DS18B20 temperature sensor - waterproof version and air temperature measuring

Posted by: @kutgv

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I was in a discussion with another engineer regarding the use of the waterproof version (sealed in that header) for air temperature measuring. For me that should not be a problem but am I right?

In my tests, the result seemed to be the same as the other air temp sensors ...

One would expect the coating to follow the temperature of the ambient air. So the worst performance would be if that coating were an effective insulator (which would be an unlikely choice of the manufacturer) in which case the sensor would simply register temperature changes "slower" than an uncoated one.

@will tx will, that was my assumption as well ... now I can tell my boss he was 'less right' than me :p

Mario

@kutgv 

Well, I'm not an engineer, so just tell him he's been "out-voted" 🙂

Hi @kutgv,

@Will is correct in suggesting insulation could increase the time for temperature changes to register, but that is not the only issue when considering 'accurate' measurements.

  The classic problem with electrical sensors (excluding thermocouples which have their own specific problems), is that they require an electrical current to flow through the sensor, and some of that electrical power flow is turned into heat inside the sensor.

Obviously, the internal circuit of a thermal sensor will be designed to minimise that heating effect, but it will not be zero. Furthermore, sensors like the DS18B20 include some 'extra' circuitry to make it more convenient to use, and that 'extra' circuitry will increase the electrical power needed and hence the heating effect.

If the sensor is in good thermal contact with the object (e.g. the surrounding air), then this heat will be efficiently conducted away from the sensor and the actual sensor temperature will be very close to that of its environment.

But if the sensor is thermally insulated from the environment, the heat from the electrical current flow will significantly raise the temperature of the 'active' part of the sensor, and it will return that higher temperature in its readings.

In many applications, the difference of temperature is small, and can be ignored or compensated for as a small offset.

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Thus the billion dollar question is: "Will it be a problem in my particular situation?"

Unfortunately, that is something that often can only be accurately determined by some careful experiments and measurements. It is possible to measure the voltage drop and current flow, and hence determine the heat dissipation. Determining the thermal resistance between the active part in the centre of the sensor and the environment can be much harder, although data sheets may include some estimates. Furthermore, in the case of the environment being air, it will depend on whether that air is static or moving past the sensor. 

Best wishes, Dave