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[Reply #30 on:]

The resistance of a PT100 sensor at 75 degrees Fahrenheit is exactly 109.3035 ohms.

But how do I calibrate my thermometer to 75F?

Such a resistor is used to calibrate the interface electronics, not the sensor. You remove the sensor and wire the resistor in it's place. Once you have validated or adjusted the electronics to read 75F you remove the resistor and reinstall the RTD. That's it, the RTD is not adjustable, it obeys that resistance chart or it's defective. To validate the RTD after you have calibrated the electronics, you can just put it into boiling water and then an ice bath and see if reads out within the stated sensor accuracy specification.

Lefty

[Reply #31 on:]

Well, I was implying that if it's spec'd at 109.3035R at 75F then you have to know when it's 75F if you expect it to be at 109.3035R (a circular problem).

As it turns out these types of resistor are accurate to 10 ppm from 15C to 50C so just having it at "room temperature" would be sufficient.

[Reply #32 on:]

But "room temperature" isn't an objective measured value  -  it's a subjective assessed value 

[Reply #33 on:]

No.  I'm saying they were shooting for approximately 100 ohms.

You wouldn't believe how many of my EE students do a calculation, apply Ohm's law, take the result and then ask me for a "1037.42 ohm resistor" (or some other absurdly exact number).

[Reply #34 on:]

Back in my days, 20% tolerance (E6 series) was considered "normal"  

Kids nowadays... spoiled by 0.1% SMD resistors.

[Reply #35 on:]

How many of us have ever used a Wheatstone Bridge? Now there is a measurement device. I still have one in my barn.

[Reply #36 on:]

This talk reminds me of the saying "man who owns one clock ... always certain of the time".

Similarly if you own one multimeter, you are always certain of the voltage / resistance etc.

When you own two, suddenly doubts arise, when they give different readings. Which one is right? Are either right? Is the component faulty?

Of course, it all gets worse if you own three.

[Reply #37 on:]

Looks like I can get a 250 ohm, 0.01% resistor for $45.50.

Maybe then I can check my meter.

[Reply #38 on:]

And Farnell offer free delivery for orders over $45.

Might be amusing to order one resistor.

[Reply #39 on:]

But when you've spent your $45 and bought the resistor you will still only be capable of establishing ONE point of calibration which is more or less a waste of time.  However, when you do establish this single point and you find there is an error between the resistor value and what your meter indicates, what do you do then.  You really were better off living in blissful ignorance.  

We generally get too hung up on quality of measurement.  Measurement is a relative thing.
If you want to share data then measurement between you and those you share must have a common basis (national standards)
However,  if you are simply playing along yourself then it matters little whether your pint pot of ale actually contains 3/4 pint or 5/4 of a pint.  If you are happy to say "today I drunk 2 pints and yesterday I drunk 3" then you know your consumption decreased by 33% irrespective of the "true" volume of the pot.

[Reply #40 on:]

Just about 50 (yes fifty) years ago at my first job at a large electronics dealer in Chicago, I worked as a parts checker. People (men only in those days) from around the world would mail in orders for their parts. A guy someplace in Nevada ordered one 1/2 watt resistor. Well, it was Friday night and none of us high school kids wanted to sort through all the mis-ordered, mis-filled trays of parts. It was Christmas eve so we decided to wrap "Mr. Resistor" a Christmas present and thanked him for his order, about 30 lbs of 'stuff'. A couple of weeks later, our supervisor called a "team" meeting, and we all got wacked. Lesson learned, never trust a person who only orders one resistor...

[Reply #41 on:]

large electronics dealer in Chicago,

Allied Electronics by any chance?

Lefty

[Reply #42 on:]

Allied Radio
100 N. Western Ave.
Chicago, Il.

Home of Knight-Kits

[Reply #43 on:]

Allied Radio
100 N. Western Ave.
Chicago, Il.

Home of Knight-Kits

Loved that outfit. Hell I learned a lot of my electronics fundamentals from their fat catalog I would send off for in the late 50s/early 60s. Saved up lunch money to make an order now and then. Lots of mental dreams flipping through those pages. Boy would I have loved to have received a 30# gift package from electronics Santa.  

Later I even built a few Knight kits. I thought they were every good or better the the comparable HeathKit at the time, they just didn't have as big a selection and model range as HK did.

Lefty

[Reply #44 on:]

You probably would have been smart enough NOT to send a Thank You note! Hahahaha!  Their "showrooms" were like going to heaven for tech savvy people back then. Audio, video (no color TV), Ham radio (they had an operating hamshack which was cool), ahh! Those were the days! Appliance operators used toasters, not cell phones and tablets.