USA calibration club

[Muxr]

Sorry I fell off the face of the earth!  Life got in the way of my hobbies.  I’ll make time this week to get the reference into the hands of the last few people who jumped on the band wagon.

I've been kind of in the same boat.. work projects took over all of my brain time. Catching up on all the stuff I missed. Good thing is my references should have all aged. 

[nikonoid]

I got the package today. Logging starts tomorrow


Sent from my iPhone using Tapatalk

[cellularmitosis]

I just sent this message to nikonoid, but I thought I'd echo it here for the benefit of the next recipients.

I've included a little Arduino shield which uses an Si7021 temperature and humidity sensor.

Here's the Python script I use to read the data from the Arduino and log the data in CSV format:

https://gist.github.com/cellularmitosis/421781b4f89636b612378f33d705f8c3

(also attached to this post)

Usage:


The script takes one argument, which is the serial port it should read from.

For example, using this script on linux might look like:

Code: [Select]

./Si7021-logger.py /dev/ttyACM0

The script will print output like so:

Code: [Select]

timestamp,temp_c,humidity
1500422021.565,25.4043,68.5087
1500422022.565,25.4257,68.5239
1500422023.565,25.4150,68.5239
...

That's a timestamp (seconds since the 1970), temperature in celsius, and humidity in percent.

On Linux, your serial port will probably look like /dev/ttyACM0 or /dev/ttyUSB0.  On Mac, it might look like /dev/tty.usbserial-DN02TIYO. 

On Linux, after you plug in the Arduino, you can run the dmesg command to figure out which device name it got associated with.  I don't know of the equivalent of dmesg on Mac, so I just open the Arduino IDE to figure it out.  I'm guessing you can do the same on Windows, but I haven't tried it.

Since the output of the script is in CSV format, you can just redirect the output of the script to a file in order to log to a file, e.g.:

Code: [Select]

./Si7021-logger.py /dev/ttyACM0 > temp-log.csv

If you'd like to watch the output while also logging to a file, you can use the tee command:

Code: [Select]

./Si7021-logger.py /dev/ttyACM0 | tee temp-log.csv

Technical info:


The Arduino spits out 9 bytes of binary at a time: a 32-bit floating point temperature, a 32-bit floating point humidity, and an 8-bit CRC.

The Python script reads in 9 bytes and checks that the CRC is valid.  If it isn't valid, it tries reading another byte until the CRC works, so even if the script starts up in the middle of a 9-byte sequence, the script will recover after throwing away the first reading.

[cellularmitosis]

For the curious, here's the Arduino sketch:

https://gist.github.com/cellularmitosis/4bc0de74b6c3dd03d57ef5ce09c51896

Drop the crc files into the same folder as your Arduino sketch and it should compile correctly in the Arduino IDE.

After I implemented this, I found out about Fletcher's checksum, which is much simpler, and will do just fine for an application like this.  I'll give that a shot for my next project.  https://en.wikipedia.org/wiki/Fletcher%27s_checksum

[bitseeker]

That's neat. Is it just the shield (with cable and sensor) that you put into the package or the Uno as well? I have a variety of Arduinos, so either way is OK with me. I just wasn't clear which it was.

[cellularmitosis]

Yup, it was light weight enough that I just threw the whole thing in there.

[bitseeker]

OK, cool. If space/weight become a limiting factor, I'm sure the whole thing could be made much smaller/lighter.

[nikonoid]

I finally have first round of preliminary results:

K2002 had been powered continuously. It sits between K2001 and another K2002 and consistently shows same internal temperature as when it was calibrated by CalMachine.

After warm up for few hours the reference read at  +1.5ppm, however now that I had it powered continuously for 5 days it slowly drifted up to about +4.5ppm. This reference may need to be warmed up for longer than few hours to perform better. This is sort of a bad news for a traveling reference.

All readings were done at 10 PLC with digital filtering off. I had drawn the 20 sample moving average line to compensate for low NPLC count and no filtering. Attached is an 8 hours run. ‘TEMP?’ is the internal temperature difference with the internal calibration temperature. The 10v reference was put inside of metal foil container along with temperature/humidity sensor (‘TEMP’ and ‘HUMID%’). As you can see my temperature difference throughout the run was under 0.5 C.

I am not sure why the shape of K2002 internal temperature change is different from the shape for Arduino temp sensor.

I am going to do another run with foil container placed on the top of warm meter to create a different temperature point for the reference.

Is there anything else you would like me to test before passing the reference on?

[bitseeker]

Wow, a 5-day warmup. However, if we plot the characteristic warmup drift of the reference over time, it could be taken into account when comparing the output voltage to other instruments, no?

[cellularmitosis]

Very interesting to hear about the 1.5 to 4.5 ppm drift over 5 days.  Feel free to keep the reference for a few more days -- I'd be interested to see how well it has really settled out.

I'll be looking specifically for a similar drift over the 5 days when the reference returns to me.

[nikonoid]

I am finally done with the reference. It went back to CM today.

After going through my records I was able to attribute a large portion of 1.5mm => 4.5ppm drift to temperature. My room temperature is fairly stable, in most cases not exceeding +/-0.5C per day, staying pretty much around 70F every day.

With that in mind the initial testing was done in open air (Run1 attachment). Then seeing a large amount of noise I enclosed the reference in foil baking tray and covered with sheet metal (Run4 - temperature not recorded, bug in a script). This did not help noise much. Then for (Run 5), the tray with reference and also temp sensor were standing on an off Keithley 2000, but to the left of working K2002, blowing some warm air in its direction. For Run 6, the tray was relocated atop of working K2001 (see photo attached). This increased temperature somewhat.  Just to test the temperature hypothesis, I let the reference be off for 24 hours (still seating on warm K2001). Then after day off I turned reference back on and is returned to expected voltage very quickly (Run 7).

It is possible that I was seeing a temperature dependency around 1ppm/C.

To make sure that noise or temperature variation are not contributing through performance of the Keithley 2002, I recorded output of Fluke 732a for 14 hours with the same setup and script (Run732a).


I have to find a place to upload Excel sessions. They are too big to fit here.

CM, please run a long session over few days to confirm. Thanks.

[bitseeker]

I have to find a place to upload Excel sessions. They are too big to fit here.

You can put them on xdevs. See CM's post followed by TiN's starting at https://www.eevblog.com/forum/metrology/usa-calibration-club/msg1276612/#msg1276612. On the FTP server, you'll see a directory for our cal club and subdirectories that CatalinaWOW and I made with our data.

[nikonoid]

Thanks, bitseeker.

I just found the "datashort" and uploaded files there. It is raw data plus a charts in excel.

[bitseeker]

Excellent!

[cellularmitosis]

nikonoid, I've received the SVR-T and have started logging.

I'm glad to see that we are using the same "q-tip" trick and the same cable, so the differences between our setups are minimized.  I also applied a shot of deoxit, both before I sent you the unit and again today.

I'll keep you posted!

[TiN]

Hey, I have a special deal for you guys

In theory I can provide xDevs.com FX 10V reference to USA Club, if somebody willing to donate LTZ1000A and set of LTZ resistors for the cause.
Rest of the parts, calibration and shipping I'll provide myself. 

[cellularmitosis]

TiN I would be happy to take that deal!

[nikonoid]

Cellular, I can chip in for the cost.


Sent from my iPhone using Tapatalk

[kj7e]

I may have an extra VHD200 13k/1k network for the LTZ1000(A)? heater divider.  Not sure how well that would fit on the FX board.

[kj7e]

Got a new toy, early Christmas gift to myself.

[RandallMcRee]

I hit the "Thanks" button only because I could not find the 'JEALOUS' one.

Looking forward to some quality logging data from that puppy.

[CatalinaWOW]

I can also chip in for TiN's generous offer.

[RandallMcRee]

Just saw TiN's offer.
I just bought some precision resistors for another project. Have some extra, so for LTZ1000A resistors in particular I can offer
two 100K RNC90Y (10ppm) and one naked Zfoil 10K (.2ppm).  But there may be better on offer.

http://www.newark.com/vishay-foil-resistors/y0089100k000tp0l/metal-film-resistor-100kohm-300mw/dp/35M4599
http://www.newark.com/vishay-foil-resistors-vpg/y070610k0000t9l/through-hole-metal-foil-resistor/dp/39T1673

I homebuilt a reference using this ebay sellers NOS LTZ1000A (you can see my review below!):
https://www.ebay.com/itm/NEW-Genuine-Linear-Technology-LTZ1000ACH-Ultra-Precision-Reference-/253083175317?hash=item3aeceec995#rwid

Money, ok too...but would prefer to donate parts. My family bought a lot of Christmas presents. Nothing left!

[TiN]

VHD will fit perfectly. Worlds reference, I like it

[kj7e]

Okay, I can provide VHD200 13K/1K.