ITTSB Blog 1/2018 Thermoelectric DIY Thermistor Tester - Minus 5~110 Celsius

[Kiriakos GR]

ITTSB Blog January 2018 - Advanced DIY Thermistor Tester – Minus 5C up to 110 Celsius

Intro:
Recently I got abscessed to discover all details about one Ni-CD battery charger for cordless drill / screwdriver, this using NTC thermistor which is installed in to the battery pack.

My research over thermistor values and exploration of thermistor operation details this lasted several weeks.

Nowadays production of Ni-CD cells at 1,2V it is still alive in China mainland when originally, Panasonic Japan was the only trusted source for cordless tools Ni-CD cells in the past 25 years.

Panasonic (Japan) Ni-CD cells design these introduced high performance standards, they are more tolerant to higher charging voltage and current.
Additionally their operating temperature and charging temperature limits was also high with out this to translate to battery cell damage.

Recently produced Made in China Ni-CD cells they are inferior compared to High performance standards’ introduced by Panasonic.
Therefore it is wise, any battery pack thermistor this to be tested (measured) at true range of temperatures because improper thermistor value, this will lead to shortened life cycle of any such battery packs.


NTC Thermistor = "NTC" means Negative Temperature Coefficient.
In simple words, as the temperature increases, the resistance reduces.
The resistance selected is the resistance at 25C

Such resistance measurement it is possible by any reliably made multimeter, but the challenge here it is you to own a special mini oven (precision heater) so this to elevate thermistor temperature at desired levels.

After several experiments I did ended up to select one Peltier / TEC as Thermoelectric heater.
Due thermoelectric properties of TEC, hot side can not stay as hot with out supplied power (Ampere), additionally when there is no current flow, hot side temperature this instantly decreases.
By supplying a larger heatsink to cold side and a smaller heatsink at hot side, our benefit this is relative fast temperature change in about two minutes of time.
Thermoelectric TEC properties are now operating as to be a scale.
By using one laboratory power supply with Fine precision at voltage setting, we can simulate a range of temperatures from ambient up to 110 Celsius.

For example setting an temperature of 30,7 or 42,3 or 70,9 Celsius, this is possible to achieved by precision voltage control, and balancing  and  stability can be succeeded by tweaking voltage source for few minutes.
When your thermoelectric scale this has balance at your preference, then it stays there as long you need it.
Thermistor it is equally fast to K type thermocouple and I did install one as you may see next to thermistor spring-loaded clip.

I do believe that supplied pictures of my project they would answer all questions regarding How-To.
Currently in the market there is no available such a Thermistor Tester apparatus.
By reversing voltage polarity hot side becomes cold side.

As thermal insulation (separator) of hot & cold sides, I have use hardened nylon this formed as carton paper, this is 100% waterproof.


Accurate measurement of thermistor at five counts resolution, this is a challenge.
Your tests leads should have low mass so to not operate as additional heatsink over the thermistor leads.

Now you are fully informed of all details, good luck about making your own Thermoelectric DIY Thermistor Tester!

[Kiriakos GR]

Few days later I am now realizing what I have just compose by making this testing device, and what I did this is a first step in the working principal of Dry-Block Calibrator which this is made for thermocouple testing and calibration.

Dry-Block Calibrator this is actually made by the use of aluminum cylindrical chamber ( oven ), additional accessory this is a pipe including several holes them drilled in several diameter so to serve actual diameter of its one temperature probe.

Additionally at Dry-Block chamber there is active air flow.

In conclusion, operating temperature of  Dry-Block Calibrator this is controlled by three points of control:
a) incoming electrical energy
b) Air flow
c) One or more accurate thermocouple inside this device.

In my project I did all the work by using manual control and somewhat this was easy to do, any upgrade of this simple design about this resembling to mini Dry-Block Calibrator this will require elevated cost and additional work, in simple words to add automation in such DIY project is not easy to do.

Dry-Block Calibrator concept actually similes to a Hair-dryer with several mini steps of temperature adjustment.
Either way I can possibly perform some improvements at a later time, by finding or making one heatsink for the hot side, this be more close to Dry-Block Calibrator concept.