Measuring incorrect voltage by multi meter. What could be reason

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1. I was measuring voltage of thermopile with Fluke multimeter 15B+. Attached is my circuit.  Its party incomplete shown but in actual its complete in hardware.

 

2. When I measure voltage by only thermopile in hand and not noumted on board, only connect probes of multimeter, I can measure voltage as per its mV vs Celsius graph.

 

3. When I connect it to board, and don't give 3.3V supply to board and connect probes across J1 connector, then also correct voltage read.

 

4.But when I give 3.3V to circuit, and connect probes across J1 connector, ideally it should be the same, because I am still measuring across the thermopile only, 

but its very off than actual values, which I was reading without giving power. Why is that so?

 

5. is it high values 1M resistor connected across? But still y probes are connected across thermopile NO?

Earlier when I was reading 10mV, not I am reading around 200mV.

 

 

 

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My guess is that R2 and R5 are present in order to detect when the thermopile is NOT connected. If the installed values are the same as the schematic, then there should be a current of (about) 1.6uA through the thermopile when it is connected. A change of 190mV with that current is huge. That suggests that the termopile has a resistance of 190mV/1.6uA = 120K approximately. So, there must be one of two problems. 

 

Either (1) the thermopile is faulty and has a high resistance. You can check this with a multimeter. 

 

Or (2) R2 and R5 are not the values claimed. Does your built circuit really have 1 MegOhm resistors?

 

Not many other possibilities.

 

Jim

 

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Wed. Jun 17, 2020 - 03:41 PM
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Do you have a part number or datasheet for the thermopile?

 

(Possum Lodge oath) Quando omni flunkus, moritati.

"I thought growing old would take longer"

 

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Either (1) the thermopile is faulty and has a high resistance. You can check this with a multimeter. 

Since the little thermopile can only output a microscopic amount of power,  I'd sorta expect it to have a high impedance, so a 100k doesn't , on the surface, seem far-fetched.

 

here's a random grab

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Wed. Jun 17, 2020 - 08:13 PM
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OK, I was thinking "thermopile = thermocouple"

 

If 50K is a reasonable resistance, then you have to be prepared for those kinds of voltage drops in that circuit. If you want to accurately monitor the thermopile output then remove R2 and R5.

 

The NAU7802 is "just" a 24 bit ADC, nothing special as far as thermopile is concerned. So, just remove R2 and R5. There MAY need to be a bias level with respect to ground. If this is the case, connect both R2 and R5 to ONE SIDE of the thermocouple output.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Wed. Jun 17, 2020 - 08:58 PM
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The NAU7802 is "just" a 24 bit ADC,

Remember we'd get excited by an 8-bit ADC??   Or maybe an old super-slow dual-slope good for 14 bits twice a second !

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Is the right-hand side of the NAU7802 not have any connections?  If AVDD and DVDD are not connected to +3.3V, then there may be lots of power going through the positive voltage reference of the NAU.

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true on the lack of NAU connections. I had assumed (BAD idea!) that it was just a matter of schematic laziness. But, if it is unpowered, it could certainly add to the bias current through the thermopile.

 

That said, the 120K that I estimated is not that far from the random 50K that candyman found.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Thanks sorted . @ka7ehk

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avrcandies  will have you for this: - edit: see #15!

(and rightly so, IMO)

https://www.avrfreaks.net/commen...

https://www.avrfreaks.net/commen...

 

 

Also this mess:

Is entirely avoidable by just putting the U1 pins in a better order

 

https://www.avrfreaks.net/commen...

 

 

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Last Edited: Sun. Jul 5, 2020 - 12:09 PM
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I am using this thermopile:

Now i am able to measure the thermopile voltage(verified by multimeter also, same reading)  & thermistor votage and then able to convert thermistor voltage into celsisus also.

 

Now I need to convert it into the object temperature. Any mathematical equatin where I can direclty put these values and get the object temperature?  Processing not an issue, since I have send both thermopile and thermistor values to PC, there made a small software. so rather than going by array search better to go with equation.

 

 


 

I downlaoded TE connectivity sheet, they have this one equation(attached also)  

 

but emisstivity of object also varies, how to keep that into accoount?

 

Most of places I have searched they all do by array search, is there equation method also?

 

 

 

 

 

Attachment(s): 

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Human skin, glass, wood, and oil-based paint all have very good emissivity that is greater than 0.9

 

Here is a lot of good info for you:

https://www.fierceelectronics.com/components/demystifying-thermopile-ir-temp-sensors

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Vindhyachal Takniki wrote:
but emisstivity [sic] of object also varies, how to keep that into accoount [sic]?

You have to allow the user to change it via some menu option.

 

Here's a random picture of an infra-red thermometer from Amazon

Link to full size Amazon Image

 

On your other questions; that APP NOTE you attached pretty much covers everything. If you're struggling with the high complexity of infra-red temperature measurement; perhaps you should investigate a digital thermopile sensor where much of the compensation is done for you. It's still a difficult job because your housing and fresnel lens will affect the readings and therefore must be calibrated.

 

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Note---moved from the OP's duplicate/locked post

 

 1. I am interfacing sensor thermopile and thermistor , whose data-sheet is in the locked post

3. Issue is thermistor tempreature either keep on falling or increasing even if room tempreature is stable. It increase/decrease gradually in steps of 0.01 in either way sometimes up, sometimes down.  I have pasted external fluke meter sensor to measure ambient its stable.

 

4. Measured by CRO the REFPN remians stable also

 

5. Is the filter capacitors? C1,C2,C3 and C8,C9 & C10? and resistors of 47ohm in series?

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Sat. Jul 4, 2020 - 11:23 PM
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Have you even tested your adc using some pots & creating voltages?

 

Your aren't serious about r1 r2 ?  That is ridiculous. Why is that not a 1k 1K  divider... 

 

The thermopile produces an extremely small voltage, lets say 0.5 mv.

 

Tie your thermopile to gnd and measure it.   3.27?  or 3.32, or 2.97?  Got rid of it, tie your thermopile to quiet gnd.  That will GREATLY improve your CMRR.

 

If the ADC has trouble around 0V (check specs)then set the 1k/100K(to 3.3) divider to give maybe 32.7 mv. Otherwise connect direct to GND!!!

 

Get rid of thermistor resistor R7, it does nothing for you.  At most you might add an optional resistor in parallel with the thermistor, but even that is not a good idea if you have a micro.

 

C1 is in a tug of war with (C2 & C3) reducing their effect, pick C1 or the others 

 

Pick a time constant to better reject 60Hz line.  (like 20ms RC constant)

ex: 10K & 2uF...or just take a lot of ADC averages.  

 

 

Do NOT use 4-way connections, your circuit will be misinterpreted, and/or your technician will wire it wrong.

 

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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I would not consider R1 and R2 THAT ridiculous, especially given the capacitors at the ADC inputs. They are only bias, and the ADC is measuring differentially.

 

I do concur that a test is really needed with a known voltage source. Perhaps even just a divider string from 3V3. When the  OP writes "falling or increasing", how big is this change relative to expected readings? 

 

Lets see, DVM full scale is 4.0V. I assume that this means +/-4.999V, to be charitable (that is, it MIGHT really be 3.999V), with a resolution of 1mV. That means 10,000 counts or 14 bits. Compared this to a 24bit ADC. Thus, the ADC can change bits and not have anything visible on the multimeter. For example, the 3V3 supply could change by 0.5mV (making it invisible to the DVM) and the thermistor voltage could change by several microvolts which would be quite a few counts in the ADC.

 

I really think that a serious assessment of measurement methods are in order. You also need to consider whether or not your test setup is sufficiently free of bi-metal junctions and other sources of microvolt-level errors. The 24bit resolution "world" requires VERY careful design and implementation to get even modest results. So far, from the circuits and observations offered, I am not confident that the OP is doing careful design.Sorry, but those are the facts, as I see them.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

Last Edited: Sat. Jul 4, 2020 - 11:55 PM
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In addition to Jim's comments - consider the type of capacitors used. In particular the Hi-K types like X7R. They are piezoelectric, so tapping the board will cause a change in measurement. They are also leaky. Having 1M bias resistors also means measurement using a standard multimeter will introduce significant errors - the average multimeter is 10M input resistance/impedance.

Low impedance sensors like strain gauges mean that bias problems are much less of an issue but with high impedance sensors, this becomes a consideration. As Jim mentions, bimetal junctions (thermocouples), flux contamination, component selection, layout (guard rings) and much more become even more critical.

 

Everything is a component with resistance and thermal effects. This contrasts with high frequency RF where everything is a capacitor and inductor.

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Are you complaining that your 0.5% accuracy Fluke 15B+ multimeter is revealing inaccuracies in your 24 bit ADC setup? Would you expect to be able to determine lengths to a resolution of 1/8" with only a 1' ruler without any minor graduations?

Ross McKenzie ValuSoft Melbourne Australia

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1. R1 & R2 are there to put device into center CMR. 1M are good trade off, as ckt are AA cell powered and PGA has much higher resistance at input.

 

2. R6 & R7 does the same also for thermistor & form a divider also.

 

3. C1,C2,C3  & C8,C9,C10 are all X7R

many of TI appnote I saw has this C1/C8 also for thermocouples and mentioned as C1 should be ten times of C2 & C3 as rule of thumb.

 

4. Variation is my room temperature is at 25.7C around and when I turn on device I display NTC temperature with two decimal like 24.72 

It keeps on up or down in 0.01 steps and for 15 min I checked it goes 0.5C change.  I checked with anther sensor manufactuere(same sensor) for calibration my device have this issue. Even interchanged the sensor also.  Calibration device dont  show error while my device does, that means sensor is ok. Issue is in my measurement or ckt.

 

5. currently taking 10 samples, arranging them in descending order, take avg.  speed of adc is 10sps. 

 

 

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Vindhyachal Takniki wrote:
It keeps on up or down in 0.01 steps

If you really mean 0.01°C of noise then give yourself a pat on the back. That is actually excellent performance.

 

Vindhyachal Takniki wrote:
and for 15 min I checked it goes 0.5C change.

Do you mean a temperature rise of 0.5°C ?

This could be harder to track down - you'll need to provide details of the thermistor, it's mounting, your power supply, and your ADC voltage reference.

 

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https://www.murata.com/en-global...

 

Thermocouples are low impedance sources, thermistors are somewhat different. Capacitor value is critical.  Be careful when making assumptions.

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@kartman ,

Didnt get your point actually.

 

 

@ N.Winterbottom , will put a fixed 100K and see if there is same variation 

 

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you want the thermistor to have a wide voltage vatriation...what is the rnage of the thermistor ohms?

 

The 100K's will give a small voltage variation, which is opposes the desired result.  That is fine except:

 

Say you set things up so from 0 to 80C  the thermistor voltages changes only 1%...that is fine with a 24bit ADC.   However, if the 3.3V varies 2% that is like traveling through two of your 0-80C ranges. The ADC can't tell whether the voltage % variation is due to temperature variation or 3.3V variation. 

Hopefully your thermistor variation is a large %.    This is where a rationmetric measurement is much better, such as you can get from an AVR ADC.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. Jul 6, 2020 - 04:11 AM
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At the risk of ever so slightly ignoring the original question: have we seen what the ADC is? There are specialist 24-bit ADC chips characterised for strain guages which include switchable gain instrumentation amplifier as the front end. Two obvious candidates are HX711 (loads of Arduino examples) or CS1238 (and friends).

 

Sampling the millivolt change in a thermocouple is really the same task as sampling the millivolt change of a strain gauge - the only significant difference is that the strain gauge is generally going to be offset to about half rail - which is great for the amps and the ADC. You'd have to bias the thermocouple to get it into a convenient range.

 

Neil

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In the original post, the schematic says NAU7802

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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My point was you seemed to justify your capacitor selection based on what TI used. Capacitor leakage is much less of a problem if the sensor is a thermocouple or strain gauge, more of a problem if you are using a thermopile or thermistor. It could be your capacitors are causing the drift.  Understand the component properties and the application. 

 

Another gotcha is the wiring - or more specifically the insulation of the wire used. Some insulation is piezoelectric. For small signal wiring, this can be significant.

 

Last Edited: Sun. Jul 5, 2020 - 11:51 PM
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Let's go back  to fundamentals please.

 

1. When in open air the thermopile produces the expected voltage (post #1)

2. When connected into the 1M/thermopile/1M UNPOWERED circuit, it measures correctly also.

3. When powered with 3.3 volts it produces about 200 mVolts. OP complains that this is not correct.

4. The thermopile's resistance at 25C is about 115K (post#14)

 

So a little simple maths shows that the resistive divider formed by the two 1M resistors and the thermopile of 115K (nominal value) drops 189mVolts across the thermopile. Tolerances on all components will produce the OP's 200mV.

 

Ohm's Law is maintained.

 

Ross McKenzie ValuSoft Melbourne Australia

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Except the second schematic (msg #14) does not have current through the thermopile. The divider is just used to bias one side of the thermopile.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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Jim,

 

I was addressing the OP's original complaint from post#1.

 

However, his bottom schematic in post#14 shows the thermopile in the resistive divider  but this time with 100K resistors instead of 1M ones. Even worse.

Ross McKenzie ValuSoft Melbourne Australia

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valusoft wrote:
However, his bottom schematic in post#14 shows the thermopile in the resistive divider  but this time with 100K resistors instead of 1M ones. Even worse.

 

#14 - The thermistor is in series with the 100K resistors, not the thermopile. ka7ehk is correct in #28.

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So what is the OP using? A Thermopile as in his post#1 or thermistor?

Ross McKenzie ValuSoft Melbourne Australia

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The thermistor is part of the thermopile sensor, used as reference - like a thermocouple cold junction. The table in OP post #11 is unfortunately incomplete, he does not want to reveal the actual part number?

The table seems to show that with an object temperature of 50C at 5-14 um and an ambient (sensor) temperature of 25C the thermopile output will be about 2.4mV.  The thermistor resistance is specified as 100Kohm @ 25C with a beta value of 3964K for 25C/100C.

 

Also in #11 the TE Connectivity application note link: THERMOPILE SENSOR FOR CONTACTLESS TEMPERATURE -  explains a lot, I doubt that anyone responding here has examined it thoughtfully judging from the responses.

 

As for the OP's question in #11:   but emisstivity of object also varies, how to keep that into accoount?

Emissivity https://en.wikipedia.org/wiki/Emissivity (also see Albedo https://en.wikipedia.org/wiki/Albedo)  depends on the object. That can be determined/accounted for by measuring the objects actual temperature to determine the correction.

Last Edited: Mon. Jul 6, 2020 - 03:43 AM
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The OP did post the data sheet in his second, now locked, post thread. Reposted here.

 

 

 

 

edit to uncheck Notify me when a reply is posted

Attachment(s): 

Last Edited: Sun. Jul 12, 2020 - 07:56 PM
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All of which goes to prove "rubbish in, rubbish out". Yet again this poster provides inadequate information and leads everyone off on a wild goose chase. What a waste of space. (and yes I know that statement is ambiguous)

Ross McKenzie ValuSoft Melbourne Australia

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Thermistor and thermopile are in same package. as in datasheet as in #33.

Right now i will try with fixed 100K to see if there any variation. if varaition still persists then issue in my board, otherwise thermistor actually varying. Will rule out one variable atleast,

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sbennett wrote:
the TE Connectivity application note link: THERMOPILE SENSOR FOR CONTACTLESS TEMPERATURE -  explains a lot, I doubt that anyone responding here has examined it thoughtfully judging

But this is a Microchip AVR forum - so not the place to come for detailed support on an unrelated manufacturer's product!

 

OP should be going to TE for support with their product!

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I display NTC temperature with two decimal like 24.72

You will NOT be measuring temperature like 24.72, maybe 24.7 vs 24.6, even then it is very difficult to get rid of flicker.  Are you meauring in a vat of oil?--maybe then, if you wait 10 hours.  An inch away the temperature will be something else (you can stir your oil bath).

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

Last Edited: Mon. Jul 6, 2020 - 09:58 PM
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Of course, with a 24 bit ADC, you can INDICATE temperature values like 24.72. Whether or not you believe that will depend on your recent consumption of chemicals of your choice. Wether or  not you believe it, the reading may have little to do with reality, especially to the right of the decimal point.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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I have removed C8 in #14. But still varation is there. But when i connect fixed 100K resistor there, I measure 25C without any fluctuation.

 

1. One reason could be this, I was reading this. Currently minimum current which will pass thorugh my thermistor at 25C is = 3.3V/(100K + 100K + 100K)  = 11uA

But i was reading this pdf , here it says to have max 1 to 5uA .

 

 

 

 

2. Can this be reason?

Thermopile, has some sort of lens in front and whatever infrared falls onto it, it concentrates on inside. There we have thermopile and thermistor?  Could it be reason, when heat gets into inside, it also heats up the thermistor? This is very random thought though. Dont know if this could be issue?

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Think about how you would prove/disprove your assumption.

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Thermistor measuring current self-heating applies to any thermistor.  Of course if the physical size is extremely small, then less wattage is needed to cause a problem. 

 

If they are talking 10ua being a problem,they should have used a 10k thermistor rather than a 100-150K.  Working with lower currents just make it easier to be influenced by noise, moisture, dirt, etc.

 

Could it be reason, when heat gets into inside, it also heats up the thermistor?

A mismatch would mean the sensor is worthless, so I doubt they designed it that way!  They assume the thermistor and thermopile work correctly together & they are closely matching in temperature. so the thermistor "knows" the thermopile temperature & allows a calculation.

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!

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Vindhyachal Takniki wrote:
I have removed C8 in #14. But still varation is there.

Self-heating will cause an upward drift n temperature. You talk of variation. Do you mean noise ?

 

Log the readings over 30 mins and post the graph to demonstrate the problem.

 

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I am able to measure thermopile(Vtp) mV & NTC(Celsius) temperature correctly as black body reference temperature when measured display correct results. 

Took thermopile reading and NTC and did curve fit value.

 

Now when I shifted to human body measurement, results are wrong, due to emmisivity. I took e= 0.95 & 0.97 values and divided Vtp by it & then tried curve fit, but values are all wrong. Can someone suggest what is right way to shift to human body?

1. Currently only dividing Vtp by e. Is it ok?

2. Or need to divide NTC also by e?

3. Or divide final temperature by e?

4.What is right method?

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Vindhyachal Takniki wrote:
Can someone suggest what is right way to shift to human body?

I don't think you do "shift to human body" I believe they are two separate non-overlapping products and should be designed as such.

 

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There is switch in these infrared thermometer for human body or surface selection.

standard in any infrared thermometer.

 

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Vindhyachal Takniki wrote:
standard in any infrared thermometer.

None I own and none I've seen.

The mechanical arrangement is also very different. The sensor is right up front on my in-ear but sits back behind a collimator in my spot thermometer.

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Use 35.0 degrees C for the skin temperature

 

see bottom of #11 for equation

 

How good are the results, aimed at a wall?  a table?  a tomato?

When in the dark remember-the future looks brighter than ever.   I look forward to being able to predict the future!