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Phil
Phil, Mechanical Engineer
Category: Electrical
Satisfied Customers: 7590
Experience:  Retired electrical contractor, 51 years experience.
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I need help to design & advice on making a sensor to

Customer Question

Hi I need help to design & advice on making a sensor to switching device, hopefully using some off the shelf components.
This is one project with one control box, sensors & controlled devices are remote from the control point, all separate & max of 1.2m away.
sensor 1) temperature sensor accurate to 0.1 degC, 28 to 40 degC range providing a temperature display with a variable setting on/off point within the range 35 to 38 degC, this set point to operate a heating element & LED if temperature is below the set value. Switching to be no less than 10 second duration in order to protect the element.
sensor 2) temperature less accurate to act as a backup to the one above operating a second heating element with LED this one will always be 0.5 degC below that of the one above.
sensor 3) humidity with temperature compensation accurate to 2% providing a digital readout, & switching of a humidifier pump & LED at settable values from 45% to 85%.Again no repeat switching until 10 seconds have passed.
Timer 4) time clock with jumpers 2hourley, 4hourley, & 12hourley, to operate a motor circuit, firstly in one direction then the other using small motors & magnetic micro switches to stop & reset.
OK now I am not an electronics buff, but I am an electrician, so should grasp the basics, I will not understand abbreviations or electronic jargon, patience will be needed & the ability to source parts in the UK.
Submitted: 7 months ago.
Category: Electrical
Customer: replied 7 months ago.
Price offered will be more than £12.00.
Expert:  Tyler Bosch replied 7 months ago.
Hi, if you are stI'll in need of assistance with your sensor design, I am available.
Expert:  Tyler Bosch replied 7 months ago.
My name is*****'ll need a few more details when you get the chance.
Customer: replied 7 months ago.
Hi ask away, I thought the initial £12 will be for sorting out questions to provide a parts list a further £30 for helping me get a working circuit, hope that's OK. obviously a time difference between us but that should not be a problem. the control is for an old incubator that no longer has replacement printed circuits. Ta John
Expert:  Phil replied 7 months ago.
Hello, I see that Tyler has not followed up on this.I will just make this note, then opt out. I designed the accurate to 0.1C temperature control system for the laboratory that the huge mirror for the Hubble space telescope was made in... I know a bit about it. I will make this remark, then opt out, I do not want to get involved in any dissent here. Your incubator would work just fine at plus or minus 1/2C...thats all it takes, it is a herculean task to design a system that controls as close as 0.1C... we kept the mirror itself closer than a 0.1C temperature but only by means of letting the room temperature swing 1 full degree C.... the mirror stayed at the median... that was the secret of it. No one alive today will be able to control the air temperature in your incubator to an accuracy of 0.1C... the histeresis in the electronic sensor circuit and natural variations in the various locations within an incubator prevent that. Change your spec to reasonable figure and this will be do-able.
Customer: replied 7 months ago.
I do not need the heating to be accurate to 0.1 degC but the sensory probe should be this accurate, allowing maybe 0.3 degC as an end result.
Expert:  Phil replied 7 months ago.
Hello again see page 3 for a class AA- class B sensor from Texas instruments and circuits. However your electric heating element has *mass* a key term.. a very key term, and the sticky in this wicket. The heating element will over run and under run the sensor accuracy. http://www.ti.com/lit/an/snaa267/snaa267.pdf Engineering on a project like this generally runs $5,000 or a lot more, and then due to these Histeresis issues, gaining accuracy to the levels you are mentioning can cost a few hundred times more than it is worth.https://en.wikipedia.org/wiki/Hysteresis My experience in this area of overly strict specifications comes from work at IBM, Texas instruments and similar firms. I had become common that the management (not trained engineers) would read a trade journal article that mentions strict specifications on temperature and humidity control... and wrote those into the purchase order. I would comply by issuing a quote for say, 20 million dollars, and mention in my closing that if they changed the spec to allow 1 degree F differential and humidity 5% differential I would bid the job at 500,000 dollars.... There is a similar situation here. Study up the references and wiring diagram I have provided, get back to me, we can go from there.
Expert:  Phil replied 7 months ago.
IBM's RISC chip plant in Burlington VT had this temperature over run and under run problem, that caused them to loose their lead in the chip business at that time...I was called in to remediate the problem. (caused by high mass heating elements that over ran the control cycles among other factors). My solution involved air mixing with pulsed damper modulation, your job here most likely does not warrant even a slight fraction of that expense or complexity. You may want to search incubator temperature tolerances for whatever you are incubating and send me a link to what you discover.

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