4°F to 212°F (-20°C to +100°C), and the meter is intended for weather-protected or unheated locations.
Yes. The standard gasket material is FPM (ordering code V), and FFKM is available as an option (ordering code T). On the differential pressure controller versions, F and B are the FFKM equivalents of E and A. The fitting and valve stem stay stainless steel 1.4404, with PTFE at the float stop and valve seat.
Measured error is ±3% (qG = 50%) in accordance with VDE/VDI 3513, with a repeatability of ±1.0% of full scale. With the differential pressure controller, accuracy is ±3.5% of full scale (E/F) or ±5% of full scale (A/B) and repeatability is ±1.5% or ±2.5% of full scale, over 10 to 100% of span.
Choose E or F when the inlet pressure varies and the outlet pressure is constant; the valve sits at the inlet. Choose A or B for gases when the outlet pressure varies and the inlet pressure is constant; the valve sits at the outlet. For liquids, A/B is preferred. Both need at least 5.1 psi (350 mbar) differential.
No special maintenance is needed when the meter is used as intended, but it should be checked during routine plant maintenance for dirt, corrosion, mechanical wear, and damage to the glass cone, with a check recommended once a year. The glass cone can be removed for cleaning or replacement, and new gaskets must be used on reassembly.
Product Specifications
Available Measuring Ranges
0.02-0.25 l/h...10-100 l/h, 2-20 Nl/h...300-3000 Nl/h
Fittings
¼" NPT Thread, Hose Connection
Material
St. Steel, Glass, FPM Seals (FFKM Optional)
Max. Media Temperature
212 °F (100 °C) (158 °F w/ Switch)
Option
1 Inductive Proximity Switch (NAMUR Relay Required)
Media Categories
Liquid or Gas
Product Features
Integral Flow Control Valve
Direct Reading Scales for Water and Air
Switching for Low Flows of Liquids and Gases
Certificates/Approvals: ATEX
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