pt100 thermistor and analog input module

[Vasilis]

hi everyone,

i am facing big problems using a pt100 thermistor. this device has 3
wires, two red and a blue one. The two red wires are connected, i.e.
using the beeper on the multimeter it beeps. I suppose that the way
this works is this: apply a tension on red and blue wires, and the
difference with respect to the initial voltage, is to be correlated
with room temperature, I.e. the thermistor reduces the voltage
applied. Right?

The second device of my configuration is an analog input module,
capable of reading 0-5volt inputs (among other things), with 12 bit
resolution. I wrote a simple driver that converts tensions to two
bytes, used a transformer on it and aknowledged that it works! fine.

What about putting the two together? I want to read the output of the
pt100 device using this module, but I fail. I cannot understand why,
applying a tension on the red and blue wires of the pt100 thermistor,
all i get is a very very small variation of the initial voltage(~4 mV)
and no further variation on increasing diminishing heat on the pt100
device. What is wrong? am i using them wrong? did i connect them
wrong? i tried putting 1,5V, 3V etc but i noticed no significant
difference....

I hope I've been clear.

Thank you,
vasilis.

 

[John Fields]

On Wed, 24 Sep 2008 07:04:51 -0700 (PDT), Vasilis
<VASILI3000@hotmail.com> wrote:

hi everyone,

i am facing big problems using a pt100 thermistor.

---
Are you sure it's a thermistor?

"PT100" is a designation usually used when referring to a 100 ohm
platinum resistance thermometer.
---

this device has 3
wires, two red and a blue one. The two red wires are connected, i.e.
using the beeper on the multimeter it beeps. I suppose that the way
this works is this: apply a tension on red and blue wires, and the
difference with respect to the initial voltage, is to be correlated
with room temperature, I.e. the thermistor reduces the voltage
applied. Right?

---
I don't think so. The wiring you're describing is consistent with a
3-wire RTD:

http://newton.ex.ac.uk/teaching/CDHW/Sensors/an046.pdf
---

The second device of my configuration is an analog input module,
capable of reading 0-5volt inputs (among other things), with 12 bit
resolution. I wrote a simple driver that converts tensions to two
bytes, used a transformer on it and aknowledged that it works! fine.

What about putting the two together? I want to read the output of the
pt100 device using this module, but I fail. I cannot understand why,
applying a tension on the red and blue wires of the pt100 thermistor,
all i get is a very very small variation of the initial voltage(~4 mV)
and no further variation on increasing diminishing heat on the pt100
device. What is wrong? am i using them wrong? did i connect them
wrong? i tried putting 1,5V, 3V etc but i noticed no significant
difference....

I hope I've been clear.

---
You have been, but first things first.

Is the device a platinum resistance thermometer or a thermistor?

JF

 

[Vasilis]

On Sep 24, 7:25 am, John Fields <jfie...@austininstruments.com> wrote:

You have been, but first things first.

Is the device a platinum resistance thermometer or a thermistor?
Dear JF,

thank you for you reply.
it is exactly what you have described in your post. a platinum
resistance thermometer.

I think I understand now that this device works with a very low
voltage, but i have no idea on exactly what voltage it is needed. What
do i have to do in order to read successfully the small voltage
variations that are provoked by the temperature variations?

Thank you,
Vasilis.

 

[John Fields]

On Thu, 25 Sep 2008 01:43:23 -0700 (PDT), Vasilis
<VASILI3000@hotmail.com> wrote:

On Sep 24, 7:25 am, John Fields <jfie...@austininstruments.com> wrote:
You have been, but first things first.

Is the device a platinum resistance thermometer or a thermistor?
Dear JF,
thank you for you reply.
it is exactly what you have described in your post. a platinum
resistance thermometer.

I think I understand now that this device works with a very low
voltage, but i have no idea on exactly what voltage it is needed. What
do i have to do in order to read successfully the small voltage
variations that are provoked by the temperature variations?

---
The usual approach is to buy an already built meter like:

http://www.omega.com/ppt/pptsc.asp?ref=DP460-T&Nav=temm07

If you want to build your own then it gets complicated.

What do you want to do?

JF

 

[Vasilis]

On Sep 25, 2:50 am, John Fields <jfie...@austininstruments.com> wrote:

The usual approach is to buy an already built meter like:

http://www.omega.com/ppt/pptsc.asp?ref=DP460-T&Nav=temm07

If you want to build your own then it gets complicated.

What do you want to do?

I would like to read with my 0-5V analog input device the values of
the thermometer. Applying a very small voltage on the two wires of the
thermometer ( 18 mV ) and heating it, the voltage between the two
wires of the termometer increases. Problem is that i cannot get a
temperature value ( a resistance value ) just from reading the voltage
change, or can I?

Thank you,
Vasilis.

 

[John Fields]

On Fri, 26 Sep 2008 01:13:51 -0700 (PDT), Vasilis
<VASILI3000@hotmail.com> wrote:

On Sep 25, 2:50 am, John Fields <jfie...@austininstruments.com> wrote:
The usual approach is to buy an already built meter like:

http://www.omega.com/ppt/pptsc.asp?ref=DP460-T&Nav=temm07

If you want to build your own then it gets complicated.

What do you want to do?

I would like to read with my 0-5V analog input device the values of
the thermometer. Applying a very small voltage on the two wires of the
thermometer ( 18 mV ) and heating it, the voltage between the two
wires of the termometer increases. Problem is that i cannot get a
temperature value ( a resistance value ) just from reading the voltage
change, or can I?

---
Yes, you can.

Consulting the platinum resistance tables at:

http://www.shelbyjones.com/OnlineCatalog/Temperature/Sensors/Thermocouples/Thermocouples_Literature/Thermocouples_RTD_Tables.htm

we find that a 100 ohm sensor with an alpha of 0.00385 exhibits a
resistance of 100.00 ohms at 0C and a resistance of 138.51 ohms at 100C

That's a difference of 38.51 ohms, and if you want to use an analog
input device to indicate zero volts at 0C and 5V at 100C, then we must
convert that 38.51 ohm resistance change into a 5V change in voltage.

To start with, we must excite the RTD electrically, but we mustn't force
too much current through it or it'll self-heat to the point where its
accuracy will be degraded.

Assuming that 1mA is OK and that we have a 10V excitation supply, we can
start like this:


10V Vin
|
[R1]
|
+--->Vout
|
[RTD]R2
|
0V

With 1mA through the RTD at 0C, we'll have, as Vout:


Vout = Irtd R2 = 0.001A * 100R = 0.1V


Now, in order to limit the current to 1mA, we must calculate the value
of R1:

Vin - Vout 10V - 0.1V
R1 = ------------ = ------------ = 9900 ohms
Irtd 0.001A


So, we now have:


10V E1
|
[9900]R1
|
+--->0.1V E2
|
[100]R2
|
0V

At 100C we'll have:


10V E1
|
[9900]R1
|
+---> E2
|
[138.51]R2
|
0V

And the output voltage from the divider will be:


E1 * R2 10V * 138.51R
E2 = --------- = ------------------ = 0.138V
R1 + R2 9900R + 138.51R


The voltage difference, then, for the temperature change from 0C to 100C
will be:


dV = V100 - V0 = 0.138V - 0.1V = 0.038V


which means that for your analog device to read 0V at 0C and 5V at 100C,
there must be something with a gain of:


Vout 5.0V
Av = ------ = -------- = 131.58
Vin 0.038V


interposed between the output of the divider and the analog device.

A difference amplifier would do it, and the circuit would look like
this:

+V>---+---------+---------------+
|R1 | |
[9900] [R3] +----|--[R6]--+-->Vout
| | | | |
| +--[R5]----+---|-\ |
| | | >------+
+---------|--[R7]----+---|+/
|R2 | | |
[RTD]R2 [R4] [R8] |
| | | |
0V----+---------+----------+----+

Working out the values of the resistors and selecting the opamp is left
as an exercise for the student.


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JF