Return to the Cloudy Nights Telescope Reviews home pageAstronomics discounts for Cloudy Nights members
Get a Cloudy Nights T-Shirt Submit a Review / Article

Click here if you are having trouble logging into the forums

Privacy Policy | Please read our Terms of Service | Signup and Troubleshooting FAQ | Problems? PM a Red or a Green Gu uh, User

Equipment Discussions >> ATM, Optics and DIY Forum

Pages: 1 | 2 | 3 | (show all)
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Instrumenting a Dob with Temperature Probes
      #5942106 - 06/26/13 08:50 PM

The "visualizing airflow" thread got me to thinking about collecting data directly from Dobs about how temperatures are distributed internally and externally.

What you would like is a temperature readout mounted on the scope that can switch among at least three probes, and have the whole kit and caboodle neither weigh nor cost very much. One probe attached to the mirror and two for air temperatures at different locations (which could be moved around to investigate air flow) wouls be the minimum set up for collecting data.

I am not an electronics hobbyist and do not have a ready knowledge base of what is available these days for putting something like this together. A little Googling for kits, or ready-built monitors, turned up little or nothing for the first, and systems costing hundreds of dollars for the second.

The absolute temperature precision is not as important as accurately measuring the differentials between the probes. A low incremental per-probe cost would be good.

Any suggestions?


Post Extras: Print Post   Remind Me!   Notify Moderator  
richard7Moderator
Not Quite
*****

Reged: 11/02/07

Loc: Sacramento
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942143 - 06/26/13 09:13 PM

1. Circuit Specialists.
2. Trailtech.
Or if you're up to some diy. Electronics DIY.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Meep_Esq
member


Reged: 01/02/07

Loc: Dunedin, New Zealand
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942144 - 06/26/13 09:13 PM

y'know I'm sure I read a website where a chappy found a cheap garden thermometer that had an inside and outside temperature probe which he used to compare tube temp with ambient.

Post Extras: Print Post   Remind Me!   Notify Moderator  
highertheflyer
professor emeritus
*****

Reged: 07/08/05

Loc: Ft. Worth, Texas
Re: Instrumenting a Dob with Temperature Probes new [Re: Meep_Esq]
      #5942248 - 06/26/13 10:29 PM

Each of the home made reflectors here is equipped with this very simple setup and fun to monitor.
It's pretty easy to find small indoor/outdoor digital thermometers for this use.
With one registering in the cooling fan flow, and one attached/buried/surface glued against the back of the mirror.
The mirror monitor probe should be encased in a cotton lined plastic housing to insulate the mirror temp probe from the outside temperature affects.

Jim


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: richard7]
      #5942451 - 06/27/13 02:34 AM

Thanks, this is exactly what I was looking for.

The first site has several interesting solutions.

Simply buying several of these LCD temp units (http://www.circuitspecialists.com/dtm0503.html), and making a simple switching mechanism to connect to different probes (patch panel maybe?) would be an inexpensive solution for a non-electronically adept individual.

(I build computers from parts, but have never become familiar with building circuits which requires considerably more expertise.)


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942454 - 06/27/13 02:43 AM

I would suggest using an inexpensive temperature sensor such as the LM35 http://www.ti.com/lit/ds/symlink/lm35.pdf. Band gap temperature sensors such as the LM35 don't require much signal conditioning and have a linear voltage output proportional to temperature that can be read by a voltmeter. There are probably inexpensive temperature USB modules that would make it easy to collect data with a laptop PC and if one had Labview http://www.ni.com/labview/ would be, for me, really easy and quick to put a program together to collect and analyze the results.

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
derangedhermit
Pooh-Bah


Reged: 10/07/09

Loc: USA
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942472 - 06/27/13 03:04 AM

Quote:

Simply buying several of these LCD temp units (http://www.circuitspecialists.com/dtm0503.html), and making a simple switching mechanism to connect to different probes (patch panel maybe?) would be an inexpensive solution for a non-electronically adept individual.



That looks like one complete unit - temp probe and display. Buy as many as you need - I'm not sure what the patch panel would be for.

It doesn't give accuracy or precision, though, that I could find. It may not do better than 1 degC.

I'd like to do the same thing you're asking about - I want better than 1F accuracy.


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: derangedhermit]
      #5942515 - 06/27/13 04:20 AM

Quote:


It doesn't give accuracy or precision, though, that I could find. It may not do better than 1 degC.

I'd like to do the same thing you're asking about - I want better than 1F accuracy.




Since we are looking for differential measurements wouldn't it be resolution less than a degree and not accuracy or precision? Also linearity.

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5942535 - 06/27/13 05:08 AM

Here is my write up of measuring temperature at various points in an SCT exposed to the sky:
SCT Temperature measurements.

I used ds1820's and custom code in a microcontroller. These devices are simple and serially addressable individually - so you can have as many as you want on a single two-wire connection. The temperature precision and accuracy were adequate for this study - and could be improved by calibrating them carefully.

A key issue in studies like these is the actual thermal/radiative environment where the 'scope is used. The cooling behavior in a wide open field with very clear skies would be very different from a tree-shaded area, or in an observatory dome. The radiative losses don't just happen on the front lens, but on the entire system including the mount.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: derangedhermit]
      #5942707 - 06/27/13 09:11 AM

Quote:

Quote:

Simply buying several of these LCD temp units (http://www.circuitspecialists.com/dtm0503.html), and making a simple switching mechanism to connect to different probes (patch panel maybe?) would be an inexpensive solution for a non-electronically adept individual.



That looks like one complete unit - temp probe and display. Buy as many as you need - I'm not sure what the patch panel would be for.o do the same thing you're asking about - I want better than 1F accuracy.




To just have one readout mounted on the scope for multiple probes, not a whole collection of them. Yes, it would involve chopping off the probes and ending up with surplus LCD panels, but they are cheap.

Getting two units would allow direct comparison for consistency between the units - much more important than absolute accuracy since it is really the temperature differentials we are interested in.


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5942716 - 06/27/13 09:16 AM

Quote:

Quote:


It doesn't give accuracy or precision, though, that I could find. It may not do better than 1 degC.

I'd like to do the same thing you're asking about - I want better than 1F accuracy.




Since we are looking for differential measurements wouldn't it be resolution less than a degree and not accuracy or precision? Also linearity.

Don Clement




Precision, but not accuracy (the absolute temperatures are really not of interest).

The LCD units I was looking at actually read-out to 0.1F, but testing would be required to show how reproducible the measurements are between units.


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5942765 - 06/27/13 09:42 AM

Quote:

I would suggest using an inexpensive temperature sensor such as the LM35 http://www.ti.com/lit/ds/symlink/lm35.pdf. Band gap temperature sensors such as the LM35 don't require much signal conditioning and have a linear voltage output proportional to temperature that can be read by a voltmeter. There are probably inexpensive temperature USB modules that would make it easy to collect data with a laptop PC and if one had Labview http://www.ni.com/labview/ would be, for me, really easy and quick to put a program together to collect and analyze the results.

Don Clement




Thanks, this the sensor you recommend looks like a very good candidate for the sort of low cost of entry monitoring.

With the 0C + 10 mV per C output I would just need a millivolt meter reading to 400 mV or so (to reach 40 C, as high as I would be likely to ever measure). Unfortunately I note that inexpensive ubiquitous digital LCD panel voltmeters seem to be limited to 200 mV (must be a widely used chip spec).

For a really serious monitoring approach a digital system would clearly be best, but the total "cost of entry" (money plus learning time) is higher.

Would the trial version of Labview work? Otherwise a license is $999, a non-starter unless I was getting into really serious data collection for other projects.


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942825 - 06/27/13 10:16 AM

Carry,

I mentioned the LM 35, http://www.amazon.com/microtivity-Precision-Centigrade-Temperature-Sensor/dp/... an analog temperature sensor that has a linear output of 10mV/*C . The LM35 is pretty easy to set up, even for the electronics challenged. All you need is an unregulated DC voltage source of 4V to 20V and a way to read the analog output voltage. A 9V battery would do fine. An inexpensive digital voltmeter can be used for readout such as http://www.harborfreight.com/7-function-multimeter-98025.html A simple analog switch would be used between sensors. Since you are interested in differences between sensors, the analog switch allows the same DAC (the digital voltmeter) to be used for all sensors. I believe this scheme has the simplicity, precision, and resolution you are looking for.

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5942848 - 06/27/13 10:31 AM

Quote:

With the 0C + 10 mV per C output I would just need a millivolt meter reading to 400 mV or so (to reach 40 C, as high as I would be likely to ever measure). Unfortunately I note that inexpensive ubiquitous digital LCD panel voltmeters seem to be limited to 200 mV (must be a widely used chip spec).

For a really serious monitoring approach a digital system would clearly be best, but the total "cost of entry" (money plus learning time) is higher.






on the spec sheet it states if there is a high difference in air temp and surface temp you have to epoxy the leads to the surface so the device won't read air temp.

also there is a way to turn it to F. they show a circuit for the change on the spec sheet.


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5943375 - 06/27/13 04:21 PM

I think this slightly more expensive multimeter from HF would be needed:
http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html

It gives (says the manual) 0.1 mV accuracy in the 200 mV range (20C/68F) but is still 1 mV in the 2000 mV range (+/- 0.1C, taking the temp up the maximum of interest).

The 7-function one lists 1% accuracy in the 2000 mV range (i.e. 20 mV or 2C, or maybe 40 mV/4C).


Post Extras: Print Post   Remind Me!   Notify Moderator  
Arjan
super member


Reged: 01/21/09

Loc: Netherlands
Re: Instrumenting a Dob with Temperature Probes new [Re: Pinbout]
      #5943384 - 06/27/13 04:27 PM

Quote:

also there is a way to turn it to F. they show a circuit for the change on the spec sheet.




Who would want this?


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5943388 - 06/27/13 04:28 PM

couldn't you just shoot the mirror with IR thermometer that way you can measure the edge, the back middle, front middle... +/-2, do you really need to read more?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Instrumenting a Dob with Temperature Probes new [Re: Arjan]
      #5943389 - 06/27/13 04:31 PM

Quote:

Quote:

also there is a way to turn it to F. they show a circuit for the change on the spec sheet.




Who would want this?




you know who else would like to do that...My Mom


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: Pinbout]
      #5943454 - 06/27/13 05:11 PM

Carry,

There is always the old standard (and still is)HP3458 8.5 digit DMM.

http://www.youtube.com/watch?v=yBb24pCUDpk


When I was a teenager I built a Heathkit IM13 VTVM
http://www.ohio.edu/people/postr/bapix/vtvms.htm

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: Pinbout]
      #5943735 - 06/27/13 08:09 PM

Quote:

couldn't you just shoot the mirror with IR thermometer that way you can measure the edge, the back middle, front middle... +/-2, do you really need to read more?




You can do the back and side of the mirror with an IR thermometer, but you can't do the front unless you put an opaque patch on the mirror surface, and you can't take air temperatures. A patch stuck temporarily in the Newtonian central "blind spot" woud be an option for getting mirror front surface temperatures.

Putting a probe on the mirror front surface as similar issues - probe covered with a bit of foam or cork help down by a clip.

Consider the back exhaust fan in a closed mirror box with a front baffle, which has been recommended for all night cooling (ensuring mirror tracks ambient air, scrubbing boundary layers, preventing dew formation, etc.).

In a closed box taking IR measurements is difficult anyway, but we want to track the air flow, using the air temperature as a proxy, and a physical feature of primary interest which the IR thermometer cannot easily measure. A temperature probe above and outside of the mirror box gives ambient, a probe just inside the baffle where the air flows off the mirror surface, an insulated probe on the top side edge and on the center back of the mirror, and perhaps one just in front of the exhaust fan could give a pretty good view of the air circulation and cooling/warming that is going on.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Tom and Beth
Post Laureate


Reged: 01/08/07

Loc: Tucson, AZ
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5943835 - 06/27/13 09:17 PM

Would something like THIS work for what you have in mind? If nothing else, the same error would be in each of your readings.

I use these for rough temp readings, such as the underside of the roof in the observatory, drive motor temp, heat issues with Observatory Computer ...


Post Extras: Print Post   Remind Me!   Notify Moderator  
Dale Eason
member


Reged: 11/24/09

Loc: Roseville,Mn.
Re: Instrumenting a Dob with Temperature Probes new [Re: Tom and Beth]
      #5943936 - 06/27/13 10:37 PM

Quote:

Would something like THIS work for what you have in mind? If nothing else, the same error would be in each of your readings.

I use these for rough temp readings, such as the underside of the roof in the observatory, drive motor temp, heat issues with Observatory Computer ...





The problem to watch out for with those non-contact thermometers is that glass even uncoated is reflective in IR. So they don't read the temp of the glass but what is reflected by it. I solved that problem by putting a piece of tape on the back of the mirror and pointed it at that.

Dale Eason


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: Tom and Beth]
      #5944143 - 06/28/13 01:55 AM

IR thermometers are useful, I have one that I have used to monitor open back Dob mirror temps.

They do not however take air temperatures, an important aspect of monitoring cooling design; air temps also allows monitoring the air circulation - it is not only directly relevant measurement, it is also a proxy for monitoring the air flow through the system.

Also, not all parts of telescope systems are directly accessible to IR monitoring (closed Dob boxes, telescope tubes).


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5944149 - 06/28/13 01:59 AM

I am sure there are five figure lab units that also make good voltmeters.

Ah the late lamented HeathKit! The website assets "the news of my death is greatly exaggerated" but that is so far the only sign of life.


Post Extras: Print Post   Remind Me!   Notify Moderator  
FHarry
member


Reged: 04/20/07

Loc: Bavaria
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5944170 - 06/28/13 02:38 AM

why not use the one-wire sensors? Super-small in size and cheap as well. If you would not like to put the necessary sensors to a bus-cable (actually, a simple cable like the Cat5 or so and all sensors connected as a bus to this cable) then you even could use the one-wire buttons called iButton. Sensor with battery in a small housing, like a coin. So you could equipp your scope with this "buttons" wherever you like. In the end, you pick them out and put them into a little reader unit and read the data. Buttons can be configured to take a measurement all n-minutes or so.

Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5944486 - 06/28/13 09:52 AM

Quote:

I am sure there are five figure lab units that also make good voltmeters.




Yes Kiethley instruments come to mind http://www.keithley.com/

Quote:

Ah the late lamented HeathKit! The website assets "the news of my death is greatly exaggerated" but that is so far the only sign of life.




I am sure there are many others that believe that the vacuum tube is dead. There are also the few of us still channeling those dead vacuum tubes for the clairvoyant sounds of long deceased vinyl.

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5944502 - 06/28/13 10:03 AM

Quote:


I am sure there are many others that believe that the vacuum tube is dead. There are also the few of us still channeling those dead vacuum tubes for the clairvoyant sounds of long deceased vinyl.

Don Clement






but the day has long been dead that you drive to the 7-11 to check out if the tubes are blown.

I still have my gt101 accoustic amp


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: FHarry]
      #5944556 - 06/28/13 10:39 AM

Quote:

why not use the one-wire sensors? Super-small in size and cheap as well. If you would not like to put the necessary sensors to a bus-cable (actually, a simple cable like the Cat5 or so and all sensors connected as a bus to this cable) then you even could use the one-wire buttons called iButton. Sensor with battery in a small housing, like a coin. So you could equipp your scope with this "buttons" wherever you like. In the end, you pick them out and put them into a little reader unit and read the data. Buttons can be configured to take a measurement all n-minutes or so.




Interesting. I did not know about the one-wire technology.

This sort of low cost, low performance digital microlan looks promising. Here is a good introductory page from a hobby component provider:
https://www.hobby-boards.com/store/pages.php?pageid=4

I found this site interesting:
http://www.digitemp.com/
Temperature monitoring software for Linux, I have two EEPC netbooks running EEBuntu that might be just the thing for this. This would solve the controller/monitor problem with looked like the most costly/time consuming part of any digital solution.

The iButtons look a lot more expensive on a per sensor basis.

Are there any books out there providing a good grounding in 1-wire network design and set-up; e.g. how to use these little digital switches and other network components I see? Something along the lines of the Arduino Cookbook for example?

This looks like a cheap and convenient and versatile enough technology, with a low enough cost of entry, to merit closer examination.


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5944591 - 06/28/13 11:00 AM

The ds1820's that I used in my sct cooling study mentioned earlier also operate in one-wire mode, allowing multiple sensors to be connected on the same input and individually addressed. The newer version, ds18b20, has 12-bit precision, or 0.06C. This sensor family worked well for the specific application of measuring temperature at different points in a telescope, with no analog signals floating around. But I was comfortable writing custom PIC code for the whole thing.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pinbout
Postmaster
*****

Reged: 02/22/10

Loc: nj
Re: Instrumenting a Dob with Temperature Probes new [Re: freestar8n]
      #5944722 - 06/28/13 12:35 PM

shouldn't be a problem to wire into an arduino or even a basic stamp?

Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5945086 - 06/28/13 03:52 PM

My thinking

Repeatability between probes is very important. So... When it comes to repeatibility as well as accuracy, it it hard to beat the old fashioned thermocouple. Thermocouple wire is relatively cheap so you can make your own probes. The voltage output is quite low so an amplifier of some sort is needed. I have an old Fluke multimeter with a seperate module that fits any multimeter and outputs 1 mv per degree either F of C. The Fluke has a millivolt scale that reads to 0.1 mv so this is 0.1 degree.

With other sorts of sensors, the repeatability is questionable between sensors.

As far as using an IR device to measure the temperature, the amount of IR emitted by a body depends on both its temperature and its emissivity so one cannot make accurate measurements of different surfaces without calibration.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: Jon Isaacs]
      #5945264 - 06/28/13 05:41 PM


Thermocouples are non-linear. One must provide a lookup table to linearize thermocouples. Some of the old Fluke DVM did do that for K thermocouples IIR. There is a simple way to match the output on LM35 linear temperature sensors using 10K trim pots.
See: http://www.investigacion.frc.utn.edu.ar/sensores/temperatura/AN-460.pdf

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
freestar8n
Post Laureate
*****

Reged: 10/12/07

Re: Instrumenting a Dob with Temperature Probes new [Re: Pinbout]
      #5945266 - 06/28/13 05:41 PM

Quote:

shouldn't be a problem to wire into an arduino or even a basic stamp?




Yes - many chips and environments would work for development - but my main point is that custom work is helped by being able to write code specific to the needs of the project.

As for accuracy, when it comes to air measurement I think a key limitation is a good shroud for the sensor so it isn't heavily affected by radiation from nearby surfaces and the sky - and is a true measure of the local air temperature rather than the radiative properties of the sensor in its environment.

Frank


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5945357 - 06/28/13 06:44 PM

Quote:


Thermocouples are non-linear. One must provide a lookup table to linearize thermocouples. Some of the old Fluke DVM did do that for K thermocouples IIR. There is a simple way to match the output on LM35 linear temperature sensors using 10K trim pots.
See: http://www.investigacion.frc.utn.edu.ar/sensores/temperatura/AN-460.pdf

Don Clement




It is true that thermocouples are nonlinear over their full range but the full range of a K type is up to 1370C. Ktypes are reasonably linear to a 1000C. In the range of interest here they are linear and in any event these measurements are relative rather than absolute.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
don clement
Vendor (Clement Focuser)


Reged: 02/02/11

Loc: Running Springs, California
Re: Instrumenting a Dob with Temperature Probes new [Re: Jon Isaacs]
      #5945444 - 06/28/13 07:31 PM

Omega is a good place to get thermocouples,connectors and other types of temperature probes. http://www.omega.com/temperature/tsc.html

Don Clement


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: don clement]
      #5945459 - 06/28/13 07:40 PM

Quote:

Omega is a good place to get thermocouples,connectors and other types of temperature probes. http://www.omega.com/temperature/tsc.html

Don Clement




That's where I get our stuff for the lab. We have a wonderful old digital thermocouple reader, it's 12 channels, switchable, reads to 0.1 degrees. But I think a data logger would be slick for this app. There are some inexpensive USB data loggers out there that would do the job.

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: Jon Isaacs]
      #5945943 - 06/29/13 03:03 AM

Quote:

Quote:


Thermocouples are non-linear. One must provide a lookup table to linearize thermocouples. Some of the old Fluke DVM did do that for K thermocouples IIR. There is a simple way to match the output on LM35 linear temperature sensors using 10K trim pots.
See: http://www.investigacion.frc.utn.edu.ar/sensores/temperatura/AN-460.pdf

Don Clement




It is true that thermocouples are nonlinear over their full range but the full range of a K type is up to 1370C. Ktypes are reasonably linear to a 1000C. In the range of interest here they are linear and in any event these measurements are relative rather than absolute.

Jon




Putting the Omega type K specs into a spreadsheet I see that average mV/C over the range of -10 to 50 C is 0.0403; over the more restricted range of interest 5 to 35 C it is exactly the same.
The maximum degree differential range shown (-10 to 50 C) is 0.039 to 0.042.
The average mV/C per temp decade:
-10 : 0.039
0 : 0.040
10: 0.040
20: 0.041
30: 0.041
40: 0.041

And it appears that over this entire range, if you used the average value of 0.040 mV/C the largest deviation would get anywhere would be 0.05 C, and the largest degree-to-degree variation 0.01 C.

From this I conclude that the output is effectively linear in this range, below the error of a typical voltmeter. Of course the voltage needs to be boosted about 100 fold for most milli-range voltmeters.


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5945975 - 06/29/13 03:57 AM

OTOH - significantly more work seems necessary to incorporate a thermocouple into device that actually reads temperature from multiple probes.

Looking on-line I see lots of sources descrbing how to make a thermocouple thermomenter, that focus on making the thermocouple probe itself, which can be bought for a few dollars, then describes using equipment costing thousands of dollars on a lab bench to actually use it. Rather topsy-turvy in emphasis.

The 1-bit technology at least has ready made components, wiring, and free Linux software designed to monitor tempertures.

The LM-35 IC package puts out enough voltage to monitor directly with a half-decenbt multimeter.

To use the thermocouples effectively it looks like getting into Arduino building and programming (I an an assembly programmer from way back so this hardly fazes me, but it is more work).

BTW - isn't compensating for the wire resistance of the wire from the thermocouple to the amp a problem?


Post Extras: Print Post   Remind Me!   Notify Moderator  
tjmck
super member


Reged: 10/06/07

Loc: Winnipeg, Manitoba, Canada
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5946087 - 06/29/13 08:03 AM

Look at Adafruit Industries. They have all the solutions you are looking for. If you want to go with K type thermocouples - $10 per, DS18B20 - $4 per, TMP36 - $2 per. Most popular controllers including Arduino and Rasperry Pi, Thermocouple Amplifier MAX31855 breakout board. They also have a data logger sheild for Arduino that incorporates a real time clock and SD card for data storage. Most importantly, they have tutorials on how to setup all of the items and a support community to solve issues.

As for measurement precision, a Class 1 K-Type thermocouple is only rated 1.5C Accuracy from -40C to +375C. For the DS18B20 0.5C Accuracy from -10C to +85C.

There a lots of ways to do what you want. I recently assembled a lab freezer temperature mapping system using an Arduino, Data Logger Shield and 16x DS18B20s that was programmed to take measurements off all 16 sensors every 6 seconds. The data was stored as a CSV file and easily read into a spreadsheet. You could also add a LCD and keyboard or touch screen and even run the whole thing from batteries if you wanted.

Tom


Post Extras: Print Post   Remind Me!   Notify Moderator  
Jon Isaacs
Postmaster
*****

Reged: 06/16/04

Loc: San Diego and Boulevard, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: careysub]
      #5946429 - 06/29/13 11:42 AM

Quote:

BTW - isn't compensating for the wire resistance of the wire from the thermocouple to the amp a problem




In general the amp will have a high input impedance so current flow in the wire should not be a problem. I buy my thermocouple wire in rolls and make my own...

How much accuracy are you hoping for?

Jon


Post Extras: Print Post   Remind Me!   Notify Moderator  
careysub
Carpal Tunnel
*****

Reged: 02/18/11

Loc: Rancho Cucamonga, CA
Re: Instrumenting a Dob with Temperature Probes new [Re: tjmck]
      #5946827 - 06/29/13 04:03 PM

Quote:

Look at Adafruit Industries. They have all the solutions you are looking for. If you want to go with K type thermocouples - $10 per, DS18B20 - $4 per, TMP36 - $2 per. Most popular controllers including Arduino and Rasperry Pi, Thermocouple Amplifier MAX31855 breakout board. They also have a data logger sheild for Arduino that incorporates a real time clock and SD card for data storage. Most importantly, they have tutorials on how to setup all of the items and a support community to solve issues.

As for measurement precision, a Class 1 K-Type thermocouple is only rated 1.5C Accuracy from -40C to +375C. For the DS18B20 0.5C Accuracy from -10C to +85C.

There a lots of ways to do what you want. I recently assembled a lab freezer temperature mapping system using an Arduino, Data Logger Shield and 16x DS18B20s that was programmed to take measurements off all 16 sensors every 6 seconds. The data was stored as a CSV file and easily read into a spreadsheet. You could also add a LCD and keyboard or touch screen and even run the whole thing from batteries if you wanted.

Tom


]

The accuracy figure quoted for the Class 1 Type K might be the absolute accuracy over the entire range, I am only interested in relative precision in 5C to 35C (41F to 95F) roughly (OC to 40C absolute limit - 32F to 104F).

Thanks, Omega had K-type thermocouples for only $3.60 per (pack of 5) and the voltage output datasheet in the range of interest suggested precision of 0.05C or better - but of course that is only one component in overall system precision.


Precision of 0.5 C or better between sensors (not overall accuracy, which could be less) would be what I am looking for. What is critical is the precision of the differential between sensors at a particular time. Neither absolute accuracy nor stability of individual sensor reading over long periods is important, they just all have to agree closely at the same time.

It sounds like I should get into Arduino at some point, just because. Not sure how soon I want to tackle it though.


Post Extras: Print Post   Remind Me!   Notify Moderator  
Pages: 1 | 2 | 3 | (show all)


Extra information
12 registered and 18 anonymous users are browsing this forum.

Moderator:  ausastronomer, richard7, Starman81 

Print Thread

Forum Permissions
      You cannot start new topics
      You cannot reply to topics
      HTML is disabled
      UBBCode is enabled


Thread views: 1098

Jump to

CN Forums Home


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics