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 Post subject: Thermo Electric MiniMIte
PostPosted: Nov Tue 12, 2019 3:22 pm 
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Hi folks

I picked this up as a package deal with things that really interested me, but I am not familiar with this. It is model 70200

I have scoured the 'Net, and found a different model, asked the company for documentation/ schematics to which they replied a nice version of "good luck".

Anyone know this piece of equipment? It clearly measures temps, but other than that I am in the dark.

It has a D-size mercury battery, that was leaking, so it came out. It was rated at 1.3V. I would like to see if it works, but am unsure as to whether a 1.5D will toast it...

Thanks!

Steve


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 Post subject: Re: Thermo Electric MiniMIte
PostPosted: Nov Tue 12, 2019 3:30 pm 
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Location: Gretna, Nebraska
I doubt you will hurt it with a 1.5 volt alkaline battery, but if you are worried about it you could use a rechargable NiMH battery. NiMH batteries are about 1.2 volts.

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Last edited by processhead on Nov Thu 14, 2019 3:00 am, edited 1 time in total.

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 Post subject: Re: Thermo Electric MiniMIte
PostPosted: Nov Tue 12, 2019 3:56 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: S. Dartmouth MA 02748-1225 USA
Its been 25 years since I used an old school "potentiometer".

The large mercury cell is used to power the bridge its current into the bridge is regulated by the battery rheostat which is set during CAL procedure. If the battery is too low or too high the calibration control may be out of range. Too high is easy, add a resistor. Should me a wire-wound temp compensated but who cares? Cal balances via battery rheostat against a mercury standard cell using the galvanometer to "0"

Seems that it has two scales neither in calibration for an typical thermocouple. To use it you will need a thermocouple conversion chart for the thermocouple type that you have, which can also be made from thermocouple extension wire. The chart will be calibrated for a specific temperature either 32 deg F (0 cel) or room temperature usually 70 degrees. you will also need a thermometer (lab mercury in glass) if using a temperature chart calibrated for room.

If the chart is calibrated for 32 deg, an insulated cup full of crushed ice is needed only a little water. Bath when agitated will give 32F as a reference, a pair of copper extension leads. Each copper lead is twisted as tightly as possible to one of the thermocouple wires, the joints, separated are placed in the ice bath. The charts are almost always written for temperature above room and above ice point, the thermocouple type will have a + and - wire. These now copper go the the appropriate EMF terminals.

In test position the the EMF terminals are shorted and the galvanometer balance with the pot. that should indicate the chart at balance and the external thermometer you provide.

Choose the MV scale and connect the external thermocouple, balance the galvanometer and the pot scale will give millivolts. Look at chart to determine the temperature at the distant thermocouple.

I would have to 'mess" with it a bit to be sure of exact operation...

There may be a mercury standard cell inside :shock: :lol: .

This is NOT the cell that powers the bridge but is used to balance the front calibration pot.

DO NOT attempt to measure that cell with anything but a 25 meg VTVM. ANY load on the standard cell will polarize it and it will be ruined. The standard cell is essential for the instrument to be internally calibrate.

Instruments of this type have been around since late 19th century, they remained so right up to hand held digital stuff was developed, what 25 ya?

Good Luck!

Chas

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 Post subject: Re: Thermo Electric MiniMIte
PostPosted: Nov Wed 13, 2019 7:34 pm 
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Joined: Mar Sun 20, 2016 11:04 pm
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Location: Mesa, AZ 85206
Thanks...I took out the D size mercury call because it was leaking...I like the idea of using the rechargeable...Still trying to figure out how actually to see if it works!

Steve


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