A Null-Balance Potentiometer

Thank You for the reply, I have talked to one of the Penn- Foster teacher and he tried to help me but all he more less said was it's in the book. I"ve read and re-read the book on this question and even had the thought as you have said, that " A " may be a possible answer to. I'am a little confused the more i read about this. Thank You for the website links, i will look at them.

Here is another website you may find helpful, as it described the operation of potentiometric voltage measurement circuits like the one in your question:

http://www.allaboutcircuits.com/vol_1/chpt_8/3.html

It is not unusual that courses teach information that is outdated, and of little practical use today.

Sometimes the specific examples are not useful today, but are the best, clearest way of getting across a vital, fundamental concept.

Sometimes text books fail on both of these counts. Miserably.

Sometimes you will feel a conflict in giving the text-book correct answer, vs. what you really have researched to be the correct answer.

When you find yourself in this situation, congratulations! You have really learned something, whatever score you got on the test.

Now, if all the students hitting CR4 were more like theo2 they might get some friendly help.

Tony's reference to voltmeters was pretty informative and shows some null voltage applications. Note that the applications involve measuring voltage not current. There may be a clue in there.

My first thought was a bridge circuit. Routinely used where small ohmic values need to be indirectly measured eg ground resistance measurements.

The answer may be hiding in your text book, you just need to look at the problem out of the box. Clever question if that is the case.

Is this application discussed at all in the text? Does a drawing accompany the question? Have you seen this demonstrated? Are there laboratory exercises in the text?

What is the Thevenin equivalent circuit of a thermocouple? Another clue for you there.....

It's an adventure.

Don't over think this problem. It can't be A) because galvanometers measure current and thermocouple output is calibrated in volts. Galvanometers only measure volts when they are part of a potentiometric measurement. The answer can't be B) because the voltage across the reference junction is equal to the reference junction voltage at reference temperature (normally 0 deg. C). This leaves only C) which is the correct answer and also describes the slide wire mechanism used to convert the galvanometer to a potentiometric indicator.

I always thought that a cold junction was required when making temperature measurements with a thermocouple since the two connection clips from any measuring instrument will be in contact with the thermocouple leads and will create a potential at their junction of dissimilar metals that will either add or subtract from the reading.

Certainly a null-balance potentiometer will not draw any current from the circuit and will read the true voltage of the source but in this case the cold junctions in the circuit due to the connection will give a false reading.

A better example would have been a 1.5 volt battery in series with a 100 meg resistor then this instrument would read the voltage across the resistor which in this case would be the open circuit voltage of the battery.

Marv

There is some truth to what you say but the original problem is correctly worded. In modern instruments, a cold junction is not required because it has been replaced by an electronic equivalent. The terminals of modern instruments are made out of the correct thermocouple metals for the intended thermocouple. If you add to the length of a thermocouple with another alloy wire, you will create new junctions that will add and subtract to the reading. Even this will not affect accuracy if both the new junctions are at the same temperature. None-the-less, it is not considered good practice.

> In modern instruments, a cold junction is not required because it has been replaced by an electronic equivalent.

I agree. A physical ice bath and separate thermocouple is not required because ice point or cold junction compensation is now done electronically.

>The terminals of modern instruments are made out of the correct thermocouple metals for the intended thermocouple.

Not true. All instrumentation I know of uses nickel plated copper terminals. The temperature effects of a junction created by a 3rd material, like nickel, is cancelled by the "law of intermediate metals". Omega's diagram explains it well.

Base metal thermocouple connectors (plugs and jacks) are made from respective thermocouple alloys.

All manufacturers assume their field terminal connections are isothermal; both terminals at the same temperature, whether that's the case or not.

>If you add to the length of a thermocouple with another alloy wire, you will create new junctions that will add and subtract to the reading.

That leads to really bad temperature errors, that is, using copper wire as extension wire for thermocouples leads to really bogus temperature values. The practice is more common than I'd like to think.

I agree 100%. You rephrased what I said. When I used the word "terminals" I meant plugs, jacks. etc. We try to NEVER use non-thermocouple extension wires, but sometimes it is necessary in field conditions. When this happens, we maintain the 2 dissimilar junctions at the same temperature.

This is an interesting thread because (to me) it seems to indicate the perils of using other peoples' language. There is something puzzling about the question. So what is it? I think the problem is in the question, namely that the null-balance potentiometer does not indicate the temperature of the thermocouple, it measures the potential of the thermocouple. This creates a situation where there is not an observable chain of logic that can be rationally examined on its face. Of course, when you are doing your own problem solving, you can phrase the questions your want to answer however you want. You can also examine the experimentally found data and check it with a separate method if actually finding the temperature is the desired goal. Can you think of a different way to phrase the question so that this "problem" would not be present? (remember, this stuff can be fun)

You are correct, the output of a thermocouple is measured is in volts. One must then go to a table to convert the measured volts to temperature. I think the problem was worded precisely as intended. Answer (A) is wrong because a galvanometer does not measure volts. It would not have been fair to provide more than one correct answer to a multiple choice question.

Hello All, Took the test and the correct answer was " C " I want to Thank You all for your input and guidance,, really made me think about and mainly Learn ! Thank You all again!! I'll probably be asking more questions down the road.