Koehler Existent Gum Evaporation Bath
The Koehler Existent Gum Bath conforms to ASTM D381 and related specifications and are available as three-unit and six-unit models.
Product Overview
Gum can form during fuel storage which can deposit on induction system surfaces, intake valves, stems and guides.
The test for gum content uses a 50mL sample which is evaporated in an aluminium block bath for a specified period under controlled conditions of temperature and flow of air (for aviation and motor gasolines) or steam (for aircraft turbine fuel). The Koehler baths conform to the specifications of ASTM D381, IP 131, IP 540 and several more. There are two models available, with either 3 or 6 sample beakers.
Existent gum represents the non‑volatile residue remaining after controlled evaporation of a measured fuel sample and is an indicator of deposit‑forming tendencies that can affect injectors, carburettors, and engine cleanliness.
ASTM D381 allows two evaporation techniques—air‑jet and steam‑jet—and the Koehler apparatus can be configured for either method depending on fuel type and specification requirements.
- Air‑Jet Method (ASTM D381 — Air Evaporation)
In the air‑jet method, a measured volume of fuel is evaporated by directing a regulated stream of clean, dry air onto the sample while it is held at a controlled elevated temperature. The evaporation removes volatile components, leaving behind any existing non‑volatile residue.
- Steam‑Jet Method (ASTM D381 — Steam Evaporation)
In the steam‑jet method, evaporation is accomplished using saturated steam directed onto the fuel sample. The steam both evaporates volatile fuel components and helps remove certain water‑soluble materials during the process.
Features & Benefits
246°C Max Temperature
±0.5°C Stability
Aluminium Heating Block
Up to 3000W Heating Power
Integrated Superheater
Easy Access Safety Top
Digital Air Flow Control
Microprocessor Temperature Control
Built-In Pressure Regulators
Key applications of Koehler Existent Gum Evaporation Bath
There are several models of the Koehler Existent Gum Evaporation baths available depending on the test requirements:
- K3378X models have 3-places
- K33900 Existent Gum Evaporation Bath has a Superheater and can hold 6 beakers.
The Koehler Existent Gum baths have the below specifications:
Maximum Temperature: 246°C
Temperature Control Stability: ±0.5°C
Bath Configuration: machined aluminium block with multiple cartridge heaters
Heater Range: K33900: 0-3000W; K33780 and K33781: 0-1500W
Superheater: Model K33900 only – The superheating chamber and condensate trap are constructed of stainless steel and have a solid state thermoregulator (0-287°C)
Safety top assembly allows for easy positioning and connection of all conical adapters to the bath
Digital flowmeter for accurate and precise air flow measurement
Microprocessor programmable high accuracy temperature control
Built-in pressure regulators and air flowmeters
K33900 6-Unit Existent Gum Evaporation Bath
About the Koehler Existent Gum Evaporation Bath
Existent gum in petroleum-based fuel is the non‑volatile residue present in the sample that remains after evaporation which can form harmful deposits in fuel systems.
Electrically heated baths are used for determining existent gum in aircraft turbine fuels by steam-jet evaporation and in motor and aviation gasolines by air-jet evaporation.
The Koehler Existent Gum Evaporation baths conform to ASTM D381 and have a fully insulated, aluminium block design which assures safe, efficient high temperature operation.
They also come equipped with air/steam pressure regulator with gauge and a digital flowmeter for adjusting air flow per ASTM specifications.
Operation
Pre‑cleaned and pre‑weighed sample cups are placed into designated openings or holders within the bath. These holders ensure proper positioning of the cups relative to the jet nozzles and maintain consistent exposure to heat.
A measured volume of the fuel sample is transferred into each cup according to the applicable ASTM D381 procedure.
Stainless steel jets are used to deliver air or steam flow to the test wells through removable brass conical adapters. Once the sample is in place and the bath is at temperature, evaporation is initiated using either:
- Air‑jet mode, in which a regulated flow of clean, dry air is directed onto the surface of the fuel
- Steam‑jet mode, in which saturated steam is applied to evaporate the fuel and wash water‑soluble components.
The jet is positioned to uniformly impinge on the sample without splashing or sample loss. Flow rate and exposure time are controlled by the test method and maintained consistently for each sample.
Evaporation continues until all volatile components of the fuel have been removed. The bath temperature ensures efficient evaporation while preventing thermal decomposition of the residue.
At the end of the evaporation period, the jet flow is stopped and the sample cups are carefully removed from the bath.
After removal, the sample cups are allowed to cool in a controlled environment to avoid moisture uptake or contamination. Once cooled, each cup is reweighed using an analytical balance.
The existent gum content is calculated as the mass of residue remaining in the cup, typically reported in milligrams per 100 ml of fuel.
Safety and Control Features
The Koehler Existent Gum Bath incorporates safety features such as:
- Over‑temperature protection to prevent overheating
- Stable electrical and thermal controls to ensure repeatable test conditions
- Materials and construction suitable for prolonged exposure to heat, fuel vapours, and steam
A fully insulated evaporation bath designed for ASTM D381 existent gum
testing in aviation and motor fuels.
Product FAQ's
The Koehler Existent Gum Evaporation baths can be used to test the tendency of liquid fuel samples such as motor gasoline, aviation gasoline (AVGAS) and aircraft turbine fuels (Jet A, Jet A-1) to form existent gum during evaporation.
Yes, the Koehler Existent Gum Evaporation Unit is specifically designed and manufactured to conform to ASTM D381. The units are also certified to related international standards such as IP 131, ISO 6246, and DIN 51784 (depending on the model and configuration).
The model range allows for parallel testing of either three or six samples simultaneously. Each sample cup is used to evaporate 50 ml of fuel sample.
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