77 degrees F = 109.735 ohms
75 degrees F = 109.304 ohms
It all looks right to me.
I think your misreading the chart. You look at the 70F row and move 6 columns (-5) right to reach 65 degrees = 109.304 ohms.
Lefty
I see what you are saying but it looks to me like the minus numbers just apply to the minus temperatures at the top of the chart. Look 8 rows up from the 70 degree row.
We had a good two hour talk with our vishay rep about these precision resistors, and got a sample card of different styles they make, its interesting stuff.
Like the ones we are using for production testing to act like shunts for current measurements (like single mA with 7 points of precision after that) are in a nutshell an array of resistors in series on a die which they fuse off to make different values.
those are the cheaper ones, like 20 bucks a pop (we ordered 100 of them, that was fun explaining to the purchasing dept "you spend 2 grand on 100 resistors!")
Yep it looks like a match i just don't see a $100 for a resistor to set that.But who no's Thanks for the math PapaG
I would use hot and cold. And thanks for posting this It sure been fun racking my brain. But maybe we should ask the suppler. LOL just to see whats what.
Osgeld:
We had a good two hour talk with our vishay rep about these precision resistors, and got a sample card of different styles they make, its interesting stuff.
Like the ones we are using for production testing to act like shunts for current measurements (like single mA with 7 points of precision after that) are in a nutshell an array of resistors in series on a die which they fuse off to make different values.
those are the cheaper ones, like 20 bucks a pop (we ordered 100 of them, that was fun explaining to the purchasing dept "you spend 2 grand on 100 resistors!")
I used some custom value Vishay parts in a product once. They can match temperature coefficients very closely so that if you use them in a voltage divider application the ratio tracks well with temperature. And yes, they can be a little spendy.
be80be:
Yep it looks like a match i just don't see a $100 for a resistor to set that.But who no's Thanks for the math PapaG
I would use hot and cold. And thanks for posting this It sure been fun racking my brain. But maybe we should ask the suppler. LOL just to see whats what.
If you look at the data sheet for the part, http://www.mouser.com/ds/2/428/vhp100-10459.pdf, you can see that it has fantastic specs. It's probably more precise than any instrumentation you would have.
I enjoyed the puzzle as well. You can check with the supplier if you like. I'm convinced.
It's probably the 'part number', misplaced in some SQL database, or put a couple in series, and get that 218.6070 ohm standard we're all looking for. haha.
PapaG:
The resistance of a PT100 sensor at 75 degrees Fahrenheit is exactly 109.3035 ohms.
But how do I calibrate my thermometer to 75F?
Substitute the resistor for the RTD and do what ever is appropriate to adjust your thermometer readout to 75F. Unless you describe your thermometer, I can't get more specific. An example of things to adjust would be to adjust the RTD current or the bridge amplifier gain.
PapaG:
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.
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.
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).
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.
