300deg. digital thermometer

[avz]

Hi all
I would like to build a digital thermometer that can measure and display up to 300deg Celsius. since the LM35 is suitable only up to 150 deg, I think that the use of a RTD such as PT100 can be the solution. the PT100 has a resistance of 100 ohms at 0 deg and 212 ohms at 300 deg. I need a circuit that can read those two values as the range and translate it to a display. I was thinking that the 7106 as a panel meter might be OK.
Thanx for your ideas and help.

 

[ericgibbs]

Hi all
I would like to build a digital thermometer that can measure and display up to 300deg Celsius. since the LM35 is suitable only up to 150 deg, I think that the use of a RTD such as PT100 can be the solution. the PT100 has a resistance of 100 ohms at 0 deg and 212 ohms at 300 deg. I need a circuit that can read those two values as the range and translate it to a display. I was thinking that the 7106 as a panel meter might be OK.
Thanx for your ideas and help.

hi,
As the base voltage range of the 7106 is 200mV, then it should be possible to supply the PT100 with a current that will give a change of 200mV over a range of 112R.
It will be necessary to ZERO the offset due to the 100R at 0Cdeg.

 

[canadaelk]

The 7106 is a voltmeter. You need to read resistance. Maybe a preamp that will also scale the output to the 7106 to display the temperature. Some voltage will go through the PT 100 and you need to take self-heating into account.
I use an IR meter (non-contact measurement) wherever I can.

 

[Boncuk]

The 7106 is a voltmeter. You need to read resistance. Maybe a preamp that will also scale the output to the 7106 to display the temperature. Some voltage will go through the PT 100 and you need to take self-heating into account.
I use an IR meter (non-contact measurement) wherever I can.

The PT100 datasheet gives every resistance value for any given temperature.

Using a supply voltage within limits there will be absolutely no self heating of the sensor.

Boncuk

 

[MikeMl]

Here is an old thread about the PT100. There are lots of others: use the search feature on this web site.

Why bother making an LCD voltmeter for the back end of the thermometer when you can buy a 3 1/2 digit panel meter for under $10?

 

[Boncuk]

Why bother making an LCD voltmeter for the back end of the thermometer when you can buy a 3 1/2 digit panel meter for under $10?

Certainly - but none to measure up to 300deg/C.

 

[MikeMl]

Certainly - but none to measure up to 300deg/C.

No, but the commonly available digital panel meters usually read +-0.1999V or +-1.999V full scale, which if you use an opamp circuit to scale the PT100 output so that 100degC makes 0 V, and 299 degC reads 1.999V would make a digital thermometer with a range from 0 degC to 300degC with a resolution of 0.1degC. You would have to add 100 to the reading.

 

[avz]

MikeMl
Thanks, I like the idea, the 7016 is not the only option. could you please draw a circuit of the suggestet application?
Thanx.

 

[MikeMl]

MikeMl
Thanks, I like the idea, the 7016 is not the only option. could you please draw a circuit of the suggestet application?
Thanx.

Here is the basis for putting the PT100 into a bridge whose output can be read directly using a +-200mV digital DC voltmeter, either panel-mounted, or using a DMM, like my Fluke75.

I used the data shown on this page to model the resistance variation of the PT100.

You need a very precise voltage reference of 10.00V. I would suggest the LM4040. The three resistors R2, R3, R4 must be precision metal film resistors. To get the values specified, I would make up those values by adding up to three 1% standard value resistors in series.

In the simulation, I optimized the values of R2, R3, R4 to get the smallest Error E2 from an ideal output V2. Note the use of LTSpice as a "calculator" to compute the error between the actual circuit and an ideal circuit. Note that I minimized the error over the entire range of 0 to 300degC. If you only need to read a subset of this range, the error could be minimized over just that range, making a much better fit over then narrower range.

The difference Diff E1 between the + and - output from the bridge is where the output would normally be taken. For example, if I wanted to use my Fluke DMM to read temperature using this circuit, I would connect the leads across + and -, and set the meter to read 200mV full scale. To get the actual temperature in degC, I would multiply the DMM reading by 1000(i.e. ignore the millivolts), and then add 100. A reading of 100mV would be 200degC.

Note that in interfacing the differential output from this circuit to a panel mounted digital voltmeter, you might run into the issue that on some of the DPMs, their inputs cannot be referenced to their own supply voltage, which means you may require two power supplies which are isolated from each other; one to power the bridge, the other to power the DPM.

 

[avz]

MikeMl
Thanks for the detailed answer. it seems that overcoming the the resistors values, it can be done. I'll try to put it together.
Thanks again.

 

[stars1]

Hi all
I would like to build a digital thermometer that can measure and display up to 300deg Celsius. since the LM35 is suitable only up to 150 deg, I think that the use of a RTD such as PT100 can be the solution. the PT100 has a resistance of 100 ohms at 0 deg and 212 ohms at 300 deg. I need a circuit that can read those two values as the range and translate it to a display. I was thinking that the 7106 as a panel meter might be OK.
Thanx for your ideas and help.

I have a Digital Food Thermometer Measuring Range between -50 — + 300 Celsius, displays temperature information clearly. Don't know if it's what you are looking for.