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Problems with resistance measurement of the USB DrDAQ

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Problems with resistance measurement of the USB DrDAQ

Postby Glovisol » Tue Nov 04, 2014 7:39 pm

I tested the accuracy of the new USB DrDAQ I received last week. The product specification is as follows:

RANGE: 0 - 1 MOhm
RESOLUTION: 250 Ohm @ 10 KOhm
ACCURACY: 5%

I tested the unit in the range 1 KOhm to 1 MOhm with 1% resistors, checked with a high precision / high resolution Standard Meter (Monacor). Ambient temperature: 19.5 °C. Results are as follows.

1) The resolution is not 250 Ohm, but 1 KOhm, as it is impossible to read even one decimal place, even after setting the format in PicoLog.

2) Readings

NOMINAL KOhm..................REAL KOhm (precision meter)............................DrDAQ KOhm
1.....................................0.998.......................................................1.0
1.8...................................1.773.......................................................2.0
2.7...................................2.710.......................................................4.0
4.7...................................4.69.........................................................6.0
6.8...................................6.79.........................................................9.0
10.0.................................10.00.......................................................13.0
15.0.................................14.86.......................................................19.0
22.0.................................22.30.......................................................27.0
33.0.................................32.93.......................................................37.0
56.0.................................56.00.......................................................57.0
82.0.................................81.60.......................................................83.0
100.0................................99.95.......................................................98.0
150.0...............................152.20.....................................................148.0
270.0...............................270.00.....................................................261.0
470.0...............................465.00.....................................................469.0
680.0...............................682.00.....................................................630.0
825.0...............................830.00.....................................................756.0
1000.0..............................996.00.....................................................891.0

This data shows the unit to be far out of specification. Most worriyng is the lack of the 1 decimal place specified, but even precision is far off. If there is drift with time as well, the resistance channel seems to be useless. I wonder if mine is just a faulty unit, or if this is a general situation.
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Re: Problems with resistance measurement of the USB DrDAQ

Postby Hitesh » Tue Nov 18, 2014 2:53 pm

Hi Glovisol,

Having run some tests with resistor, it seems that there is a limitation with the values returned by the driver.

The scaling is set to kOhms, but the driver returns 16-bit integer values although PicoLog displays readings with two decimal places.

I will report this to our Development Team.

Update: The driver has been updated to provide floating point values and fix resistance calculations which resolves the issue. A new version of PicoLog will shortly be available for download from our software download pages and the SDK will also be updated.

Regards,
Last edited by Hitesh on Fri Jan 16, 2015 11:00 am, edited 1 time in total.
Reason: Update to indicate a driver fix.
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Re: Problems with resistance measurement of the DrDAQ - UPD

Postby Glovisol » Mon Jan 26, 2015 10:44 am

Hi Hitesh,

Very many thanks for your good news. I shall look closely for the Picolog/SDK update.

Glovisol
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Re: Problems with resistance measurement of the USB DrDAQ -

Postby Glovisol » Tue Feb 10, 2015 1:16 pm

The new version for PICOLOG ( VER. r_5_24_2) appeared on Feb. 5th. After installation I did a preliminary test and RESULTS ARE EXTREMELY GOOD SO FAR. Readings are to 3 decimal places. I tested the range 1 KOhm - 10 KOhm. From the KOhm readings obtained, I inserted the following Reference Table.

1.38.................1.0
1.86.................1.5
2.39.................2.0
3.09.................2.7
4.25.................3.9
6.00.................5.6
7.19.................6.8
8.41.................8.2
10.34................10.0

With this correction readings between 1 KOhm and 10 KOhm are now well within 1.5%.

Sincere compliments to the PICOLOG Design Team, as the software modification transformed the Resistance measurement from useless to very useful !!

I shall test the entire range to 1 MOhm and give the results in a future post.
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Re: Problems with resistance measurement of the USB DrDAQ

Postby Glovisol » Tue Feb 10, 2015 8:15 pm

THE NEW PICOLOG VERSION r5_24_2 WORKS JUST FINE!

I have taken the time to collect measurement data on the Resistance channel of the DrDAQ, after downloading the new version of PICOLOG. Operating conditions were as follows:
- USB Voltage to DrDAQ: 5.06V.
- Sampling interval: 1 Sec. (smaller sampling intervals do not allow the three digit resolution).
- All sample resistors checked with:
- Monacor LCR-400 LCR meter, having an accuracy of +/- 0.3% in the range 1 KOhm - 1 MOhm at stated ambient temp.
- Ambient temperature: 19°C.

All recovered data is presented in the enclosed PDF File "New Ohm 1", ready for download. Data is organised as follows.

COLUMN A: Measured values, KOhm.
COLUMN B: True values, KOhm.
COLUMN C: Uncorrected error %. This error is as high as 39% @ 1 KOhm and -3.84% @ 1 MOhm.

The above data can be used to compile a Reference (scaling) Table in Picolog, or to compile a Reference (scaling)Table in a .DDS File, once the measurement range (e.g. min. and max. values) has been chosen. I have also curve fitted the above data to obtain correction equations for the three ranges: 1-10 KOhm, 10-100 KOhm and 100-1000 KOhm. These are shown as follows:

COLUMNS E,F,G,H for the 1-10 KOhm range.
COLUMNS I,J,K,L for the 10-100 KOhm range.
COLUMNS M,N,O,P for the 100-1000 KOhm range.

The correction (scaling) equations must be inserted in the "Parameret Scale" window in PICOLOG and are as follows:

1 - 10 KOhm range: R = (0.9975*Rmeas) - 0.3843
measurement error is less than +/- 0.5% throughout the range.

10 - 100 KOhm range: R = (1.0019*Rmeas) - 0.4747
measurement error is less than +/- 0.5% throughout the range.

100 - 1000 KOhm range: R = (1.0429*Rmeas) - 0.8.0581
measurement error is less than +/- 0.5% throughout the range, save for 100 KOhm, where it is -3.64%.

If different ranges from the ones shown are needed, the presented data can be used to configure adequate tables and curve fitting equations.

To conclude, the DrDAQ, with the new improved PICOLOG software, now yelds very satisfactory performance in resistance measurement and control.
Attachments
New Ohm 1.xls
Meas. and Scaling data for the DrDAQ
(35.5 KiB) Downloaded 71 times
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Re: Problems with resistance measurement of the USB DrDAQ

Postby Glovisol » Wed Feb 11, 2015 3:52 pm

ERRATA CORRIGE

THE NEW PICOLOG VERSION r5_24_2 WORKS JUST FINE!


Please amend the following sentence:
The above data can be used to compile a Reference (scaling) Table in Picolog, or to compile a Reference (scaling)Table in a .DDS File, once the measurement range (e.g. min. and max. values) has been chosen.


as follows:
The above data can be used to compile a Reference (scaling) Table in Picolog, once the measurement range (e.g. min. and max. values) has been chosen.


The reason being that .DDS files can only be applied to the EXTERNAL INPUTS of the DrDAQ and NOT to the RESISTOR input.

Apologies by Glovisol
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Re: Problems with resistance measurement of the USB DrDAQ

Postby Glovisol » Tue Feb 17, 2015 4:18 pm

ERRATA CORRIGE

There is a mistake in the text of the 100 - 1000 KOhm scaling equation previously presented, although equations shown in the EXCEL file are correct. Correct equations in improved form following the PICOLOG rule are shown below.

1 - 10 KOhm range: (0.9975*X) - 0.3843

10 - 100 KOhm range: (1.0019*X) - 0.4747

100 - 1000 KOhm range: (1.0429*X) - 8.0581
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