Temperature measurement

[belinao]

Hello everyone,

I am designing a product that needs to measure the temperature of a liquid sample inside a small brass tube. This tube is about 5 cm long and it is not possible to drill it and insert a temperature sensor inside (which would be the easiest way), the temperature measurement would have to be from outside the tube. I have heard of temperature measurements using (inductance), like having a wire surrounding the tube, but i cannot find any references about that.

Does anyone have a clue about that?

Thanks in advance.

[kayvee]

As brass is a good very thermal conductor, surely the easiest way would to attach a temperature sensor to the outer surface of the brass tube, and assume inner temperature = outer temperature.  Or am I missing something in your application?

[belinao]

Hello,

And thanks for responding.

I think I forgot to mention. The liquid sample is going to be heated kind of fast, about 30 degrees in less than a minute, so it would take some time until the liquid and brass temperatura to become equal.

[kayvee]

Yes, that's complicates it a bit more.  Thermal imaging? ..budget notwithstanding

[pickit2]

I have seen it done, a few years ago, with pressure sensor. The temperature of liquid in a sealed vat was measured by a pressure sensor, and proscess controled by reading. there must have been other things taken into account too for it work.

[TomJackson69]

You still have problem measuring the liquid temperature in the tube even you have fast response temperature probe; because the liquid temperature rise so fast, faster than the tube can transfer the heat. At the time you measure temperature on the tube surface, the liquid temperature already changed.

Is the brass tube come from you or from customer? there is a quick solution if the brass tube come from you. If it is from you than you can make a short tube with threaded on both end (use standard fitting size). Drill a small hole (for the themperature probe) and that should be easy to measure temperature in more real time.

If that is not applicable than use a very THIN type J thermal couple and attach the probe end to the surface of the tube. That way the thin probe will not dissipate that much heat through it.

Try pickit2 idea to see it it works.

Tom

[kayvee]

The OP stated a hole could not be drilled in the tube.  There are a number of variables which have not been stated.  How about measuring and plotting the internal/external differential temperature vs. time, but this method relies on come constants which are unknown (to me, at least).

[Jagi]

TomJackson69 and Pickit2 both have good ideas to solve the problem, but the basic problem is that all sensors have a certain "hysteresis" associated with them. The rate of change of temperature of the liquid in your application is pretty much faster than most sensors can accommodate. If your application can tolerate a delay of 0.5 to 1 second measurement error then your would be very good off using a "K-Type" thermocouple. Of course you'll need a wheatstone bridge or some other configuration to detect the change in temperature based upon the change in voltage/current. The thermocouple can be mounted at the the end of the tube using heat resistant glue (no hole need to be drilled). The following website http://www.smartsensors.com/tcouples.htm has info and sales for the thermocouples.

[belinao]

Hello everyone,

and thanks again for your attention,

I have a thermocouple here with me, so I am gonna try the ideas you suggested. After I have some results, I will post them here, for everyone, okay?

pickit2 do you have any reference material about this method? It would be really helpful. Thanks.

The system that I have seen once is of a milk analyzer. It had a brass tube with a  thin insulated copper wire (as of a inductor coil) surrounding it´s whole extension, and surrounding it there was a Nickel-chromium wire, to heat the liquid. The coil and the resistance (nickel-chromium) were connected in series, at one side was 12V, and the other a MOSFET to control them,  and their connection point (juntcion point) was connected to a comparator input. There was also another comparator connected to the brass tube (?) - that one tricked me most. I was really curious about how it worked, seemed to be some inductive method. Has anyone seem something similar to that?

[pickit2]

seen this and it maybe helpful.
http://www.dongda-sensor.com/en/product-show.asp?id=183

[tAhm1D]

Hi belinao,

You should have a look at the LM35:
Here's the datasheet:
http://www.datasheetcatalog.org/datasheet/nationalsemiconductor/DS005516.PDF

[Ikaros]

To add a good sensor the LM335 (see at http://www.national.com/mpf/LM/LM335.html).

Also ifyou do not want to get involved with analog electronics, use the all digital DS18B20.

Remember to add some heat transfer paste between tube and sensor.