The KOBOLD VKP is a viscosity compensated plastic variable area flow meter (rotameter) that measures both water and oil with a single spring-loaded float, mounts in any orientation, and requires no recalibration when the fluid changes. It is designed for inexpensive flow measurement of liquids in domestic services, cooling water, lubrication systems, solar heating, and OEM applications.
Flow ranges span 0.5...5 to 5...26 GPM on water scales and 0.5...4.5 to 3...20 GPM on oil scales, with accuracy of ±5% of full scale (4000 and 5000 series) and ±7% on the contamination-resistant 6000 series. The polysulfone body is rated to 230 PSI (15.9 bar) and 250°F (121°C), and every VKP is built at KOBOLD's Pittsburgh, Pennsylvania facility.
How Viscosity Compensation Works
The VKP's sharp-edged orifice float automatically compensates for viscosity changes from 1 to 400 cSt, so one meter reads accurately whether the line carries cooling water or hydraulic oil. Unlike standard rotameters that need different calibrations for different fluids, the VKP moves between water and oil service without adjustment.
This eliminates the calibration delays and inventory duplication associated with conventional variable area meters. Direct-reading scales calibrated specifically for water or for oil let operators read actual flow rates straight from the scale, with no conversion calculations, reducing errors and training requirements.
Universal Mounting in Any Orientation
A spring-loaded float design maintains calibration whether the VKP is installed vertically, horizontally, or at any angle. Traditional rotameters depend on gravity and demand vertical installation with upward flow; the VKP's spring returns the float regardless of position, opening applications where conventional variable area meters simply cannot function.
That includes mobile equipment and vehicles, where movement and orientation changes would render a gravity-dependent meter useless, and compact OEM machines where space constraints prevent vertical rotameter installation.
Made in USA, BABA-Compliant Manufacturing
Every VKP rotameter is manufactured in KOBOLD's Pittsburgh, Pennsylvania facility, ensuring full Build America, Buy America (BABA) Act compliance for federal infrastructure projects. Government contractors specifying flow meters for water treatment plants, municipal facilities, and infrastructure upgrades can select the VKP knowing it meets these procurement mandates.
American manufacturing also delivers faster delivery times, readily available replacement parts, and direct technical support from engineers who understand both rotameter technology and application requirements.
Construction and Ratings
The VKP pairs a transparent polysulfone housing and float with stainless steel internals, including a 301 SS spring and PH 15-7 Mo retaining ring, rated to 230 PSI (15.9 bar) and 250°F (121°C); with the optional reed switch installed, the maximum temperature is 185°F (85°C) according to specifications.
That pressure rating exceeds typical plastic flow meter limits by over 50%, allowing the VKP to replace more expensive metal meters in moderate-pressure applications, and the temperature rating significantly exceeds the 140°F limit of standard plastic rotameters, which is why solar heating systems specify it. The transparent housing gives quick visual verification of float position that electronic meters cannot match, and with just two primary components, the housing and the spring-loaded float, there is little to maintain or fail. Standard 1" MNPT connections install directly into existing lines, while optional union fittings allow quick removal for inspection without disturbing piping.
Optional Reed Switch for Automated Monitoring
An optional bi-stable reed contact turns the visual meter into a flow monitor, triggering alarms when flow drops below critical thresholds in lubrication systems or exceeds limits in cooling circuits. The contact is rated to maximums of 40 W / 40 VA, 230 VAC/VDC, and 2 A according to specifications.
The switch operates contactlessly through a magnet in the float, hermetically separated from the flowing medium, and wires through a DIN 43 650 C connector with cable gland. On VKP-4... and VKP-5... models, an adjustable contact option (suffix R) places the switching point anywhere along the scale.
Model Options for Clean and Contaminated Liquids
The VKP series covers three model families: water scale, oil scale, and a contaminated liquids series built around an externally grooved float. An inverted flow direction option (suffix D) accommodates top-to-bottom flow, and union fitting suffixes add brass or stainless steel connections in 1/2" and 3/4" NPT sizes:
- VKP-4020 to VKP-4100: water scale, 0.5...5 to 5...26 GPM
- VKP-5018 to VKP-5075: oil scale, 0.5...4.5 to 3...20 GPM
- VKP-6032 to VKP-6100: contaminated liquids, 2.0...8.0 to 5.0...26 GPM (water scale)
The 6000 series float carries external grooves that resist particle jamming with solids up to 400 micrometers, maintaining reliable operation in real-world conditions with filtered fluids that would clog sensitive rotameter designs.
Common Questions about VKP - Plastic Flow Meter and Switch
Inside the cylindrical housing, flowing liquid raises a float with an orifice against the strength of a spring; each float position corresponds to a flow value read at the float's upper edge. Because the spring, not gravity, provides the restoring force, readings hold steady across fluid viscosities from 1 to 400 cSt.
Yes. Dual direct-reading scales are calibrated for each medium, and the compensating float keeps both accurate with no recalibration. A system can run cooling water in the morning and hydraulic oil in the afternoon on one instrument, which reduces how many meter types a facility needs to stock.
No. Mounting is universal: vertical, horizontal, or any angle in between, with no readjustment for installation position. The spring-loaded measuring principle is independent of orientation, which suits mobile equipment, vehicles, and tight OEM installations where a gravity-dependent rotameter could not be used.
Water scale models run from 0.5...5 GPM up to 5...26 GPM, and oil scale models from 0.5...4.5 GPM up to 3...20 GPM. The contaminated liquids series spans 2.0...8.0 GPM through 5.0...26 GPM on a water scale, per the datasheet's model table.
Accuracy is ±5% of full scale for the 4000 and 5000 series and ±7% of full scale for the 6000 series, with ±1% full scale repeatability. Reading the value at the float's upper edge, on a scale matched to the medium, helps keep operator readings consistent.
Maximum pressure is 230 PSI, and the temperature limit is 250°F (121°C) for the meter alone or 185°F (85°C) with a switch fitted. The measuring range itself can be overrun without harm with non-pulsating flow, but the rated operating pressure must never be exceeded.
Choose the 6000 series, whose float carries grooves on its exterior to protect it against contamination from deposited particles. This substantially reduces the danger of the float clamping and permits measurement of polluted liquids with particle sizes up to 400 µm; larger impurities can cause blockage and measurement errors.
Yes. Production takes place at KOBOLD's facility in Pittsburgh, Pennsylvania, and BABA (Build America, Buy America) compliant models are available for federal procurement. Domestic manufacturing also shortens delivery and keeps replacement parts, such as seals and reed contacts, readily available.
The standard fitting is a 1" MNPT threaded connection. Union fitting options include 1/2" NPT male or female and 3/4" NPT male, in brass (suffixes N115, FN115, N120) or stainless steel (N215, FN215, N220), letting the meter drop out of the line for cleaning or service.