resisters to adjust NTC thermistor resolution?

[knighty]

I've got a machine with an NTC 12 thermistorin it

it's set to run at 82'C, the controls shut it down and lock it out if it gets to 87'C, it's totally safe at that temp and higher, that's just the temp we like to run at

I need to double check measurements, but going by this chart... https://www.vectorcontrols.com/download/General/temperature_curves_V1-4.pdf

I'm going to test a 2k resister in series with the thermocouple, that should give a lower reading to the controls

That should drop the 82'C reading to about 45'C ish

I can see what it reads at 82'C actual temp and set it to whatever reading it gives


I'm slightly worried I'll have the same problem, when temp reading gets up, it'll think it's 5'C over and still trip out

I'm struggling to visualise it, but I'm thinking I could pt one resister in series and one in parallel and it will reduce the resistance change in the circuit of the thermocouple?

resister in parallel will reduce resistance in circuit

the one in series will put it back where it should be?


anyone know about these things who can tell me if I'm on the right track?

I essentially just want a few 'C more wiggle room before it errors out


EDIT:

it should be ~1.7kOhm at 82'C (or there about)

if I put one 1.7kOhm resister in parallel with it, that brings total resistance to 0.85kOhm

then a 0.85kOhm in series with all of it... that'll bring it back to 1.7kOhm where it should be... but the changing resistance of the thermistor should have less effect on total resistance?

 

[sako243]

If you weren't aware

Resistors in series
R(total) = R1 + R2 + R3 etc...

Resistors in parallel
1/R(total) = 1/R1 + 1/R2 + 1/R3 etc...

Work out the parallel combination first and then add the series resistance. Or some equations will sort you out to see if it's possible. Pick two data points for your NTC and substitute in.

 

[waddycall]

I’m guessing the answers “no” but is there no way of adjusting the controller? What is it PID?

 

[knighty]

I’m guessing the answers “no” but is there no way of adjusting the controller? What is it PID?

nope, nothing I can do there, it's an all in one circuit board that does the lot

 

[knighty]

If you weren't aware

Resistors in series
R(total) = R1 + R2 + R3 etc...

Resistors in parallel
1/R(total) = 1/R1 + 1/R2 + 1/R3 etc...

Work out the parallel combination first and then add the series resistance. Or some equations will sort you out to see if it's possible. Pick two data points for your NTC and substitute in.

I did an edit..

it should be ~1.7kOhm at 82'C (or there about)

if I put one 1.7kOhm resister in parallel with it, that brings total resistance to 0.85kOhm

then a 0.85kOhm in series with all of it... that'll bring it back to 1.7kOhm where it should be... but the changing resistance of the thermocouple should have less effect on total resistance?


does that look right?

there's something about the way the resistance of the thermistor changes that's melting my brain

 

[waddycall]

Is it a thermocouple (produces a voltage when it’s heated) or a thermistor (resistance changes according to temperature)?

 

[knighty]

Is it a thermocouple (produces a voltage when it’s heated) or a thermistor (resistance changes according to temperature)?

you're right! just edited to fix it :-o

 

[m_c]

Your best bet is to make a spreadsheet to run the figures at various temperatures/resistances.

The main issue is most thermistors aren't very linear, so doubling the resistance isn't likely to half the temperature.
Also, is the controller fixed to cut the temperature at 5deg over, or a percentage over?

 

[mike 109444]

"there's something about the way the resistance of the thermocouple changes that's melting my brain"
Is the thermocouple / control system liner ?
If not then that would screw with your expectations

 

[mike 109444]

Also be aware that any "resistors" you use will have a tolerance. When we use a fixed resistor to set up our thermomixing pcbs we use a precision (1%) resistor inplace of the thermocouple and even then we have to run cycles with a total of 12 thermal couples spread around the wash basin. The "normal" cycles we run are in the range of 30 to 40 degs but the "selfdisinfect" cycle which is >81 degs but < 85 degs for 15 mins. The system work by keeping the heating element on till 83 is reached then heater relay is turned off then back on at 82. This continues through the 15 min cycle after which the heater is turned off and the system continues circulating the water till it reaches 80 at which point the water is allowed to drain and cycle completes. There is a second thermom couple that is monitored and set to stop the machine if it reaches 90 degs but due to the on and off cycling around the 82/83 this does not happen. The above is controlled by a plc controller that the programsrun in. This is all for the medical devices I work on which are endoscope washers (AER's) They are tested to the above every 3 months.

 

[m_c]

Here's a quick snapshot of how adding parallel/series resistors looks resistance wise with 16k, either added in series to the thermistor, in parallel to the thermistor, and combined (series value added to parallel value), based purely on the actual temperature.
(temp scale is reversed to get the VLookup working simply)

The chart shows the resultant resistance, and with sub 20deg values removed-

And now I've done that, I need to go and do some other stuff before I can get my brain around creating a chart showing resultant temp from the calculated values.
The Temp data in the above shows the rough value that the controller will see with the various resistors added.

 

[waddycall]

Can you use a potentiometer with a suitable range and adjust to suit on the machine?

 

[m_c]

The other option would be something to take the thermistor value, then output an artificial resistance for the controller.
You'd need to figure out how what kind of input the controller is looking for, but a basic arduino (or other similar microprocessor dev board) with a PWM output (or even an analogue output), should be enough to manipulate what the value the controller sees.

 

[m_c]

Should probably attach the spreadsheet for if anybody else want's to improve it

 

[daleyd]

I’ve just had a read through this - temp measurement and control is what I do for work, but NTC isn’t a sensor type we use or see much…almost everything we use is rtd or thermocouples.

I’m not sure I’m understand the problem correctly - are you saying the thermistor is faulty and not giving the correct resistance value out? In which case can you not change it completely for a new one? I can’t see why you couldn’t add resistance but if it’s so far out I would be suspicious of it being completely u/s?

I don’t think I’ve ever seen anything that can convert an incoming signal to resistance (I have a couple of hand held calibrators that can simulate rtd resistance but I’ve never seen an instrument that could do it as a conversion), but then it’s not something I’ve looked for either - if the Ntc is manipulating a voltage then you could possibly do something as a variable voltage/current output but because of the non linearity that also becomes tricky.

 

[m_c]

@daley from what I understand, the equipment cuts out when it goes more than 5degC above the set temp, and knighty would like to change it so there's more head room before it cuts out.

 

[knighty]

@daley from what I understand, the equipment cuts out when it goes more than 5degC above the set temp, and knighty would like to change it so there's more head room before it cuts out.

Yes this

 

[knighty]

Also, is the controller fixed to cut the temperature at 5deg over, or a percentage over?

Good point... I've got no idea... think I need to test more....

 

[daleyd]

@daley from what I understand, the equipment cuts out when it goes more than 5degC above the set temp, and knighty would like to change it so there's more head room before it cuts out.

Ah, ok I understand now, thanks.

 

[daleyd]

There seem to be 3 important figures that you need to either work out and/or measure:

1. Resistance of the NTC at the current running setpoint (82 degrees?) - your new circuit total resistance should equal this at the same temp.

2. Resistance of the NTC at the current trip setpoint (87 degrees?) - the new circuit total resistance should equal this - but at your *new* trip setpoint

3. The NTC resistance at whatever new temperature you want it to trip at - this will be part of the calculation to get the total resistance as in part 2. (Depending on which way you work the calculation)

You could either put a trip temperature in and work out resistance’s, for your new trip point, or use the resistance values and work out what temperature it’s going to trip at…

If you have an upper limit you definitely don’t want to go over then you could start from there, I’d think one of the 2 resistance will be a compromise - the running setpoint is probably going to be the one you want most accurate and you could have a bit of leeway with the trip setting?