Thermocouple into a rotating tank

[Gustavo Raush]

Hi:

Someone could address how can I solve the mechanical/electrical problem
to measure temperature from 20 thermocouple set into a 3 litre rotating
tank which contains a electrically heated water?.

I have found that MOOG company offers a slip rotating rings but the
contact resistance is about 60 mOhms. May be this contact resistance is
a little high for micro-volts sources like thermocouples.

Any suggestion will be helpfully accepted.

Thanks at all.

 

[DJ Delorie]

Scott Seidman <namdiesttocs@mindspring.com> writes:

Not to mention that the connection through the slip rings is another
thermocouple!!

As long as it's the same thermocouple on both wires, it should be OK.

I don't know what kind of accuracy you need, but for best accuracy,
the thermocouple and thermocouple amplifier must be on the same side
of the slip rings.

That means you're accuracy becomes limited by the cold junction sensor
in the amplifier. If that were acceptable, a simple thermistor could
be used instead of a thermocouple.

Heck, if that were OK, put a battery powered MCU with some DS2762's
and a zigbee radio inside the tank and skip the slip rings completely.
Or an MCU with 20 thermistors.

 

[Gustavo Raush]

Scott Seidman escribió:

Gustavo Raush <gustavo.raush@upc.edu> wrote in news:g62gbc$5un$1
@defalla.upc.es:

I have found that MOOG company offers a slip rotating rings but the
contact resistance is about 60 mOhms. May be this contact resistance is
a little high for micro-volts sources like thermocouples.

Not to mention that the connection through the slip rings is another
thermocouple!!

I don't know what kind of accuracy you need, but for best accuracy, the
thermocouple and thermocouple amplifier must be on the same side of the
slip rings.

Hi Scott:

Thanks for your quickly response.
The needed accuracy is about 0.3 to 0.5 şC, worst values are not allowed
because several heat transfer numerical models must to be validated.
Do you know some published article which could be explaining
experimental setup like this?. Not exactly the same but which could be
given details for an infrastructure with similar problem to measure.

Thanks.

 

[Gustavo Raush]

GregS escribió:

In article <g62i2q$7bu$1@defalla.upc.es>, Gustavo Raush <gustavo.raush@upc.edu> wrote:
Scott Seidman escribió:
Gustavo Raush <gustavo.raush@upc.edu> wrote in news:g62gbc$5un$1
@defalla.upc.es:

I have found that MOOG company offers a slip rotating rings but the
contact resistance is about 60 mOhms. May be this contact resistance is
a little high for micro-volts sources like thermocouples.
Not to mention that the connection through the slip rings is another
thermocouple!!

I don't know what kind of accuracy you need, but for best accuracy, the
thermocouple and thermocouple amplifier must be on the same side of the
slip rings.

Hi Scott:

Thanks for your quickly response.
The needed accuracy is about 0.3 to 0.5 şC, worst values are not allowed
because several heat transfer numerical models must to be validated.
Do you know some published article which could be explaining
experimental setup like this?. Not exactly the same but which could be
given details for an infrastructure with similar problem to measure.


If you used nice high value 100K thermistors slip ring resistance would be no
problem, just noise, and noise can be filtered. You can calibrate a temperature
to a very close resoluton, but the problem is temperature range.
Whats you range ? I lost track of the total number of channels needed, but I think your biggest problem
is getting a calibrated system using high value thermistors of 1K or more. Most systems
on the market are not that.

greg

Hi Greg:

Your idea it not so bad. Could be is the best solution because the data
acquisition units will be used is a well-known Agilent 34970+34901
module (Digital Acquisition Switching Unit+Customizing-Module). This
DASU+Module tandem allows a wider range of thermistor sensors where the
10K value is permitted. It is not clear if the 100K is also included but
I think 10 K is OK. Accuracy reported by the maker is 0.08şC.

About the calibration sensors, in my case it is not a problem because I
will be able to carry out a good one.

Thanks by the shown interest.

 

[DJ Delorie]

Scott Seidman <namdiesttocs@mindspring.com> writes:

As to whether the extra thermocouples would do anything bad, if the
'couples were wired directly into the rings, you wouldn't have the same
thermocouples. Same if you extended the 'couples with copper wires.

If both slip rings are made of the same metal and are the same
temperature, and you use a suitable thermocouple extension wire on the
outside, it works. The voltages induced by the slip rings cancel out,
just like any other metal connector you use with thermocouples. I'm
not sure what it would do for the accuracy, though - that would depend
on any thermal gradient across the slip ring assembly.

You can do it with copper wire if you take into account the
temperature of the slip rings when converting the thermocouple
reading.

 

[DJ Delorie]

Scott Seidman <namdiesttocs@mindspring.com> writes:

So, if you have a chromel-alumel thermocouple for example, and you
attach it to a slipring of connector metal X, the chromel-X junction
will exactly cancel the alumel-X junction??

A common misconception in thermocouple design.

The junction is irrelevent. It's the temperature change ALONG THE
WIRE that makes the voltage change.

A length of wire with a temperature gradient along it will act like a
uV battery. The uV/C ratio depends on the metal. A thermocouple is
two of these in series, meaning the voltage at the junction is
different than the voltage at either connection. The chromel wire has
a voltage change of X, the alumel wire has Y, you measure X-Y. If you
had a third wire Z, you'd measure X-Y-Z (or X-Y+Z depending on how you
interpret things). If you know the combined voltage and the
temperature at all but one junction, you can calcuate the temperature
at the remaining junction.

So, if the slip ring is isothermal, and you have two copper conductors
to it (assume those have the same gradiant as each other) and a K
thermocouple on the inside, what you have is FOUR seebeck effects
happening, in series. The three junctions are the two slip rings and
the tip of the thermocouple.

Since the two copper wires have the same composition and temperature
gradient, they'll have the same voltage across them. Since we're
measuring differential voltage, and the copper is providing a
common-mode voltage, they cancel out.

The only catch is, you have to know the temperature of the slip ring.

In the pdf you reference, this is Figure 12.

 

[Spehro Pefhany]

On 21 Jul 2008 20:42:17 GMT, the renowned Scott Seidman
<namdiesttocs@mindspring.com> wrote:

DJ Delorie <dj@delorie.com> wrote in news:xn63qz6njw.fsf@delorie.com:


Scott Seidman <namdiesttocs@mindspring.com> writes:
As to whether the extra thermocouples would do anything bad, if the
'couples were wired directly into the rings, you wouldn't have the
same
thermocouples. Same if you extended the 'couples with copper wires.

If both slip rings are made of the same metal and are the same
temperature, and you use a suitable thermocouple extension wire on the
outside, it works. The voltages induced by the slip rings cancel out,
just like any other metal connector you use with thermocouples. I'm
not sure what it would do for the accuracy, though - that would depend
on any thermal gradient across the slip ring assembly.

You can do it with copper wire if you take into account the
temperature of the slip rings when converting the thermocouple
reading.



So, if you have a chromel-alumel thermocouple for example, and you attach
it to a slipring of connector metal X, the chromel-X junction will
exactly cancel the alumel-X junction??

Yes, more generally the error will be roughly (roughly because it's
not quite linear) equal to the difference in temperatures between the
pieces of metal X. This is probably your biggest issue.

I thought you need a second chromel-alumel at a ref temp wired in, then
you could have two chromel-X (or alumel-X, your choice) isothermal
junctions that cancel each other. Actually, I'm pretty sure of this, and
there's an example at http://www.omega.com/temperature/z/pdf/z021-032.pdf

Modern instrumentation allows a variable 'reference' temperature which
is an isothermal connection point where the thermocouple wire is
connected. That temperature is measured and used to compensate for the
so-called "cold junction" temperature (often digitally these days,
which allows for nonlinear comnpensation. You don't need to worry
about it unless you're designing the signal conditioner.

I dislike doing precision thermocouple designs, but they're really not
magic. In any case, thermistors are easier, but don't have the same temp
range, and thermocouples can be faster.

The 60m ohm is no problem at all. Modern instruments have input
impedances in the M ohms, but you might have issues with noise from
the slip rings, depending on your desired bandwidth. Keep in mind that
most instruments pass a small current (I typically use something like
500nA) through the sensor to detect failure. That will result in
0.1°C error for every 8 ohms of resistance (or resistance change).


Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com

 

[Pieter]

On Mon, 21 Jul 2008 19:18:34 +0200, Gustavo Raush
<gustavo.raush@upc.edu> wrote:

Hi:

Someone could address how can I solve the mechanical/electrical problem
to measure temperature from 20 thermocouple set into a 3 litre rotating
tank which contains a electrically heated water?.

I have found that MOOG company offers a slip rotating rings but the
contact resistance is about 60 mOhms. May be this contact resistance is
a little high for micro-volts sources like thermocouples.

Any suggestion will be helpfully accepted.

Thanks at all.

What temperature range do you need?

A thermocouple mesaures voltage, so the resistance of the rings is not
important. But they will form a thermocouple too, where your
thermocouple wires are connected to the rings. If this is all at the
same temperature, it should all cancel out, but it is likely to give
problems.

Some other options are PT1000 elements or the modern precize NTC
thermistors. Or maybe placing some electronics (that can handle the
temperature range) in the tank and transmit the signals digitally? I
would go for this last option, but notice that the electronics will
have to take it's own temperature into account.

Pieter

 

[Jamie]

Gustavo Raush wrote:

Hi:

Someone could address how can I solve the mechanical/electrical problem
to measure temperature from 20 thermocouple set into a 3 litre rotating
tank which contains a electrically heated water?.

I have found that MOOG company offers a slip rotating rings but the
contact resistance is about 60 mOhms. May be this contact resistance is
a little high for micro-volts sources like thermocouples.

Any suggestion will be helpfully accepted.

Thanks at all.
How about using IR instead of contact?

you could simply mount each receiver near the moving
area of interest..
the other way would be to seal a complete control unit inside
and have a set of slip rings to supply the power and the control
unit can wireless transmit the values..


http://webpages.charter.net/jamie_5"

 

[Jamie]

Scott Seidman wrote:

DJ Delorie <dj@delorie.com> wrote in news:xnabgb6t0c.fsf@delorie.com:


Scott Seidman <namdiesttocs@mindspring.com> writes:

Not to mention that the connection through the slip rings is another
thermocouple!!

As long as it's the same thermocouple on both wires, it should be OK.


I don't know what kind of accuracy you need, but for best accuracy,
the thermocouple and thermocouple amplifier must be on the same side
of the slip rings.

That means you're accuracy becomes limited by the cold junction sensor
in the amplifier. If that were acceptable, a simple thermistor could
be used instead of a thermocouple.

Heck, if that were OK, put a battery powered MCU with some DS2762's
and a zigbee radio inside the tank and skip the slip rings completely.
Or an MCU with 20 thermistors.


All good points. I have a number of slip ring systems, and have often
threatened to replace them with telemetry based systems if any of the
rings broke. I'd still need rings for power, as batteries wouldn't be
an option, but that's sort of a different beast, and the ring count would
still go way down.

As to whether the extra thermocouples would do anything bad, if the
'couples were wired directly into the rings, you wouldn't have the same
thermocouples. Same if you extended the 'couples with copper wires.
I've always found thermistors to be more worry-free from such design
considerations.

RTD's also may be a suitable replacement over using thermocouples.

The RTD's can be commutated easily over slip rings.

Just place a small cap across the connection.

http://webpages.charter.net/jamie_5"