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FGBS-001 température accuracy


gab696
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Question

Hi,

the reported values of my DS18B20 sensors are not very accurate. They are 1.5C° up the reality.

When i measure the voltage on TD wire I only have 2.3V not 3.3V as indicated in the universal sensor data sheet. 

the module is powered 12V 1.5A supply.

This device has not parameters to adjust the offset...

Thanks for your help.

 

 

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It is a digital sensor... It should either be accurate or completely wrong, not 1.5 °C too high, so it is good you post your question here. Something clearly isn't in spec. ... The sensor should have +/- 0.5 °C accuracy. I own several of them and they meet all specifications.

 

Here are a few ideas in random order...

 

- Maybe the DS18B20 is accurate, and your reference is inaccurate? How do you measure this? What is your reference? Where is your reference?

 

- I have heard counterfeit DS18B20 exists, they may have wrong or no calibration and other issues. Check if you source is reputable. For comparison, this is a real DS18B20 from a reliable seller:

 

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- Maybe it is due to the value of parameter 12. I quote from the manual: "Parameter No. 12 Insensitiveness to temperature changes. This is the maximum acceptable difference between the last reported temperature and the current temperature taken from the sensor." - please check it. If you want accurate results, you can poll the device (see advanced tab).

 

- I think by TD you mean pin 2 of the sensor = DQ in the datasheet. It says input logic high should be at least 2.2 V so 2.3 is marginal, but valid. Please also check VCC = pin 3 it should be between 3.0 and 5.5 V.

 

Does this help?

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  • Inquirer
  • Thanks for your quick reply.

     

    I am talking about TP wire on the universal sensor (sorry for the mistake it's not TD). 

     

    on TP i measure only 2.3V. I think it's not normal. but I've checked this on 2 Universal Sensors and i found the same voltage.

     

    The parameter 12 is on 0 to send values on every change.

     

    My reference is a professional temperature sensor.

     

    I've added the universal sensor on the z-wave network with DS18B20 sensors on a small lenght wires (30cm) then i've used longer cables (2m) to place the sensors on the right place. 

     

    Do I need to exclude and include the universal sensor again to calibrate the sensors with the longer cables ?

     

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    On TP you should have 3.3 V +/-0.15 V so 2.3 is not good and below spec for the sensor. If you disconnect the sensor, does the module output at least 3V? Is it possible you did not connect all three pins to the correct wire of the module?

     

    It is a digital sensor. It sends ones and zeroes over a so called "1-wire bus". Length of the cable does not influence the result. Supply does not influence the result, unless it is very noisy. Or maybe, in this case, because it is too low. I have not tested a DS18B20 on a 2.3 V supply...

     

    There is no calibration, because this device is calibrated "for life" at the factory.  You did not say, but I guess your module is a "Fibaro Universal Sensor". But that is not important, because it is a digital sensor so your UBS does not change the accuracy at all... You can try an older Fibaro door/window sensor, if you do not believe me. The newer ones no longer connect to an external sensor.

     

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    You might want to try a 9-Volt battery, because it is essentially "noise free". Then check VCC on pin 3 of your sensor. For testing only, because it will drain your battery in less than a day. If you get better results, you might want to try a different power supply. If it still measures °C too high, you might want to buy another DS18B20...

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  • Inquirer
  • thanks for the very detailed explanation. I will try with others sensors. But can you check your module voltage on TP. I have 2 universal sensors here and the 2 outuput 2.3V not 3.3 has expected.

    thanks !

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    Mine outputs 3.2 V unloaded. It can supply 3.0 V when sourcing 2.5 mA which should be enough for a DS18B20 (or even four, as depicted in the manual of the UBS), because the standby current of the DS18B20 is only a uA and conversion current is 1.5 mA. I do not know how often the UBS samples the sensor, I do not have one connected right now. But I bet it is not very often so current consumption measured with a multimeter should be a lot less than 1 mA. I do not have a sensor attached at the moment, I can redo the measurements with a DS18B20 connected tomorrow. What numbers do you get?

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