>>> This is why you have two sensors, one exposed to the moving water and
>>> one to measure the temperature of the water, but not actually within
>>> the water (e.g. on the outside of a metal pipe).

>> Most of the time you have both sensors in the medium but heat only one.
>> Sensors on the outside of the pipe are usually not so good, as the
>> temperature they measure depends also a lot on the temperature around
>> the pipe. Changing the ambient temperature would then give you some
>> "phantom flow" :)

> The temperature on the outside of a thinwall metal pipe would be barely
> influenced by ambient temperature, unless the water flow had stopped, in
> which case the water temperature will also be affected.

I didn't say it was impossible... I just said "most of the time" :)

Those sensors come basically in three variants:

One is for bigger pipes, where you drill a hole in the pipe and insert a
sensor. Those pipes are usually way to thick for what you say. Besides,
there wouldn't be an advantage in having to mount two sensor heads (one
inside the pipe, the other on the outside). So they just place both sensors
in one head and facilitate mounting and increase precision.

The other type is integrated with a small piece of pipe and is meant to be
placed inline with the pipe you want to measure (you cut the pipe, take out
a short piece, and replace it with the sensor). To make this sensor
versatile, they don't really use very thin wall pipes; for example
something like 1 mm steel walls in a 10 mm diameter pipe. That's also too
much of a pipe wall for your method in most cases, and there's no advantage
to be gained with the imprecision that a sensor on the outside of the pipe
causes. So here they also usually expose the reference sensor to the medium
in this configuration.

Then there are custom sensors, where the pipe system and the sensor are one
design and the medium and other circumstances are well-defined, and there
probably are cases where they do what you describe. But I think they are
the exception rather than the rule.

In order to avoid deposits and other bad stuff, the temperature difference
between the sensors in most cases is not too high; it's in the range of
single digit degree C. Even a 1mm steel wall can introduce a quite visible
error here. You have to take into account that the medium not necessarily
has a uniform temperature; there may be gradients. Measuring outside of the
wall, you introduce a low pass between the medium and the reference
measurement that is not present in the other sensor, and this will create
phantom flow readings -- unless of course you can integrate over a time
that's much longer than the time the pipe system needs to get to
equilibrium. But that reduces the application span of a sensor.

Gerhard

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