Topic: Is it possible to modify THGR122NX with wired thermocouple

[beakmyn]

I've purchased a WMR918 station a few years back and it's been performing ok given its shortcomings. Had to replace the anonometer after a nearby lightning strike took out the board. I wedged a Wiznet serial to ethernet adapter inside the console so that I can pipe the data to a laptop in the basement running Cumulus and APRSISCE/32

Last year I bought THGR122NX to supplement my temperature readings and I was thinking of purchasing another one and modifying it to have a wired thermo-couple that I can put in the pool.

It seems the THWR288A has long been discontinued...

I didn't see anything about anyone modifying the temperature sensor and I wondered if this had been done. I haven't pulled mine apart to see if this is possible. Anybody looked into this?

 

[Old Tele man]

I believe that unit uses a thermister to sense temperature, which is totally different device than a thermocouple (a two-dissimilar metal device). Short answer: NO.

[beakmyn]

I believe that unit uses a thermister to sense temperature, which is totally different device than a thermocouple (a two-dissimilar metal device). Short answer: NO.

That's what I was afraid of. I figured it was a thermistor or similar onchip solution. Oh well.

[Bushman]

So solder a sealed thermistor to the device. They cost a few bucks.

[Old Tele man]

So solder a sealed thermistor to the device. They cost a few bucks.

...but, be sure the thermistor you're installing has the SAME resistance value as the original, otherwise the readings will be WAY WAY off.

[beakmyn]

I'll look into that once I get it open to see if that's an option. I have concerns about matching the thermistor and lead length compensation. Something I'll have to test.

[Bushman]

I've homebrewed a few thermistor sensors and lead length hardly seems to matter.  This was with a 10K hm thermistor.

[DanS]

Yeah, get a little terminal block and a small handful of thermistors around the same resistance you measure the original one (removed from the circuit). Then go through them until you zero in on the correct temp range. I did this a few months back. Thermistors were about 30 cents each and I measured a bunch, picking out the values I wanted.

[aweatherguy]

I just posted another topic on this subject. Check the OS Hardware list for the post.

[beakmyn]

Thanks. I'll take a look at that, although I may be taking the plunge and getting a Davis unit.

On a another note it looks like my internal barometer has failed is there a fix for that? It's stuck at 31.01, tried resetting and such, checked for dust and bad solder joint, nothing.

[DanS]

I guess since you already checked for bad solder joints you already know where the actual barometer sensor chip is (appears something like attached). Have you tried lightly blowing it out in case something got wedged inside? Just a thought.

[gadget_guy]

A couple of years ago I took the thermister off the PCB, used some thin coax to extend it placed it in a waterproof plastic tube and buried it in the ground to measure soil temperature.  Would something like that work for you.  It was a pretty easy mod and it's worked for years now.

[aweatherguy]

Here's something I noticed with a THGR122NX sensor in playing around with thermistor mods. I had attached a large fixed resistor to simulate a very cold temperature (-40C). Then I added 6-feet of standard flat 4-conductor telephone extension cord between the sensor and the resistor. The temperature reading went up by 0.3C!

Don't know why. I might guess they are using an AC signal to measure the resistance -- perhaps as a means to eliminate DC offsets in the measurement process. Maybe the capacitance of the 6-foot cord altered the measurement. Just guess however but I repeated the experiment a couple of times and it seems to be real.

This effect may be lessened at higher temperatures where the thermistor has a much lower resistance...but you might watch out for it anyway. To test for it, replace the thermistor with a fixed resistor equivalent to the coldest temperature you expect to measure. Some examples for the THGR122NX (approximately)

16C=14k-ohms,
7C=20k-ohms,
0C=26k-ohms,
-10C=40k-ohms,
-20C=64k-ohms,
-40C=175k-ohms.

Note the reading, then re-wire it so the resistor is connected to the sensor through your length of cable instead of directly at the sensor. See if the reading changes.

[beakmyn]

I guess since you already checked for bad solder joints you already know where the actual barometer sensor chip is (appears something like attached). Have you tried lightly blowing it out in case something got wedged inside? Just a thought.

Tried that also. I'll take the soldering iron to the board when I get a chance. Been reading up on the thermistor posts. I'll do some experimenting.

[aweatherguy]

Here's a eye-opener. I hooked an o-silly-scope up to the thermistor on my THGR122NX sensor to see what is happening when the sensor measures temperature. I used a 10Meg-ohm, 10x probe with 16pF of capacitance. A PNG image with the voltage-vs-time waveform is attached to this post.

Yikes -- they ARE using an AC signal to measure the thermistor's resistance. I've only shown the very beginning of the waveform here (first milli-second or so) but it lasts for about 0.1 seconds and looks like this all the way through. The frequency is about 12kHz.

This explains why cable capacitance creates temperature errors. For those who aren't electronics-savvy, a capacitor looks like a resistor when AC is present -- and the larger the capacitor and the higher the AC frequency, the lower the equivalent resistance presented by the capacitor. This "distorts" the apparent resistance presented by the thermistor and the OS sensor winds up thinking that it's warmer than it really is. For those who are electronics savvy, my apologies -- I've taken great liberties in this analogy in the hopes that it will suffice for the current topic.

The next obvious experiment was to connect various-sized capacitors across the thermistor and see what happens. These measurements were made at about 17C where the thermistor has about 14k-ohms of resistance. The capacitance values and temperature errors I got are as follows:

Capacitance (pF)	Temperature Error (C)	Note
16	0.1 or less	10x scope probe capacitance
95	0.3	1x scope probe capacitance
350	0.7
1000	3.2

Someone mentioned using coax to extend the thermistor away from the sensor. I don't know what kind of coax was used, but for example, RG58/U coax has a capacitance of 28pF/foot. Using just a foot of this coax can create small errors at 17C (maybe 0.1 to 0.2C), and using 10 feet would probably create 0.5C or more of error.

Now that the measurement waveform is known, it is safe to conclude that this problem gets worse at low temperatures and better at high.

I also looked at a THGN801 sensor -- measurement waveform is almost identical.

I'd like to thank everyone who posted on this topic -- I never would have figured this out otherwise. I hope you all find this interesting and useful.

[aweatherguy]

Rats...wrong again    I hate it when this happens.

It turns out that the amount of temperature error caused by extra capacitance across the thermistor does NOT depend on the temperature and you need to keep added capacitance below 10-30pF to avoid temperature errors.

There is good news though -- now I understand exactly how OS measures thermistor resistance.

For those electronics hobbyists out there -- do you remember the old "555" timer/oscillator integrated circuit (https://en.wikipedia.org/wiki/555_timer)? Well, that's almost exactly how OS measures the thermistor. They have created a resistance-to-frequency converter using a thermistor in place of the resistor. Then they simply measure the oscillator's frequency to determine the resistance.

One end of the thermistor is alternately connected to ground or battery, charging and discharging a capacitor. In my THGR122NX, the capacitor is approximately 10nF (or 10,000pF). Adding a capacitor across the thermistor distorts the results -- by an amount roughly equal to the ratio between the added capacitor and the internal 10nF capacitor. To avoid temperature errors due to the added capacitance, it must be kept below about 0.1 to 0.3% of the 10nF capacitor -- hence the 10-30pF limit.

[aweatherguy]

Another thing I've discovered about thermistors -- getting them too hot during soldering can instantly cause a permanent shift in temperature readings. I spent some $$ on 0.05C accurate thermistors and was not careful heat sinking them during installation. They got too hot and now they are reading 1 degF off or more. From now on I'm going to just crimp them in place to eliminate the risk of over-heating.