PID with pressure sensor instead of thermocouple - Page 3

#21: Post by **dsc** » March 24th, 2009, 10:52 am

Hi Greg,

thought about that too, although I will have to get me some small dia copper tubing, solder and some compression fittings to make the ends fit the boiler/PT. Oh and I will need a torch as well to join everything together. Compared to this the TC seems like such an easy option.

I do have a different kind of question, when setting the temperature of the water inside the boiler to get the correct pressure, does it matter how much water is in there? Will setting the PID to 120*C result in the same pressure with 2L and 5L of water in the boiler? My brain tries to convince the rest that the pressure will be higher with the bigger amount.

Regards,
dsc.

#22: Post by **Bluecold** » March 24th, 2009, 11:13 am

No, the pressure won't be higher. Water turns to steam at a certain temperature and pressure. At normal atmospheric pressure, this is 100C. If the water is heated to 100C in an enclosed space (boiler), part of the water turns to steam. This steam creates pressure. That pressure keeps the rest of the water molecules together. The pressure rises until there is equilibrium between steam pushing the water molecules together and heat pushing the molecules apart.
Check this steam table for temperatures and relating pressures.
The reason that larger boilers have more steaming power is because there is more water to turn into steam.

http://users.rcn.com/erics/ · #23: Post by **erics** » March 24th, 2009, 11:14 am

Tom -

Of course I can always use the TC and not bother at all.

Sounds like the best plan of them all - save your PT for more interesting adventures.

Depending on the Fuji PID you have, e.g. the PXR3 series, you can't switch between a thermocouple/RTD input and a 0-20 ma input - at least that's how I read their manuals. BTW, exactly what PT do you have?

#24: Post by **dsc** » March 24th, 2009, 1:13 pm

Hi guys,

Roeland - thanks for the explanation:)

Eric - I do have another PID which offers 4-20mA input, so I was planning to use that (actually I already used it for group head pressure, but it was too slow). The PT I have is an older Vogel Pumpen model, I couldn't find any info about it online but it works well (as far as comparing dial gauge and PT values goes).

Regards,
dsc.

#25: Post by **cafeIKE** » March 24th, 2009, 3:01 pm

If I remember my gas law correctly, when using a TC to control the water temperature, the false pressure in the boiler will be high until the air is purged. The vapor pressure of the steam is added to the vapor pressure of the air. When using a pStat, the water temperature is low because the pStat measures the pressure of the steam and the air. Once the air is gone, only the steam pressure controls the pStat and the water temperature rises.

#26: Post by **dsc** » March 25th, 2009, 6:58 am

Hi Ian,

thanks for the info. Am I correct to think that air is purged via the vacuum breaker on top of the boiler when the machine heats up?

As I stated in my Elektra thread I might switch to PT control, but I will have to make a new pipe to connect the boiler with the PT (compresssion female - standard 1/4" BSP female).

Regards,
dsc.

#27: Post by **shadowfax** » March 25th, 2009, 10:45 am

dsc wrote:thanks for the info. Am I correct to think that air is purged via the vacuum breaker on top of the boiler when the machine heats up?

That's correct, as I understand it--the whole point of a vacuum breaker. I don't see my pressure run high at the beginning on my TC-controlled boiler (my barometer still works)--it's the same before i steam and after it stabilizes again. It does seem like it takes slightly longer to recover after steaming, but then you could turn my machine OFF and probably steam several lattés worth of milk, so that's not really much at issue...