troubleshooting thermocouple thermometer

It is long overdue for me to look into the discrepancy of temperature measurements amongst the channels of my 10-channel Fluke thermocouple thermometer.

I have use this meter on and off over the years. While I have noticed long ago the readings on some channels are way off. I have never invested the time to get to the bottom of it as I just pick and use the channel which give me close enough readings. I used boiling water and ice bath to determine which are reliable. Here in Portland, Oregon the elevation is very close to sea level so the measurements should be 100C and 0C respectively.
A lot of the thermocouple wire ends are in pretty rough shape from use. I had always assumed this are the main cause of the bad reading for some channel.
inside this vintage meter; what jumps out at me is the hand writing on the label say J-115V which suggest to me it intents to convey this meter is set for J type thermocouple; the post stamp size plug-in PCB is probably set the thermocouple type

 on the bottom of the case it also indicate J type as the production configuration

 this is the slide-out thermocouple terminal PCB

 first off I take the reading of all channels with boiling water; this channel is dead on 100.0 C

 the same good channel also read very close to 0.00C on ice water bath

I am very impressed with how well this meter hold calibration. The last time it was calibrated was at least 20 years ago.

As I went through all the channel a pattern emerged. All the bad channels read about 160F with boiling water. To be sure I check the terminals and the fused end and the reading does not change. I now suspect these channels has wrong thermocouple wires. Checking the EMF vs temperature chart suggest they are likely type K.
EMF vs temperature chart on Wikipedia

In all there are six channels with the wrong thermocouple wires. This equipment is commonly used in electronic industry to perform environmental tests. I wonder how many useless test reports were generated because of them. Garbage in garbage out reports just like those cranked out by US public health authorities' diet recommendation over the last half century. The recent UK study found little or no correlation with consuming saturated fat and heart disease is only the tip of the icebergs of our suspicions of most of these recommendation are based on bad science. You remember the analogy they once give that your artery with a diet of saturated fat is like bacon fat clogs up your kitchen drain? I had always believe in eating everything with moderation. I've never subscribe to the low-XYZ products. If I drink milk, I want homogenized, not the one the agribusiness already skimmed off the butter fat. When I cook, I used heavy cream and not the so called half and half. The biggest problem with American diet is high consumption of sugar and highly refined starch as well as food-like-matter (aka processed food products that consist of 90% of supermarket isles). Cold pressed olive and rapeseed oil (and a few others including natural coconut oil), most other mass produced vegetable oils are not what they cracked up to be. Most are processed with harmful chemicals.

Next I set out to validate my coffee making technique.  I use this piston type coffee maker to make coffee when I am camping. It is probably one of the best and yet simplest way to brew coffee without involving complicated equipment (like vacuum pot or high quality espresso machine). Ideal coffee extraction temperature is around 198F plus or minus a few degrees depends on many other factors. To achieve this I always note the elevation of my location.

After bringing the water to a boil I pour the water into the piston which already been filled with a serving of finely ground coffee. I raise the kettle and pour in a thin stream from about 18 inches if I am at near sea level. Having a kettle that has a well made spout make this very easy.
no water yet so 73F reflect the room temperature of my kitchen

pouring with a thin stream

 i am very impressed of my estimate of how high to pour - this reads 196.2F

 here you can see within just a few seconds the temperature already dropped by 2 degrees F

 here in another test run i poured a thicker stream from about the same height

 the temperature reads about 8 degrees F higher!

In my recent road trip I made coffee at many different elevations to as high as 7000 feet. I always adjust the pour height, the stream size, and dwell time and my cups of coffee had been quite consistent. These were done without the benefit of a thermometer measurement.
i stripped off the wrong thermocouples from the meter

To confirm my suspicion the wrong thermocouples are K type I put them on the cheaply made Omega meter in the photo below. This meter uses the more common general purpose type K thermocouple. The readings are consistent to my known type K on the Omega.

i bought this consumer grade meter very long ago before finding the Fluke; this one costed me a good fortune at the time but the thermometer is very inaccurate from the beginning; i will never buy another product from Omega again

p.s. the reason I often prefer Fahrenheit instead of Celsius in common measurement is a degree in Fahrenheit is 5/9 of that in Celsius. Our body is sensitive to very minute temperature in relative terms. It is very inaccurate in absolute terms. If you have a digital thermostat you may notice a one degree (f) change in setting is in fact quite pronounce.