Temperature monitoring

Omega sells industrial process control stuff that will do it, but its usually overpriced.

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Michael A. Terrell
Central Florida

How long does the sensor cable have to be? Does the adjustment have to track a scale or can it be set by trial and error until it's at the desired set point? How much current do the contacts have to handle? Do you need a display of set point and/or actual temperature? How much hysteresis between on/off conditions?

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The first thing that popped into my mind was a programmable thermostat like I have in my house. Mine is a Hunter model that cost me about $17. It is run on a single AA battery (or maybe it's AAA, I don't remember). The temp can be set in either F or C and when the temp gets

1 degree below what is set then the unit switches on the furnace until the temp is 1 degree above the set temp and then it turns off again. Wouldn't be too difficult to remove the sensor to make it remote I don't think.

Cheers, Lawrence

That's tough.

What are you trying to use it for? You could try a Dallas DS1822 or related part and a microcontroller such as a Pic or AVR, that hardware will be cheap and should meet your specs but it will require some firmware to be written.

There are very few mechanical thermostats which can work that close in differential. The only one I can recall used a mercury thermometer with leads embedded in the glass tube at the desired temperature.

Even a design with electronics needs careful design to get that close of a differential. However I wonder if the spec. needs to be that accurate since this appears to be an anti freeze system.

The Dallas sensors I refer to have a straight digital output so there's no fiddly analog design to deal with. Still, that is quite a tight range.

If he provides some more detail about the intended application it will be easier to give suggestions.

On Apr 11, 3:07 pm, James Sweet wrote:

This is the application: A nearby apple orchid apparently is in danger of having the trees damaged due to an early Spring frost. Every year around May they have gotten this box out. Inside is a temperature monitoring control unit and what appears to be a low power radio transmitter. There is also a probe, a thermistor I suspect mounted inside a small piece of PVC pipe which is connected to about 50 feet of shielded cable. This is placed out in the field. The transmitter pages out via an antenna which is mounted in the orchard. The whole business is powered by a 12.0 volt wet cell. This antenna was described to me as "being about ten feet tall". So I'm assuming its a CB unit of sorts but until I run an SWR test I won't know for sure. The temperature control unit has a switch with positions from 26 to 35 degrees F. You select the temperature you want to alarm at and a contact closure occurs. This powers the transmitter and sends a signal to the pagers. The critical temperature is 31 degrees F. This control unit was damaged by water and is useless. I tried to trouble shoot it but there are house numbers on the semiconductors and the manufacturer is out of business. I took a chance and connected the transmitter to a CB antenna and I have determined that the RF portion of this Rube Golgberg works. However, since I didn't like the idea of hours of transmitter on time when I rebuild it I intend to use a one shot timer after the new temperature control unit so that I'll get a 10 second closure and then power will be removed from the transmitter. When the temperature warms the timer will reset and be ready for the next shot. Apparently during this two to three week period a dip to 31 degrees requires they get some huge fans out to warm the trees. This is the reason for the page. As much as I'd like to spend the time designing a system to do this the customer is a commercial orchard and probably will be willing to spend a few bucks to just have the situation resolved. I hope this clarifys things a little. Thanks, Lenny.

I doubt you need accuracy of 1 degree, but you should be able to achieve something close. Have a look at this part

It's a complete programmable thermostat on a chip.

If you want to avoid programming, have a look at this one

1 degree accuracy, linear output in degrees F, all you need then is a comparator, some precision resistors and a precision voltage reference.

may be a starting point.

Lenny -

If I understand the application, you don't need accuracy, but you do need repeatability. In other words, if you want the unit trip when the true temperature is 31 degrees, but the unit tells you that the temperature is 27 degrees - who cares? Just set the trip point indicator at 27 degrees. Or even (dare I say it?) put a new label on the trip point switch. Essentially what you are doing is calibrating the unit.

Repeatability is a different animal. To follow the above example, if the true temp is 31 degrees, and the unit says 27 degrees, the next question is - what does it say tomorrow? And the day after? Does it still say 27 degrees? If it consistently says 27 degrees, then you have what you need. But if the reading drifts from day to day, you are in trouble.

Did that make sense? Accuracy, repeatability, and resolution are not the same thing.

Bill Jeffrey

Probably can be done as a one-off design for under $150.

PICAXE microprocessor (under $10) DS18B20 temperature sensing chip ($5) one 2n2222 transistor (under $1)

78L05 regulator (under $3) one relay (depends on the load) about 60 lines of code (priced by the hour or by the project)

The PICAXE would read the DS18B20 chip every minute (or whatever) and compare the temperature to the set point. If the temp is low, it would operate the relay to power the transmitter for 10 seconds (or whatever). At this point, the PICAXE can set a flag that the alert has been sent, and continue to check the temp every minute. If the temp is still low after 15 minutes (30 minutes, 1 hour, whatever), it can turn the transmitter on again (same 10 seconds or longer).

If you want spares, you could duplicate the circuit as many times as you wish.

John

Email for details: picaxe at jecarter dot com