Need to measure Ambient temp, product's own temperature won't let me - Page 1
Forum section: electronics / projects

author: atwoz
author_rank: Regular Contributor
timestamp: October 24, 2013, 10:28:01 pm
content: Hello Everyone!I am designing a product that need to monitor the ambient temperature but since the product itself generates some heat I can't get a valid reading. The problem is that the product is small 5cm x 5cm x 5cm and it needs to be all contained inside the product. I have been breaking my head thinking of a way to get a reading of the ambient temperature, but can't come up with anything.One of the biggest limitations is price, since it's a consumer product, every cent counts so I can't really add some fancy cooling method. By the way, the product temperature is 35C average.Am I lost here? Or is there some crazy way of doing what I need?Any thoughts?

author: Fsck
author_rank: Super Contributor
timestamp: October 24, 2013, 10:32:00 pm
content: more of a clue of what it is would be better.could you just sink whatever produces heat in your product to the case,or maybe tacked to some aluminum fins or something through a cutout? i'm assuming that if it maintains ~35C in a sealed enclosure it doesn't produce much heat though..

author: atwoz
author_rank: Regular Contributor
timestamp: October 24, 2013, 10:39:34 pm
content: Hello!Yes, it's a sealed 5x5x5cm ABS enclosure. It has wifi connectivity and needs to read the ambient temperature and send it to a base station. I can't really modify the case that much. I added some holes but they didn't really help. I have some crazy thoughts, like adding the sensor (a thermistor) in the power cable and chaning the connector from a DC barrel jack to a mini or micro USB, using the 2 unused pins for the thermistor. This however involves having a 4 wire cable, and modifying the cable to add the thermistor some where in the cable.My current solution, is to give the product a "tail" with the sensor, so the sensor is outside the enclosure and decoupled from the heat. It works, but looks ugly.Thanks!

author: IanB
author_rank: Super Contributor
timestamp: October 24, 2013, 10:43:34 pm
content: You have to separate the sensor from the heat.You could remove the sensor from the case so it is not contaminated by internally generated heat.Or you could suck air in from outside, blow it over the sensor, and then exhaust the air. (Untidy and complicated, of course.)Possibly you could embed the sensor on the surface of the case and surround it with insulation like expanded foam or polystyrene on the inside. That way the sensor will be much closer to the external temperature than the internal temperature.

author: Fsck
author_rank: Super Contributor
timestamp: October 24, 2013, 10:53:59 pm
content: you said "contained within the product" which is the major problem.the best way to thermally decouple would be to drill a hole in the case, use an encapsulated sensor attached to wiring and put the entire sensor outside of the case, say ~3mm away from the surface of the case, the question would be if you could deal with the bump.as a modification of IanB's method would be to again drill a hole in the case but mount the sensor within the hole, insulating it from the case with some insulation on the case-side of the sensor to help shield it. this could leave you with a case without any exterior topological protuberances.

author: poorchava
author_rank: Super Contributor
timestamp: October 25, 2013, 05:02:49 am
content: Maybe you could measure the thermal resistance of heat source (as it has wifi i assume heat source is some sort of processor/soc) to ambient. I think it will not vary that much. You can also measure the power draw of the circuit and power dissipated as heat.tsrc-tamb=Pd/Rth and you know power dissipation, source temperature and thermal resistance so in theory you could calculate ambient temperature.The question is how much will thermal resistance vary from product to product.

author: nowlan
author_rank: Frequent Contributor
timestamp: October 25, 2013, 05:19:41 am
content: I am thinking aquarium thermometer, which could hang externally.

author: amyk
author_rank: Super Contributor
timestamp: October 25, 2013, 05:21:07 am
content: Quote from: atwoz on October 24, 2013, 10:39:34 pmWho cares if it looks "ugly" if it works? That's the best way to isolate the sensor. You can't break the laws of physics...

author: atwoz
author_rank: Regular Contributor
timestamp: October 25, 2013, 05:49:27 am
content: Thanks everyone, I think I will have to give it a tail.. Calculating the temperature using the power used could be rather hard, I would have to do power measurements in real time since the soc would not use the same power all the time, and other factors would come into play too.

author: poorchava
author_rank: Super Contributor
timestamp: October 25, 2013, 08:12:15 am
content: I assume you can change the circuit, right? Add some sort of current monitor and that's it. I think for the first approximation you could assume that all energy drawn by the device is dissipated as heat, you can correct that in software later.

author: cavlovic
author_rank: Contributor
timestamp: October 25, 2013, 06:40:44 pm
content: Why don't u make a separate chamber inside case, that uses a corner of it, with many small holes for ventilation and a bit of airflow? If budget allows, maybe even put some kind of thin thermal insulator inside the chamber so that inside plastic doesn't transfer heat to the sensor.edit: something like this, but of course much smaller with holes.