Previous in Forum: instrumentation code and standard   Next in Forum: Nullifying of noise pollution
Close
Close
Close
6 comments
Anonymous Poster

grounded thermocouple

03/28/2009 4:28 AM

hi friends,

while going through types of t/c(thermocouple) ,i came to known about grounded junction t/c is used for temp of high preasure fluids.my doubt is- how the emf exists if the measuring junction is grounded.

thanks,

Reply
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Power-User
South Africa - Member -

Join Date: Nov 2008
Location: Johannesburg
Posts: 295
Good Answers: 12
#1

Re: grounded thermocouple

03/30/2009 2:43 AM

Hi there,

A grounded junction is when the internal conductors of the thermocouple are welded directly to the sheath material. This forms a sealed integral junction.

So your hot junction now has a greater measuring area. Remember the measuring junction is not grounded - so you will have EMF.

Regards,

Craig

Reply
Guru
United Kingdom - Member - Indeterminate Engineering Fields - Control Engineering - New Member

Join Date: Jan 2007
Location: In the bothy, 7 chains down the line from Dodman's Lane level crossing, in the nation formerly known as Great Britain. Kettle's on.
Posts: 32049
Good Answers: 838
#2

Re: grounded thermocouple

11/11/2022 5:33 AM

Well, the voltage of one of the leads will be negative with respect to earth/ground, and the other will be positive.

Simples.

__________________
"Did you get my e-mail?" - "The biggest problem in communication is the illusion that it has taken place" - George Bernard Shaw, 1856
Reply Score 1 for Good Answer
Guru

Join Date: Jul 2010
Posts: 669
Good Answers: 176
#3

Re: grounded thermocouple

11/12/2022 10:39 AM

The EMF potential is not just at the hot end. A thermocouple's EMF is generated 'across or along the gradient', the gradient being the difference in temperature between the hot end and the cold end. A difference in temperature is needed to generate the EMF: the difference being the difference in temperature between the hot end and the cold end of the pair of thermocouple wires.

A thermocouple with a meter of thermocouple wire leading to its 'cold end', sitting on a desk top in an office environment that has stabilized to the ambient temperature, is likely to be isothermal, that is, at the same temperature at both ends. Reading the thermocouple's EMF, voltmeter will read 0.0mV at the because there is no temperature gradient; the thermocouple is isothermal and there is no difference in temperature between hot end and cold end and therefore no EMF potential is generated.

When the thermocouple's hot end junction is 'grounded' to some other metal, then the potential of that metal, with respect to the device reading the signal at the cold end, is added algebraically to the EMF potential creating a common mode voltage between the grounded hot end and the AI (analog input) cold end. That common mode voltage can and does affect how the analog input reads the EMF signal.

Battery powered thermocouple calibrator/simulators typically read a thermocouple signal as well as generating a mV signal simulating a thermocouple. Battery powered devices "float" with respect to earth ground, so are less subject to influences from the ground potential at the hot end.

If the isothermal thermocouple in the office environment (described above) generates zero mV, how does an electronic temperature instrument read 20.3C (or whatever) for the temperature of the hot end?

All thermocouple thermometry requires a cold junction reference measurement at the measuring cold end. The cold junction measurement measures the temperature of the connection point where the thermocouple wires connect to the analog input, with some temperature sensing element that is not a thermocouple, like an RTD or a thermistor.

The cold junction temperature is then added to the thermocouple temperature measurement to produced the reported temperature value.

Shorting the inputs of a thermocouple analog input will produce the cold junction measured temperature value because the thermocouple measurement will be zero mV due to the shorted input.

Reply
Guru

Join Date: Mar 2007
Location: by the beach in Florida
Posts: 33233
Good Answers: 1807
#4

Re: grounded thermocouple

11/12/2022 12:10 PM

I think it has to do with the relationship between the voltage/current level and the resistance of the circuit...so the normal resistance of the metal conductor however slight is enough to counteract the low voltage current generated, so it's more the flow of the current through the wire itself than what would normally be considered a shorted connection on a higher current circuit, a capacitive circuit results from the dissimilar metals....

__________________
All living things seek to control their own destiny....this is the purpose of life
Reply
Guru
Philippines - Member - New Member Engineering Fields - Instrumentation Engineering - New Member Engineering Fields - Control Engineering - Who am I?

Join Date: Oct 2006
Location: Northern Mindanao, Philippines
Posts: 2127
Good Answers: 51
#5

Re: grounded thermocouple

06/25/2023 9:15 AM

A grounded thermocouple has the thermocouple junction welded or touching the tip of the sheath. This allows the thermocouple to sense the heat faster. Unfortunately, this also "grounds" the sensor, which can create a ground loop and introduce noise into the signal.

If you're going to use a grounded thermocouple, make sure the probe (the sheath, the head, the process connection) are ungrounded, i.e., isolated from ground. Or you can ensure that the measuring instrument and the surface that the probe is inserted into is at the same potential.

One manufacturer I know separated the tip of the sheath from the rest of the probe (using a ceramic insulator). It solved the ground loop issue and still maintained the benefits of the grounded thermocouple.

regards,

Vulcan

__________________
Miscommunication: when what people heard you say differs from what you said. Make yourself understood.
Reply
Guru
India - Member - Sensors Technology Popular Science - Cosmology - Dream, Think and Act United Kingdom - Member - New Member United States - Member - New Member Canada - Member - New Member

Join Date: Aug 2006
Location: AM-51, Deen Dayal Nagar, Gwalior, Madhya Pradesh, MP 474001, India
Posts: 3416
Good Answers: 32
#6

Re: grounded thermocouple

06/25/2023 10:38 AM

Grounded junction will form a common mode voltage and if high voltage gets connected then your amplifier will also get higher common mode voltage and can damage it. Heater power supply can leak into thermocouple.

Floating thermocouple is much safer.

__________________
Prof. (Dr.) Shyam, Managing Director for Sensors Technology Private Limited. Gwalior, MP474001, India.
Reply
Reply to Forum Thread 6 comments
Interested in this topic? By joining CR4 you can "subscribe" to
this discussion and receive notification when new comments are added.

Comments rated to be "almost" Good Answers:

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Copy to Clipboard

Users who posted comments:

craigza (1); Iris (1); PWSlack (1); Shyam (1); SolarEagle (1); Vulcan (1)

Previous in Forum: instrumentation code and standard   Next in Forum: Nullifying of noise pollution
You might be interested in: EMF Meters, Thermocouple Wire, Thermocouple Connectors

Advertisement