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### Topic: 109.3035 ohm resistor (Read 9772 times)previous topic - next topic

#### be80be

#15
##### Mar 04, 2013, 02:05 am
Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

#### retrolefty

#16
##### Mar 04, 2013, 02:59 am

Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Try 65 degrees F = 109.304 ohms from chart:
http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf

Lefty

#### be80be

#17
##### Mar 04, 2013, 03:40 amLast Edit: Mar 04, 2013, 03:48 am by be80be Reason: 1
I been racking my brain about this and I no I seen these things used in some thing just don't no what it was
But I don't think it's to set for 75 F cause it didn't add up on any chart I seen.

#### Papa G

#18
##### Mar 04, 2013, 03:54 am

Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Well:

25 * 9 / 5 + 32 = 77 degrees F

(75 - 32) * 5 / 9 = 23.9 degrees C

From the chart at http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf :

77 degrees F = 109.735 ohms
75 degrees F = 109.304 ohms

It all looks right to me.

#### retrolefty

#19
##### Mar 04, 2013, 04:01 am

Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Well:

25 * 9 / 5 + 32 = 77 degrees F

(75 - 32) * 5 / 9 = 23.9 degrees C

From the chart at http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf :

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

#### Papa G

#20
##### Mar 04, 2013, 04:07 am

Where did you find that a PT100 is 109.3035   It's 109.73 at 25C could you show how you did the math for Fahrenheit

Well:

25 * 9 / 5 + 32 = 77 degrees F

(75 - 32) * 5 / 9 = 23.9 degrees C

From the chart at http://www.tnp-instruments.com/sitebuildercontent/sitebuilderfiles/pt100_385f_table.pdf :

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.

#### retrolefty

#21
##### Mar 04, 2013, 04:30 am
Your right, I was too quick to see the structure correctly that changes between negative and positive values.

Lefty

#### Papa G

#22
##### Mar 04, 2013, 04:36 am

Your right, I was too quick to see the structure correctly that changes between negative and positive values.

Lefty

Easy mistake, Master Po.

#### Osgeld

#23
##### Mar 04, 2013, 04:44 am
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!")

#### be80be

#24
##### Mar 04, 2013, 04:47 am
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.

#### Papa G

#25
##### Mar 04, 2013, 04:51 am

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.

#### Papa G

#26
##### Mar 04, 2013, 05:00 am

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.

#### Learning

#27
##### Mar 05, 2013, 04:36 pm
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.

#### Chagrin

#28
##### Mar 05, 2013, 10:13 pm
The resistance of a PT100 sensor at 75 degrees Fahrenheit is exactly 109.3035 ohms.

But how do I calibrate my thermometer to 75F?

#### Papa G

#29
##### Mar 05, 2013, 10:25 pm

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.

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