Is a drop in brew temperature normal with PID controllers?

#1: Post by **SiliconLunch** » January 23rd, 2015, 4:50 am

Hi there,

I've owned a Lelit PL41TEMD espresso machine for a couple of years and am pretty happy with it. I use it every day. The machine is fitted with a PID temperature controller & nice blue LED display, which is advertised to "ensure stable brew water temperature which is vital for great tasting espresso".

The machine seems to operate properly. For example, when I let it heat up to operating temperature, the PID controller stabilizes the brew temperature at exactly 94°C. However, as soon as I start extracting a shot, the PID display rapidly drops until it reaches around 78°C, which it stays at until the shot is fully extracted. As soon as the pump is turned off, the machine quickly brings the brewing temperature back to a stable 94°C.

I have an engineering background, so to my way of thinking, the Lelit machine seems to have limitations. I've read on these forums that the ideal temperature & pressure to extract espresso is 94°C @ 9 bar. I assume this means that the temperature and pressure should remain at these levels throughout the 30 seconds or so the entire shot is being extracted. (Please correct me if any of these assumptions are wrong). What seems to be happening with the Lelit is that the thermoblock is being brought to the ideal brewing temperature prior to activating the pump, however, once the water starts flowing through the thermoblock from the tank (tank water temperature is about 15°C), the heating element is not powerful enough to bring this up to 94°C. So that's why the delivery is around 78°C.

I have confirmed this with a digital instant-read thermometer; When the brew pump is initially activated, the water coming out of the head is 94°C. However, after a few seconds, it drops to around 75-78°C, which is confirmed on the PID's LED display.

I am wondering what experience the community has with this potential issue, and whether it is common for many machines to have the brew water temperature drop like this? I had (perhaps naively) thought that a machine equipped with a PID controller would do a better job at delivering a temperature-stable stream of hot water through the brew head, but maybe I'm wrong?

I would expect professional grade machines to be able to to this, either through a much stronger heating element (to heat water from the tank on the fly), a bigger thermoblock, or else via a reservoir that was pre-heated to 94°C with enough capacity for at least a couple of shots.

Any thoughts, anyone?

All the best,
Nico

#2: Post by **DaveC** » January 23rd, 2015, 6:09 am

The problem is the machines specific design, simply adding a PID doesn't make a machines brew water temperature accurate, or stable.

#3: Post by **DeGaulle** » January 23rd, 2015, 7:02 am

I don't have experience with PID controllers on espresso machines, but do have an engineering background too.
Compared to a conventional thermostat the PID is designed to respond quicker (i.e. not wait until the temperature drops below a certain dead band) and modulate the heat delivery in proportion to how much the actual temperature varies from the temperature you have set on the dial. However it is always REACTIVE, so it senses a temperature disturbance and then responds. Since the boiler volume on a Lelit is only 250 ml, the fresh water influx does give a considerable temperature disturbance. A bigger size boiler would help reduce the size of such disturbances.
One thing the PID does not do on this occasion is give you a flat temperature regardless. Having said this, a drop to 78 degr. C throughout the extraction seems like a lot to me.
Try one or two brief flushes to trick the PID into heat delivery, allow the temperature to recover and then pull the shot, see if that gives you a better result.
Else it may be the PID tuning has been optimised for small heating cycles when the machine is idling, but sluggish under load. If the tuning is set more sensitive, you may have some slight temperature oscillations when the machine is idling, but perhaps a better response when you are extracting.

#4: Post by **another_jim** » January 23rd, 2015, 7:41 am

SiliconLunch wrote: ... the Lelit machine seems to have limitations ... What seems to be happening with the Lelit is that the thermoblock is being brought to the ideal brewing temperature prior to activating the pump, however, once the water starts flowing through the thermoblock from the tank (tank water temperature is about 15°C), the heating element is not powerful enough to bring this up to 94°C. So that's why the delivery is around 78°C.

Your analysis is correct. This is a problem with very cheap espresso machines (i.e. less than about $500). Better machines do not use thermoblocks (except the 1800 watt Quickmills, 220V only), since they have to heat the water on the fly. Instead they have brew boilers that store sufficient water for one shot, and have enough power to recover in about a minute. This requires at least 3/4 liter water and about 1200 watts, or more water with less heat. The best machines add preheating devices to the water entering the brew boiler. An alternative is to have an HX in a very large steam boiler.

Cheap machines can be slightly tweaked by starting the shot at at 98c, and using a fine grind/larger dose to slow it down, so it takes around 35 to 40 seconds. That lets the thermoblock keep up and may keep your water above 88C at the end. The initial high temperature is dissipated by the cold coffee. Putting the machine on steam and preheating the portafilter with a flush also helps. You can nurse the machine to a good shot using these tricks (too cool will taste weak and soury, too hot will taste aggressive and bitter), but it will get old very quickly.

On cost-is-no-object machines, the temperature can be rock steady during the shot, or follow an identical trajectory each time. Usually the non straight line trajectories is a curve that rises, then falls. But the temperature will always remain in a 90 to 95 range.

#5: Post by **Bikeminded** » January 23rd, 2015, 7:55 am

SiliconLunch...it is timely you should post this. I have a Lelit PL041TEM and have been trying to address the same just this week. I don't think it is PID related, more a short coming of the small boiler on the unit.

I start a shot at 202F (94C) and by the end of a 25g weighed shot, the display reads in the 180-185F (82-85C) range.

Something I just started experimenting with last night/this morning was switching on the steam mode for about 10 sec, switch off, then immediately start the pump (shot). By doing this the heating element is just starting to turn on/heat the water.
I guess it is a different version of temperature surfing that is used on other machines. Only have done this a few times, but I see the display pop up to maybe 205F (96C) as I am starting the shot and around 195F (90C) at the end of 25g. It continues to fall another 5 degrees after the shot ends, then recovers.

One of the reasons I bought the PL041 was it's quick heatup time, and being more of a newbie at that time did not know the negative side of that.

I am interested on what other folks more experts in the field think on how this temperature variation has on shot quality.

#6: Post by **Bikeminded** » January 23rd, 2015, 7:57 am

Jim...you and I must have been typing at the same time. Thanks for the insight/affirmations.

January 23rd, 2015, 9:07 pm

Thanks to everyone who has responded. I'm pleasantly surprised at the quality of the responses, the S/N ratio on this forum is exceptional - hard to find these days!

The truth of the situation is actually a bit worse than I initially wrote, as I re-tested the machine's performance again this morning with my Thermapen. Leaving the pump running, the steady-state brew water temperature converges closer to 50°C than the 78°C I originally wrote. The tank water temperature is still around 15°C (winter here in Tokyo). So in terms of the on-the-fly heating power of the machine, the delta is only about 35°C above tank ambient. On reflection, that's pretty poor, if the target is to deliver 94°C !

Disappointed to hear that ultimately it's a shortcoming of the machine's design. The machine wasn't exactly cheap at around AUD $1000, but I suppose it's also not in the league of Cimbali or Marzocco. So you get what you pay for, and it's an interesting learning experience I will be sure to apply when I look to upgrade one day.

In the meantime I'll try out some of the tweaks suggested.

Thanks again for the great discussion.
Nico

#8: Post by **another_jim** » January 23rd, 2015, 9:28 pm

SiliconLunch wrote: ... I re-tested the machine's performance again this morning with my Thermapen. Leaving the pump running, the steady-state brew water temperature converges closer to 50°C

This is inaccurate: the faster the flow, the less the thermoblock can rasire the temeprature. The steady state temperature of the machine is at espresso flow rates -- about 90 grams per minute. Technically this only requires a 500 watt heater if it managed to transfer 100% of its heat to the water. In practice, high end thermoblocks are 1500 to 1800 watts.

You should measure with a TC mounted in a basket filed with coffee; or google on Scace device.

http://users.rcn.com/erics/ · #9: Post by **erics** » January 23rd, 2015, 9:54 pm

For Nico -

Are you sure this machine is fitted with a thermoblock? All references I have seen refer to it having a 250 ml boiler and, according to 1st Line, this temperature drop is considered normal. Of course, I cannot locate that exact reference now but I do remember reading same. This temperature DOES NOT reflect the temperature of the water presented to the coffee. For that, you would need to do as Jim S. advised.

January 23rd, 2015, 10:56 pm

Hi Eric,

Actually, I don't know for sure whether the machine has a thermoblock, I was just assuming. Would the exploded diagram (link below) assist in identifying?

http://espresso.lelit.com/wp-content/up ... L41TEM.pdf

Specs are here:
http://espresso.lelit.com/modello/pl41tem/

To Jim: thanks for clarifying the error in my measurements. I can see now how a free-flow rate is quite a bit different to the espresso flow rate. Before I go for my Scace device (

), how trustworthy would you think the PID LED temperature display would be, when the filter basket is full and we're extracting at the espresso rate? I guess another relevant question would be, where exactly is the Lelit TC located?

Cheers,
Nico