LED digital panel thermometer - advise for a sensor?

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geb

Newbie level 4
what is red water found in thermometer

Heillo,
I've been trying to find a reasonably priced pair or LED DPMs for passively monitoring fahrenheit in a water cooling loop.

All I find are very expensive industrial or laboratory units.
I don't need programmability or control functions. Simply a thermometer.

I've found a few of this sort of unit
http://webtronics.stores.yahoo.net/led9vinpowsu.html

I've asked at three forums prior to this. I've asked each of the parts suppliers.
I've searched deep into the web for information on how to mate a transducer to one of these DPMs for measuring temp.

I'm astonished and extremely frustrated that this seems so difficult to achieve.
These measure millivolts. A transducer produces millivolts. Some transducers are more linear than others. Some are more/less accurate. I don't need much accuracy. I'm trying to simply monitor gross changes in temp in this cooling loop.
I have to use red LED dpms.

Can anyone help me figure out what type and value of transducer to use with the above unit and perhaps tell me if it's possible to wire them together? I'm not competent enough to design a circuit. It appears that the above unit is already a designed circuit for displaying voltage in four red led digits. Is this not correct?

Help me!!!!!!

Thank you so much for any assistance.

rowokii

Member level 4
panel termometer

There are plenty of cheap LCD thermometer with probes... try searching for "digital aquarium thermometer"... sells for less then $10 unless you go with fancy models. If you really want LED digital panel meter, you could use the$10 0-200mV range LED meter and connect a analog temp sensor like LM34.

http://www.national.com/ds/LM/LM34.pdf

It sends out 10mV/F analog output. Look at figure 2 of the data sheet. It gives simple circuit that does what you want. You might have to buffer the output with a opamp if your meter loads down the sensor too much. You want what's called unity gain or voltage follower - look at first figure on this web page. It shows you what you want.

http://ourworld.compuserve.com/homepages/Bill_Bowden/opamp.htm

Cheers

geb

Newbie level 4
dpm into thermometer circuit

rowokii

Thank you very much for that. Since I have limited skill with circuits, your suggestion is the first that has offered a glimmer of hope that I might be able to do this.

Cyberdyne gauges offers an LED water temp gauge for automobiles, with sender, for about 60 USD delivered. A pair for 120 USD. That's the best deal I've found so far. I have little doubt that this gauge uses something quite similar to the ten dollar display I linked above, and another ten dollars for the sender.

Under other circumstances I'd happily pay that to put this gauge in my automobile. But for this project I squirm at the thought of paying more for the meters than any other part (almost beyond the combined cost of all else in fact).

Is the op amp used to "trim" the circuit for accuracy? Do you expect that I might get within at least five degrees F by straightforwardly combining the LM34 with this DPM? What would be a suitable range/part number for the op amp you mentioned? And lastly, once this is accomplished, is it difficult to swap out the LM34 with a comparable transducer for measuring PSI?

Thanks so much again.

rowokii

Member level 4
cyberdyne transducer

geb,

I just realize you need to divide the output by 10 to use with 0-200mV meter. I believe they have 0-200mv meters that has built in dividers that are selectable so you can read 0-2V. This should give you temp range of 0-200 degrees F. How hot do you expect your measurement to get?

I mentioned opamp to buffer the output of the sensor since I wasn't sure how much load the sensor can drive. I re-read the datasheet and it looks like it might be able to drive it without buffering. So, I would test it out without the op-amp first.

You should not have to trim the sensor... it has something like +/-2 F accuracy over the entire temp range, so you should be good.

You do need to think about how to mount the sensor and how to protect it and the sensor wire... liquids won't be very good for them.

Good luck.

geb

Newbie level 4
thermometer with panel metercircuit

rowokii

This is ultimately for a coolant circuit for a TIG welder. I may actually exceed 200 degrees, but not by much. The catalog spec for the LM34DZ-ND gives a temp to 212, so I guess 200 limit isn't sacrificing much for this device. Other versions of the LM34 go to 300F.

In the spec pdf for that led meter there is a table to..
"Select the max. measuring range and decimal point jumping as follows.."

The accompanying chart suggests..

max measured voltage/shortcircuit voltage divider section/decimal pt fixed method

200mv-------/------shortcircuit .2v joint ---------/--------- jump P3
20v---------/-------shortcircuit 20v joint ---------/---------jump P2
200v--------/--------shortcircuit 200v joint--------/--------jump P3
500v--------/--------shortcircuit 500v joint----------/--------- -

Does one of the above correspond to the 0-2v selection you described?

Can you recommend a pressure transducer that is roughly equivalent to the LM34 that I could use for PSI measurement?

geb

Newbie level 4
lm34dz thermometer

Eventually I got a pair of the following gauges second hand from someone.
Is it possible to assume an appropriate sensor for this gauge without knowing
the complete specifications on it? I can post them here as I find them eventually.

and from their website...

"RED Water Temperature
Requires 2 1/16” hole for mounting with back clamp provided. Range is 70-260F. Features ‘Hi” and “Lo” scrolling warning and user adjustable “flashing” warning. Available with high and low memory recall feature. Requires Cyberdyne temperature sender shipped with gauge."

I'll be using these gauges for my project but am unable to use the temperature senders that the company provides. I need to make my own. This will be a low temp water cooling circuit.

Would one of the LM34 units described at National's site most likely work OK for this?

<http://www.national.com/mpf/LM/LM34.html>

Would the plastic LM34DZ TO-92 version be fine or would the 'metal canned'
LM34DH TO-46 be better?

Or are there suggestions for a better commonly available sensor component? I've sent a few emails to the cyberdyne company asking for specifications but have got no word as yet.

Thanks for any help.

geb

Newbie level 4
lm34dz temperature thermometer

dammit i wish this didn't have to be such a problem.

I dismantled the sending unit that came with the gauge and discovered a little wafer/disc thermistor inside. It's about 1.5mm thick and 3mm across, black composite with silver cladding on eithe side. Measures about one ohm at room temp.

Is there any way of reliably measuring this component in order to track down a replacement thermistor of equal performance?

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