The information below pertains to the VBM Domobar Super Dual Boiler PID Version 3.
Disclaimer: Due to different environmental variables, results may differ as with any testing scenario.
1 Domobar Super Double Boiler technical details – 2011 version
1.1 Coffee boiler technical details
- Coffee boiler: 0.5 liters
- Coffee boiler element: 900 watts total
- Thermic insulation by silicon glass
- Working temperature for testing: 93° C (200° F)
- Safety Thermostat stops power at 110° C (230° F)
- The coffee boiler is situated vertically, and at the time of this writing, this is the smallest coffee boiler that we know of on a prosumer/commercial dual boiler espresso machine – as with heat exchangers, less water standing water means superior cup profiles. After many months of testing, Vibiemme found this configuration (boiler size and wattage) to provide the optimal temperature stability with both scenarios: steam boiler on or steam boiler off.
1.2 Steam boiler technical details
- Steam boiler: 1.9 liters
- The steam boiler is placed horizontally
- Steam boiler element: 1,200 watts total
- Pre-heating exchanger for coffee boiler*: 45 ml (approx. a double espresso)
- Thermic insulation by silicon glass
- Working Pressure: 1.2 bar
- Safety valve working max. pressure: 1.5 bar
- *Inside the horizontal steam boiler’s lower part where the water is located, there is a heat exchanger pipe which pre-heats water for the coffee boiler: water outlet temperature 97° C (208° F) for pressures higher than 1 bar. The pre-heating exchanger pipe is horizontal in the horizontal steam boiler to avoid any of the higher temperature steam transferred by the upper portion of the vertical steam boiler as found on other double boiler machines. In theory, the
heat exchange pipe portion in the steam portion of a vertical steam boiler will turn into steam, and this can cause an unwanted variation in the temperature of the water to the coffee boiler when the machine has been idling too long.
1.3 Electronic box and Gicar probe
- User controlled setting via front panel: 1°C or 1°F
- Probe sensitivity: 1/10° C or 1/10° F
- Stainless steel probe in coffee boiler
1.4 PID Settings for Testing (and adjustable by user)
- Temperature: 200° F
- P: 1.0
- I: 0.00
- D: 0.5
- Offset: 19° F
1.5 Fresh water supply configuration
- The machine has a dual water-supply system. The user can use the internal water reservoir or connect to an existing water line (water softener recommended). Both options on the same machine utilize a rotary pump. A mechanical lever, located under the machine, allows the end-user to choose their inbound water supply.
1.6 Waste-water drainage system
- The machine has a dual drainage system: the end-user can drain the waste water into the drip tray (and manually empty it out) or connect it directly to an existing drain system.
2 Thermic stability trial of the Domobar Super Double Boiler: tools, procedures and validation
- Important Definition: What is thermic stability? Thermic stability is the one of most important features on a PID double boiler espresso machine; it is the difference among the maximum temperatures recorded in a shot sequence. Differences between 2° C and 3° C have to be considered as acceptable, between 1° C and 2° C good, below 1° C excellent: these values create the thermic stability range, which depends on the manufacturing features of the machine.
- Thermic stability values can be detected through a measuring instrument for temperatures (SCACEII), placed at the group exit, following a world renowned testing pattern as outlined on the next section.
2.2 Procedures: the trial was made referring to a world class and standardized testing pattern
Test Point 1 2 3 – 4 5 – 6 7 – 8 9 – 14 Idle Interval
10:00 5:00 2:00 1:00 0:30 0:10
- The “Test point” corresponds to one shot of 50 ml double espresso brewed in 25 seconds.
- The “Idle Interval” is the time between one shot (end) and the beginning of the other.
- The test is made by recording the maximum temperatures of test points 1-14, respecting the idle intervals.
- The differences among the maximum temperatures recorded from test point 1 to test point 14 give as a result the values of the thermic stability range.
- Besides checking the Domobar Super Double Boiler thermic stability values, the test aims to confirm the empirical model of the E61 group isothermy in an idle state, published on home-barista isotherm.blogspot.com. This model states that when the idle Interval is longer than 2 minutes, the temperature difference between the red part (where the thermosiphonic circulation takes place in an idle state) and the blue part (the farthest and coldest part), is approx. 7° F. This is the basic principle of the pre-shot, suggested to all users of E61 espresso coffee machines.
3 DOMOBAR SUPER Double Boiler Version 3 – THERMIC STABILITY TEST RESULTS
- Confirmation of the empirical on E61 grouphead isothermy and Domobar Super Double Boiler stability test’s report
3.1 Confirmation of the empirical model on group E61 isothermy
- The thermic stability tests made on the Domobar Super Double Boiler confirm the isothermic model on group: the E61 group must be “taken to the right temperature” when the idle Interval is longer than 2 minutes. The temperature difference between Test Point 1-14 maximum temperatures and the thermic stability range (test point 6-14) is 7° F (without pre-shots). This data has to consider as a project feature of the E61 group, so the meaning of the thermic stability data has to take this feature into consideration.
3.2 Domobar Super Double Boiler stability test’s report
- Domobar Super Double Boiler stability range with steam boiler ON: 1° C (1,5°-1,8° F)
- Domobar Super Double Boiler stability range with steam boiler OFF:1° C (1,5°-1,8° F)
4 DOMOBAR SUPER Double Boiler THERMIC STABILITY TEST: Consumption and absorptions
- Maximum absorption with steam boiler ON: 1600 watts
- Maximum absorption with steam boiler OFF: 900 watts
5 HOW DOES THE NEW DOMOBAR SUPER Dual Boiler WORK?
The Domobar Super Dual Boiler can be used in different power configurations:
- Steam boiler ON and Coffee boiler ON
- Steam boiler OFF and Coffee boiler ON
Both configurations are available with:
- Water fills through existing direct water connection or plumbed in line (a water softener is always recommended)
- Water fills through water reservoir
- Water drains into the drip tray
- Water drain through existing drainage system
In case of both boilers ON (thanks to coffee boiler water pre-heating function), thermic stability range of 1° C can be kept even on shots taken every 10 seconds with less wattage utilized within the coffee boiler. VBM can finally state that the testing pattern was successfully passed with up to 900 watts absorption. In the case of thesteam boiler OFF, thermic stability range of 1° C with varying watts absorption of up to 900 watts.
The steam boiler performances are the same of the Domobar Super HX steam boiler – this is a vast improvement over the previous 2 versions. The double boiler version lower wattage is counterbalanced by the steam boiler insulation.