Two temperature sensors or one?

Hi,
I am doing some work and i need to justify the use of two temperature sensors. I grow orchids and i am planning on making my own control system yet i need to justify why i want to use two temperature sensors instead of one so can anyone please advise.
Thanks in Advance
Dean

 

I caan see the logic in having a soil and air sensor. I think it depends on what conditions you will be growing them in

 

Hi Dean. You must have some sort of reason in mind to have raised the question.

A control system implies that you are going to have a heat source, and use a sensor to control it, ie switch it on or off. Perhaps you could tell us a little bit more about the proposed system.

You can only use one sensor for control purposes. In the same way that you can only have one driver in a car. If you have two they will try to take the system/car in different directions.

But temperatures are not uniform. I have a heated propagator, and I bought a seperate thermostat with a thermocouple to measure the temperature. Even though my propagator had a lid there were big temperature gradients inside it. The temperature at the bottom of a pot of compost was at least 10C higher than the temperature at the top of the pot of compost, and the air temperature would be different again. Personally I like to have several means of measuring temperature as a way of exploring and understanding the temperature differences you will get in any system you care to think about.

 

Hi Dean
Its not easy to answer your question without a little more info. You mentioned the sensors are for a control system, but you didnt say what its for
I have 3 sensors that work as a group, thats the minimum number that will do the job.

Hi Peter

A differential thermostat wouldn`t be much cop with just one sensor :wink:

 

Hex - how do you have three sensors working as a group?

I assume that you normally use just one sensor to measure the absolute temperature, rather than temperature differences. I know that thermocouples really measure temperature differences, but they usually have a pseudo cold junction, using a thermistor or equivalent in the box, which effectively means that they have a single sensor measuring absolute temperature.

 

Hi Peter
Your right. Mines a homebrew controller as i couldnt find anything to fit the bill off the shelf.
I have one remote sensor dedicated to measuring the air temperature at plant level which connects to a basic thermostat. The output from the basic stat controls the way the differential thermostat compares its 2 remote sensors, one buried a metre deep in the ground and the other mounted high in the roof :wink:

 

I certainly agree you can't have too many temperature measuring devices. It would be most interesting to know how temperatures at different heights below and above ground vary, and also how they change from still nights to windy nights.

In still air you must get considerable gradients - like frost at ground level, but no frost 18 inches higher or so, where the Met Office does its official measuring. Doing a Google on this subject, I found that the reason was due to loss of heat by radiation from the ground. I always assumed that it was because cold air was more dense and therefore settled on the ground, leaving the air above slightly warmer. But its the same reason that you can get frost on a car roof even when the air temperature doesn't go below freezing point. The car roof is loosing heat by radiation (on a clear night), and so its temperature can fall below the temperature of the surrounding air.

 

I haven`t checked the ground temp recently but it was sitting at a cool 13C during the summer.