Dishwasher temperature logging

[Halcyon]

I decided to put my EEVblog 121GW multimeter to good use and log the temperature inside my dishwasher (I just shoved the probe through the top seal of the door). I almost always use the "pots and pans" cycle, even for general washing but have always been curious to see what kind of temperatures this cycle reaches. The dishwasher is a Siemens SN46M583AU and on the front panel, it claims that the pots and pans cycle is 70°C. I sometimes use the "Vario Speed" option which approximately halves the wash time (presumably at the expense of increased energy and/or water usage), but I've noticed that items come out a lot hotter on this quicker wash than the normal wash (to the point they are too hot to touch straight after the wash cycle finishes).

Is this because the longer wash has a dwell time after the heating phase to let everything cool off or does it actually dry at a higher temperature?

So I ran the pots and pans cycle twice: Once with "Vario Speed" disabled (default) and again with it enabled. I set my 121GW to log temperature at 10 second intervals using the included Type K thermocouple.
Some points to also consider:

• The starting temperatures were slightly elevated as the unit hadn't returned to room temperature before starting each cycle.
• Both washes were conducted with actual items in the dishwasher to avoid wasting energy/water. I split the load so that roughly the same mass and types of items were in the same positions.
• Logging was terminated as soon as each cycle finished and the machine switched off.

With Vario Speed OFF, maximum temperature was 71.0°C
With Vario Speed ON, maximum temperature was 72.9°C

Basically no difference at all.

If anyone wants to make a chart, feel free. Raw data is attached (rename to.csv)

[tom66]

I have a Bosch dishwasher with "Vario Speed" (funny, I wonder if the Siemens product is just a rebadge given the same name of the feature?)  It advises that this can 'shorten' the cycle by up to 1 hour if it detects the dishes are only lightly soiled.

In order to do this, it uses a turbidity sensor, which you can see if you take the drain filter out - it's essentially an infrared source and a receiver, measuring the approximate opacity of the drain water.  One of the first things a modern dishwasher does is a cold pre-rinse, before the tablet drops.  This gets the majority of the surface dirt off the plates.  The turbidity sensor then assesses the level of dirt and this causes the controller to adjust the wash profile.

This is one reason that not only is pre-rinsing dishes before you put them in the dishwasher unnecessary, but it can also make the dishwasher less effective at the actual cleaning process.  Because the dishwasher makes a guess at how dirty the plates are and uses more or less heat to remove the remaining dirt.  If you've essentially almost cleaned the dishes before putting them in, it will mispredict and you will be left with dirtier dishes, than had you not rinsed at all.  I'd also question why you use a dishwasher in the first place because the idea is to make doing dishes less of a chore and if you're rinsing the dishes by hand it's not much more effort to wash them up too.

Since all modern dishwashers implement this - even on their most aggressive 70C setting - I can only guess that 'Vario Speed' is a marketing term, perhaps it allows it to shorten it by a bit more. But to get the EU energy rating, it pretty much needs this feature to work on the 'Eco mode' all the time.  Yes, there is a European standard set of dirty dishes, with a precise amount of contamination to apply to each dish before measuring the efficiency of the machine.

[Twoflower]

About the Siemens and Bosch feature set match: That's no coincidence: They're called nowadays BSH (Bosch - Siemens Haushaltsgeräte or Home Appliances).

[duckduck]

This is one reason that not only is pre-rinsing dishes before you put them in the dishwasher unnecessary, but it can also make the dishwasher less effective at the actual cleaning process.  Because the dishwasher makes a guess at how dirty the plates are and uses more or less heat to remove the remaining dirt.  If you've essentially almost cleaned the dishes before putting them in, it will mispredict and you will be left with dirtier dishes, than had you not rinsed at all.  I'd also question why you use a dishwasher in the first place because the idea is to make doing dishes less of a chore and if you're rinsing the dishes by hand it's not much more effort to wash them up too.

Also, some dishwashers expect that the dishes will have some grease on them to lubricate things somewhat. Pre-washing could potentially shorten the life of these units.