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Date: 14 Jan 2007 11:57:50
From: C.Hughes
Subject: Thermocouple grounding
I'm ordering parts to PID my rancilio silvia. I've found thermocouples
available with bare conductor junction, or with an overall stainless
steel sheath. I expect the best response would be achieved using the
bare junction type directly in contact with the boiler wall.

However, would this give any electrical problems assuming the
controller ground was bonded to the same mains ground connection as
silvia? Or would the signal from it be lost by grounding the junction?


If so, is there an insulating material that could be used to isolate
the junction from the boiler while retaining good thermal transfer? Or
is it better to just go with the sheathed thermocouple and accept the
slower response? Thank you for any advice.

regards,
CJRH





 
Date: 18 Jan 2007 02:02:58
From: The Other Funk
Subject: Re: Thermocouple grounding
Finding the keyboard operational
C.Hughes entered:

> I'm ordering parts to PID my rancilio silvia. I've found
> thermocouples available with bare conductor junction, or with an
> overall stainless steel sheath. I expect the best response would be
> achieved using the bare junction type directly in contact with the
> boiler wall.
>
> However, would this give any electrical problems assuming the
> controller ground was bonded to the same mains ground connection as
> silvia? Or would the signal from it be lost by grounding the
> junction?
>
>
> If so, is there an insulating material that could be used to isolate
> the junction from the boiler while retaining good thermal transfer?
> Or is it better to just go with the sheathed thermocouple and accept
> the slower response? Thank you for any advice.
>
> regards,
> CJRH
While the grounded boiler shell shouldn't matter, as others point out, there
is a thermal paste that can be used to insulate the probe from the boiler.
It is going to be very difficult to find in very small quantities though.
Bob


--
--
Coffee worth staying up for - NY Times
www.moondoggiecoffee.com



 
Date: 15 Jan 2007 03:54:57
From: C.Hughes
Subject: Re: Thermocouple grounding

Jack Denver wrote:
> My (cheap Chinese) PID did not like a grounded probe. I was able to take my
> (homemade) probe apart and wrap the junction in a little piece of electrical
> tape, making it ungrounded. (Above the junction the thermocouple wires are
> insulated) The whole thing sits immersed down in my boiler so the tiny bit
> of insulation added by the layer of tape shouldn't take much way from
> response time. Likewise if you had a surface mounted TC that was clamped, if
> your PID didn't like the grounded signal then a little piece of electrical
> tape around the junction would do the trick I think. I never played with
> filtering settings on my PID - I don't even recall if there were any, 'cause
> that little piece of tape cured all my problems.
>
>
> "jggall01" <jggall01@yahoo.com> wrote in message
> news:j09lq2l9rgaulrt2ehcna5u03h8sslmnsv@4ax.com...
> > On 14 Jan 2007 20:16:56 GMT, Ian Smith <ian@astounding.org.uk> wrote:
> >
> >>On 14 Jan 2007 11:57:50 -0800, C.Hughes <cjrhughes@hotmail.com> wrote:
> >>
> >>> I'm ordering parts to PID my rancilio silvia. I've found thermocouples
> >>> available with bare conductor junction, or with an overall stainless
> >>> steel sheath. I expect the best response would be achieved using the
> >>> bare junction type directly in contact with the boiler wall.
> >>>
> >>> However, would this give any electrical problems assuming the
> >>> controller ground was bonded to the same mains ground connection as
> >>> silvia? Or would the signal from it be lost by grounding the junction?
> >>
> >>Shorting in the vicinity of the junction doesn't matter. If you have
> >>three metals, the response of a pair of junctions (A-X in series with
> >>X-B) is the same as a single junction (A-B). In this example, A is
> >>one thermocouple wire, B is the other, and X is the boiler. As long
> >>as the boiler wall surface is all at the same temperature, shorting it
> >>doesn't affect matters.
> >>
> >>Dragging it all to a particular voltage also doesn't matter, assuming
> >>it's a remotely half-decent PID. If 'X' is at a particular voltage,
> >>that puts an offset on the signal back to the PID, but the PID should
> >>have a high common mode rejection and just ignores that - it looks at
> >>the difference between the two wires. It should also be very high
> >>input impedance, so won't allow current to flow.
> >>
> >>So, in answer, you want a bare junction thermocouple. Either a bead
> >>or a washer or a disk, it doesn't much matter as long as you clamp it
> >>down to the boiler wall.
> >>
> >>regards, Ian SMith
> >
> > I'm in agreement with Ian on this.
> >
> > I will add that I have seen only 1 controller in over 200 that had a
> > problem with common mode rejection with a grounded t/c. It was one of
> > the very inexpensive Chinese controllers.
> >
> > If by chance you do have problems with a slightly noisy signal from
> > your grounded t/c, many controllers will let you dial in a little
> > digital filtering.
> >
> > Jim
> >
> > --
> > Posted via a free Usenet account from http://www.teranews.com
> >

All very useful information, thanks. The controller I've got is a
Eurotherm model 2132. I think Eurotherm is a good quality manufacturer
(?) so I'll try the grounded junction option first.



 
Date: 14 Jan 2007 16:02:44
From: daveb
Subject: Re: Thermocouple grounding
don't bother

dave
205



 
Date: 14 Jan 2007 20:16:56
From: Ian Smith
Subject: Re: Thermocouple grounding
On 14 Jan 2007 11:57:50 -0800, C.Hughes <cjrhughes@hotmail.com > wrote:

> I'm ordering parts to PID my rancilio silvia. I've found thermocouples
> available with bare conductor junction, or with an overall stainless
> steel sheath. I expect the best response would be achieved using the
> bare junction type directly in contact with the boiler wall.
>
> However, would this give any electrical problems assuming the
> controller ground was bonded to the same mains ground connection as
> silvia? Or would the signal from it be lost by grounding the junction?

Shorting in the vicinity of the junction doesn't matter. If you have
three metals, the response of a pair of junctions (A-X in series with
X-B) is the same as a single junction (A-B). In this example, A is
one thermocouple wire, B is the other, and X is the boiler. As long
as the boiler wall surface is all at the same temperature, shorting it
doesn't affect matters.

Dragging it all to a particular voltage also doesn't matter, assuming
it's a remotely half-decent PID. If 'X' is at a particular voltage,
that puts an offset on the signal back to the PID, but the PID should
have a high common mode rejection and just ignores that - it looks at
the difference between the two wires. It should also be very high
input impedance, so won't allow current to flow.

So, in answer, you want a bare junction thermocouple. Either a bead
or a washer or a disk, it doesn't much matter as long as you clamp it
down to the boiler wall.

regards, Ian SMith
--


  
Date: 14 Jan 2007 16:52:15
From: jggall01
Subject: Re: Thermocouple grounding
On 14 Jan 2007 20:16:56 GMT, Ian Smith <ian@astounding.org.uk > wrote:

>On 14 Jan 2007 11:57:50 -0800, C.Hughes <cjrhughes@hotmail.com> wrote:
>
>> I'm ordering parts to PID my rancilio silvia. I've found thermocouples
>> available with bare conductor junction, or with an overall stainless
>> steel sheath. I expect the best response would be achieved using the
>> bare junction type directly in contact with the boiler wall.
>>
>> However, would this give any electrical problems assuming the
>> controller ground was bonded to the same mains ground connection as
>> silvia? Or would the signal from it be lost by grounding the junction?
>
>Shorting in the vicinity of the junction doesn't matter. If you have
>three metals, the response of a pair of junctions (A-X in series with
>X-B) is the same as a single junction (A-B). In this example, A is
>one thermocouple wire, B is the other, and X is the boiler. As long
>as the boiler wall surface is all at the same temperature, shorting it
>doesn't affect matters.
>
>Dragging it all to a particular voltage also doesn't matter, assuming
>it's a remotely half-decent PID. If 'X' is at a particular voltage,
>that puts an offset on the signal back to the PID, but the PID should
>have a high common mode rejection and just ignores that - it looks at
>the difference between the two wires. It should also be very high
>input impedance, so won't allow current to flow.
>
>So, in answer, you want a bare junction thermocouple. Either a bead
>or a washer or a disk, it doesn't much matter as long as you clamp it
>down to the boiler wall.
>
>regards, Ian SMith

I'm in agreement with Ian on this.

I will add that I have seen only 1 controller in over 200 that had a
problem with common mode rejection with a grounded t/c. It was one of
the very inexpensive Chinese controllers.

If by chance you do have problems with a slightly noisy signal from
your grounded t/c, many controllers will let you dial in a little
digital filtering.

Jim

--
Posted via a free Usenet account from http://www.teranews.com



   
Date: 14 Jan 2007 18:19:28
From: Jack Denver
Subject: Re: Thermocouple grounding
My (cheap Chinese) PID did not like a grounded probe. I was able to take my
(homemade) probe apart and wrap the junction in a little piece of electrical
tape, making it ungrounded. (Above the junction the thermocouple wires are
insulated) The whole thing sits immersed down in my boiler so the tiny bit
of insulation added by the layer of tape shouldn't take much way from
response time. Likewise if you had a surface mounted TC that was clamped, if
your PID didn't like the grounded signal then a little piece of electrical
tape around the junction would do the trick I think. I never played with
filtering settings on my PID - I don't even recall if there were any, 'cause
that little piece of tape cured all my problems.


"jggall01" <jggall01@yahoo.com > wrote in message
news:j09lq2l9rgaulrt2ehcna5u03h8sslmnsv@4ax.com...
> On 14 Jan 2007 20:16:56 GMT, Ian Smith <ian@astounding.org.uk> wrote:
>
>>On 14 Jan 2007 11:57:50 -0800, C.Hughes <cjrhughes@hotmail.com> wrote:
>>
>>> I'm ordering parts to PID my rancilio silvia. I've found thermocouples
>>> available with bare conductor junction, or with an overall stainless
>>> steel sheath. I expect the best response would be achieved using the
>>> bare junction type directly in contact with the boiler wall.
>>>
>>> However, would this give any electrical problems assuming the
>>> controller ground was bonded to the same mains ground connection as
>>> silvia? Or would the signal from it be lost by grounding the junction?
>>
>>Shorting in the vicinity of the junction doesn't matter. If you have
>>three metals, the response of a pair of junctions (A-X in series with
>>X-B) is the same as a single junction (A-B). In this example, A is
>>one thermocouple wire, B is the other, and X is the boiler. As long
>>as the boiler wall surface is all at the same temperature, shorting it
>>doesn't affect matters.
>>
>>Dragging it all to a particular voltage also doesn't matter, assuming
>>it's a remotely half-decent PID. If 'X' is at a particular voltage,
>>that puts an offset on the signal back to the PID, but the PID should
>>have a high common mode rejection and just ignores that - it looks at
>>the difference between the two wires. It should also be very high
>>input impedance, so won't allow current to flow.
>>
>>So, in answer, you want a bare junction thermocouple. Either a bead
>>or a washer or a disk, it doesn't much matter as long as you clamp it
>>down to the boiler wall.
>>
>>regards, Ian SMith
>
> I'm in agreement with Ian on this.
>
> I will add that I have seen only 1 controller in over 200 that had a
> problem with common mode rejection with a grounded t/c. It was one of
> the very inexpensive Chinese controllers.
>
> If by chance you do have problems with a slightly noisy signal from
> your grounded t/c, many controllers will let you dial in a little
> digital filtering.
>
> Jim
>
> --
> Posted via a free Usenet account from http://www.teranews.com
>




    
Date: 14 Jan 2007 21:39:35
From: I->Ian
Subject: Re: Thermocouple grounding
On Sun, 14 Jan 2007 18:19:28 -0500, "Jack Denver"
<nunuvyer@netscape.net > wrote:

>My (cheap Chinese) PID did not like a grounded probe. I was able to take my
>(homemade) probe apart and wrap the junction in a little piece of electrical
>tape, making it ungrounded. (Above the junction the thermocouple wires are
>insulated) The whole thing sits immersed down in my boiler so the tiny bit
>of insulation added by the layer of tape shouldn't take much way from
>response time. Likewise if you had a surface mounted TC that was clamped, if
>your PID didn't like the grounded signal then a little piece of electrical
>tape around the junction would do the trick I think. I never played with
>filtering settings on my PID - I don't even recall if there were any, 'cause
>that little piece of tape cured all my problems.
>

That's some electrical tape! Immersed in the boiler and still sticking
to the probe, sealing out the water to maintain electrical insulation.
Amazing.


     
Date: 15 Jan 2007 09:44:37
From: Jack Denver
Subject: Re: Thermocouple grounding
No no no, the probe has a hollow stainless sheath - the thermocouple wire is
inside the probe and remains dry.


"I- >Ian" <someone@nowhere.com> wrote in message
news:3o4mq25cun3h951or4m3e0ohfeb2fgnri0@4ax.com...
>
> That's some electrical tape! Immersed in the boiler and still sticking
> to the probe, sealing out the water to maintain electrical insulation.
> Amazing.