UT61E drift and recalibration

[joeqsmith]

I've thought about running a test like this as well.   I checked a UT210E's TC when someone posted about being concerned with drift.   If you can run it and get some numbers, please post them.   

[MosherIV]

Hi

So I fianlly had time to run the experiment properly.

I first moved my Thurlby 1905a 5.5 digit dmm and the AD594JN voltage reference to the patio door.
I decided to measure the UT61E, Keysight U1232a and the clasic DT830B
I measured the reference with all the dmm under test (26°C and 60% relative humidity):
1905a 2.5V=2.4987v, 5V=5.0039V, 10V=9.9968V (control)
UT61E 2.5V=2.498V 5V=5.003V 10V=9.997V
U1232A 2.5V=2.498V 5V=5.003V 10V=9.99V
DT830B 2.5V=2.49V 5V=5.00V 10V=9.98V

[MosherIV]

I then moved the box with the 3 dmm under test into my hot conservatory. Left them there for over 1 hour to acclimatise.

Then turned on the meters and measured again :
1905A 2.5V=2.4987V 5V=5.0039V 10V=9.9968V(control 26°C)

DMM under test, measured 37°C sorry no humidity measurement in conservatory.
UT61E 2.5V=2.499V 5V=5.005V 10V= 10.001V
U1232A 2.5V=2.498V 5V=5.004V 10V=9.99V
DT830B 2.5V=2.50V 5V=5.00V 10V=10.00V

Not sure what to conclude, yes the temperature did affect the reading but only by 3 counts worst case but that is within specification.   

The Keysight shows no change what so ever, but then I would expect that.

The DT830 only chnages by 2 counts worst case. Amazing when you consider how cheap it is 

(Sorry the patio door is very dirty  )

[joeqsmith]

DT830B 2.5V=2.50V 5V=5.00V 10V=10.00V
The DT830 only chnages by 2 counts worst case. Amazing when you consider how cheap it is 

(Sorry the patio door is very dirty  )

Thanks for running this again.  10mV / count would have guessed it would be solid.  Even your 61E does not look too bad. 

[Rango]

An Amprobe AM-270 for high voltage use or a used Fluke.

GDT = Gas Discharge Tube. You can use those or spark gaps or MOVs. Spark gaps and GDT operate at 1KV and above usually and I have no plans to take this meter anywhere near there so I'll stick to a 275V MOV (clamps at 600VDC).

For input protection adding 3 of those MOVs would be sufficient. They are also 30pF so it would not bump capacitance that much but one can always Delta capacitance anyway correct? I posted more indepth question here but don't want to repost in tow post so will just post a link here
https://www.eevblog.com/forum/testgear/uni-t-ut61e-multimeter-teardown-photos/msg1349602/#msg1349602

Vishay VDRS - E Series Metal Oxide Varistor 30pF 5A, Clamping 1200V, Varistor 750V (178-059)
http://uk.rs-online.com/web/p/metal-oxide-varistors/0178059/?searchTerm=178059&relevancy-data=636F3D3126696E3D4931384E525353746F636B4E756D626572266C753D656E266D6D3D6D61746368616C6C26706D3D5E5C647B362C377D7C5C647B31307D7C28283F69292852537C5253207C52532D293F5C647B337D285C73293F5B5C732D2F255C2E2C5D285C73293F5C647B332C347D292426706F3D3126736E3D592673743D52535F53544F434B5F4E554D4245522677633D4E4F4E45267573743D313738303539267374613D3031373830353926

[Per Hansson]

To get rid of the last of the temp drift I have resorted to a thermistor (Vishay NTCLG100E2104JB Farnell 116-4823) in series with 1Mohm across R16 (9.09k on the schematic but measured at 7.55k). This gives a count of 2 drift at the moment, I need to fiddle more, dropping the 1M to 947k should do the trick.

As the details for the external Vref are listed earlier I have left that bit out. The 780k ohm resistor (calculated value without external Vref) would be 680k + 100k in series and would probably need adjusting to match each meter, but it would be a good starting point. With the external Vref I am using 947k but again this would probably need adjusting for each meter. If you can get hold of the thermistor without paying a fortune for postage it would be a very cheap way of improving the meter.

As the input protection on my TUV / GS rated UT61E died I'm ordering some parts on Elfa.
I noticed the LT1790BCS6-1.25 voltage reference is on sale on their site so I ordered the necessary components as my meter is quite old and has the requisite pads on the PCB...
As can be seen from the quotes by stephenlm324 above this will really require a thermistor too, so I ordered that as well however it's on backorder.
That will give me a good way to see the before and after response though so I do not complain
But one thing has me confused: in the first quoted message above stephenlm324 says to put the thermistor and resistor in parallel with R16.
But in the second quoted message there is a schematic attached (hotlinked below) because someone asked for clarification, and here the thermistor and resistor is shown in parallel with R16+VR1.
Can someone that has done this modification please clarify which it is?

[Serg65536]

My new UT61E in all AC modes very slowly returns to 0. It takes 2 minutes to drop down from several volts AC to 0. Is this normal or should I return it? (probes are short circuited while waiting)

[Per Hansson]

My new UT61E in all AC modes very slowly returns to 0. It takes 2 minutes to drop down from several volts AC to 0. Is this normal or should I return it? (probes are short circuited while waiting)

They do behave this way, here is an old thread about it with more examples:
https://www.eevblog.com/forum/testgear/uni-t-ut61e-does-not-show-zero-in-ac-range-with-shrten-test-leads/msg2766422/#msg2766422