Charlie's Weblog

The Rancilio Silvia is fantastic. But what about this issue of temperature (in)stability? How important is it to temperature surf and how do you do it? Is a PID controller the answer? A few weeks ago, I pidded a brand new Silvia. Now I’ve gotten around to conducting some temperature tests and this is what happened.

These are the questions I was hoping to answer for myself. Hopefully there are others that will find the results helpful:

1. How much “swing” is there in the boiler temperature?
2. How does the temperature swing affect espresso extractions?
3. How effective is temperature surfing in overcoming the temperature swing?
4. How effective is a PID controller in overcoming the temperature swing?
5. Any other benefits of fitting a PID to a Silvia?

Some notes on the experiments:

• I only conducted these experiments once and all of them on the same near-new Silvia. As far as I can tell, it’s a typical Silvia (and I have used a few over the last year or so), and none of the results surprised me, but I am unable to draw any conclusions about the variance in the Silvia “population”. So I can’t say how different another Silvia is likely to behave. If anyone would like to add to the sample size, let me know.
• The PID is a Watlow SD3C with steam control from MLG. Jetblack Espresso “donated” the Rancilio Silvia . It is now back on the demo bench 😦
• For experiments with PID “disabled” I reinstated the Silvia’s temperature control via it’s brew and steam thermostats and unhooked the PID’s control of the heating element. The PID was still indicating where it would energise the element via the little light on the PID display, however the message wasn’t going anywhere. The PID is basically a fancy boiler temperature guage
• The thermocouple is mounted at the top of the boiler. If this temperature is different from the steady state group head temperature, there will be some temperature “smoothing”. This is where the Silvia’s liberal use of brass helps. However, I did not attempt to quantify this effect. One day I will get my hands on a Scace device and do the next series of experiments. For now, it’s a matter of measuring actual top of boiler temp and taste testing the resultant espresso/latte’s.

1. How Much Temperature Swing is there in a Rancilio Silvia?

On my Silvia, I measured a 32 deg C (58 deg F) swing in the boiler temperature:

Min: 85 deg C (185 deg F)

Max: 117 deg C (243 deg F)

Procedure: I flushed the machine until the boiler element light came on and waited until the temperature “topped out”. Here is the video:

But what’s the effect of such a swing?

2. The effect of temperature swing in espresso extraction

This test was intended to show the a worst case scenario – pulling a shot at the bottom end of the boiler temperature cycle and then steaming milk after the light goes out. The results are pretty subjective, basically it came down to watching and then tasting the shot. The shot was predictabley pale and sour and the resulting latte was weak. Here it is:

3. Temperature Surfing the Rancilio Silvia

I use a reasonably consistent method of temperature surfing when I use the Silvia. I hit the brew button until the boiler element light comes on and then prepare the portafilter – grind, dose, tamp – by which time the light has gone off, then I insert the portafilter and hit the brew button. My reasoning is that this is repeatable and anything that increases consistency is a good thing when we’re trying to control all the variables that go into the espresso making process. Here is that process, but with the addition of the PID showing us the temperature as the espresso extraction proceeds. Remember the PID is NOT controlling the boiler element.

Using this method of temperature surfing, the boiler is at 116C (241F) when I hit the brew button which is arguably too high. The temperature drops to 106C (223F) by the end of the shot so the intra shot temperature swing is 10C (18F).

The resulting shot had a crema with some very dark tones and lighter tones. The shot was a little burnt/bitter.

(i) (Advanced) Temperature Surfing

The good news is we don’t have to leave it there. So long as the procedure is repeatable it can still be used. The obvious addition is a small cooling flush before brewing which should bring the boiler off its maximum temperature just enough. The other commonly employed technique is to wait a certain number of seconds after the element light goes off – but I’m usually too impatient for that technique 😉

This is what happened:

The boiler hits 116C (241F) again just as the portafilter is ready, this time however, the brew button is activated for about 2 seconds which drops the boiler temperature to 110C (230F), then the portafilter is inserted and brewing begins. By the end of brewing, the temperature is 99C (210F): a swing of 11C (20F).

The crema seems to be a more consistent reddy-brown. The shots aren’t tasted but the latte’s taste good.

The other “trick” employed is to start steaming the milk while the element light is on (as close to possible as it going off – which is a trick). This keeps the element energised which keeps the steam pressure as high as possible and the steaming operation as quick as possible. This is a lot easier with the temperature gauge in place, but in practice it works ok to start steaming about 30 to 40 seconds after hitting the steam switch.

Put all this temperature surfing stuff on top of getting everything else right – grinding, dosing, tamping etc and it’s a process. Which is why the PID controller is so popular.

5. PIDing the Rancilio Silvia

A PID controller keeps the boiler temperature at a predetermined setpoint so we don’t have to temperature surf. Fitting a Rancilio Silvia with PID is a popular modification and this video will illustrate the benefits.

Here are a couple of lattes prepared with the PID controller activated:

Things to note:

• Silvia is at 106C when shot starts (which is the setpoint)
• Intra shot temperature minimum is 98C
• Intra shot temperature swing is only 8C as opposed to 10C without PID
• While steaming milk, PID re-energises the element if the temperature gets down to 147C

Conclusions

On a standard Silvia there is enough of a temperature swing to produce the occassional sour/underextracted or bitter/overextracted shot. A simple temperature surfing method will ensure sour shots don’t happen, while a more advanced temperature surfing technique is required to ensure bitter/burnt shots are history.

It is possible to improve the steam performance of the Silvia with a relatively simple temperature surfing technique.

For all the advantages of the most elaborate temperature surfing techniques, the simple (albeit more expensive) option is to fit a PID kit. The other advantage to having a PID kit is that the temperature readout allows the barista to be more involved with the process, for example you could spend a couple of days doing temperature experiments and then writing a blog entry about it …