DS18B20 probe inquiry

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madsci2020
 
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DS18B20 probe inquiry

Post by madsci2020 »

I'm working on a project that uses your temperature sensor, DS18B20 probe, and I would like to know whether the entire metal portion of the probe needs to be submersed in liquid or only just the tip needs to be submersed in order to get a proper reading. Is the location of the sensor itself near the tip of the probe similar to a thermocouple?

Thanks.

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adafruit_support_mike
 
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Re: DS18B20 probe inquiry

Post by adafruit_support_mike »

The datasheet seems to talk about everything except the actual temperature sensor, but given the error curve and the tolerances, my guess is that it's a variable current-density PNP transistor sensor.

The relationship between a transitor's base voltage and the current flowing through it depends on temperature. At 25C, increasing the current by a factor of 10 raises the base voltage by 59.16mV. That value changes by 198uV per degree C. Both values are physical constants, and will hold for any piece of silicon across a fairly wide range of temperatures.

To make a thermometer, you can use a pair of matched transistors and send ten times as much current through one as through the other, then measure the difference between their base voltages. Another approach is to use a single transistor, alternately send 1x and 10x current through it, and compare the two voltages. That eliminates the problem of having to find/make matched transistors, but requires a bit more work to store values that exist at different times so you can compare them.

The DS18B20 uses an ADC to convert the temperature to a digital value, and almost all ADCs use sample-and-hold circuits at their front end. It would be easy to make a version that samples the 1x base voltage on one input and the 10x base voltage on the other input.

That means the actual temperature sensor is probably very small and buried inside the chip's plastic package, and probably isn't particularly close to the tip of the protective can.

Experiments have shown that the dominant thermal path to the die of an IC goes through the GND pin, which makes sense when you look at the way chips are built. The die is usually bonded to a metal paddle that leaves the body of the chip as the GND pin, and all other connections are made from metal pads in the lead frame to metal pads on the top surface of the chip. The wires that carry those signals are made of extremely fine gold wire (you can see a piece if you look into the end of an LED with a magnifier), and thin wires don't carry much heat. The GND connection is several times larger than all the other connections combined, so that's how most heat enters and leaves the chip.

The can around the sensor is probably made from stainless steel, which is a relatively poor heat conductor compared to other metals (silver, copper, and aluminum are the three best, in that order), but it's still a metal and transfers heat better than most other materials. The can is small enough that the temperature at any two points on its surface will be the same within a few seconds of a temperature change, and it's reasonable to think the temperature sensor's GND line has some kind of thermal connection to the can.

It's better to submerge the whole can in a liquid if possible, to eliminate heat transfer from the liquid, through the can, and to the air around the non-submerged part of the can. It will also avoid thermal gradients between the top surface of the liquid and the air touching it.

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dhilipkumar
 
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Re: DS18B20 probe inquiry

Post by dhilipkumar »

In our project ,we need to calculate the Board temperature for that we required the High Temp Waterproof DS18B20 Digital temperature sensor.we are working on Medical project so everything should be certified(ie Rohs,Reach,UL,CE).I have searched in Maxim website ,in that they mention only DS18B20+ hold all the Rohs,Reach,UL,CE complaint.
Can you provide the same cable set up with DS18B20+.
We want to know ,Is the stainless cable and Metal hold the Rohs,Reach,UL,CE complaints?

Please do the needful.

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Franklin97355
 
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Re: DS18B20 probe inquiry

Post by Franklin97355 »

We want to know ,Is the stainless cable and Metal hold the Rohs,Reach,UL,CE complaints?
Product 642 has a PTFE jacket and standard heat shrink tubing and carries no certifications.

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dhilipkumar
 
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Re: DS18B20 probe inquiry

Post by dhilipkumar »

Thanks.

Can you customize this part for RoHS, REACH, UL, CE compliant.
(ie) DS18B20+ hold all the RoHS, REACH, UL, CE compliant.

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nalina
 
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Re: DS18B20 probe inquiry

Post by nalina »

IS this part (complete assembly including Maxim sensor, cable, stainless steel tube, heat shrink) compliant for RoHS, REACH, UL / CE?

If not, can you make custom cable assembly with RoHS, REACH, UL and CE compliance?

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Franklin97355
 
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Re: DS18B20 probe inquiry

Post by Franklin97355 »

We do not make these and can not customize them for you, sorry.

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TWAUTERS
 
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Re: DS18B20 probe inquiry

Post by TWAUTERS »

Dear Madam, Sir,

Would it be possible to provide me with a ROHS certificate for the "Waterproof DS18B20 Digital temperature sensor
PRODUCT ID: 381"?
Note: I am looking for a certificate for the entire assembly, not just the sensor itself (maxim integrated products can provide this one). Thecabling and all used materials must comply with ROHS if used in Europe.

Thanks in advance!

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