Re: [volt-nuts] Update on 720A
Update: After changing the resistor I added in series with R1008 (A decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is slowly increasing in value back towards nominal) and re-calibrating again, the A decade is now within 1ppm of linear (using the 10V range and nominally 10V input). I note that when I'd done this, with the A decade set to 0.9 (reading 9.00V on the 3458A), the actual input voltage needed was 10.40 or so. Is that to be expected? Two of the positions on the B decade wouldn't quite adjust for a null when calibrating it. In this situation I think I have two options: 1) Add series resistors to compensate for slightly low value 9.898k resistors in the relevant two positions of the decade and recalibrate. 2) Adjust R203 to change the bridge balance slightly to the 10K ohm decades and recalibrate. Your thoughts on which approach to take is much appreciated. Thanks Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
Thanks Dave -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Bill Gold Sent: 07 August 2017 19:22 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A David: Ignore my previous post. In thinking about this while out shopping this morning I know the answer. The output resistance of the 720A is about 66k ohms. Your voltmeter has an input resistance of 10 megohms. You are simply loading the output of the 720A. The reason that the 0.1 ratio has about the expected voltage is because the 10 Volt range your voltmeter probably has a very high input resistance. You are on "autorange" and when the output is set for 0.2 ratio the voltmeter is now on the 20 volt range which is now 10 megohms. I get the same results as you do when measuring 0.9 ratio on my setup. 89.920 volts. Check your 720A by applying 10 volts and then do your ratio checks on the first decade. I just haven't used a 720A for a while and I didn't remember about the output resistance. When you get to become a "well aged citizen" it takes a while to recall things. Bill - Original Message - From: "David C. Partridge" To: "'Discussion of precise voltage measurement'" Sent: Monday, August 07, 2017 1:49 AM Subject: [volt-nuts] Update on 720A > I've added resistors in series with R1008 and R1044 to bring the resistance > for those parts of the A divider to a value that it would calibrate OK (two > of the resistor sets in the oil bath very low in value). I also had to > remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313 > and R314 was too high to allow me to calibrate S2. I also replaced R1041 > as it was worn out (almost impossible to get a null). > > I've successfully calibrated the C decade as per the maintenance part of the > manual. > > Sadly the beast isn't giving me a nice linear voltage sequence when using > just the A decade. > > Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to > zero except Decade A. > > Decade A Output V > 0.1 9.999,88 > 0.2 19.967,8 > 0.3 29.936,7 > 0.4 39.903,8 > 0.5 49.874,7 > 0.6 59.855,6 > 0.7 69.852,4 > 0.8 79.871,3 > 0.9 89.919,6 > > See also attached graph. > > Any suggestions on where to look for trouble will be most welcome ... > > All sections of the A decade attenuator now measure about 9.999,2 ohms. > > It was suggested on eevblog that the next decade (or a subsequent one) was > loading it, but with all the other decades set to 0 I don't quite "get" how. > > Thanks > Dave > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
Drats! That was most of the problem! Moving down to 10V where the 3458A has an input impedance of >10GOhm, and I see much more sensible results. Thank you Dave -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of wb6bnq Sent: 07 August 2017 11:19 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A Hi Dave, I think you need to review how to calibrate the 720A and Kelvin Varley dividers in general. The loading you need to worry about is the load you put on the output terminals, irrespective of the particular decade. I do not know how you are measuring the output, but it would appear, from your list of values, that your method of measuring the output is loading the divider, particularly as the value of error increases as you step up the "A" range. The divider was designed to look into an infinite impedance. I.E. a null detector driven from an equally stable and known DC source. Like, for example a 332 or 335 Fluke DC voltage standard. Any loading is going to drag down the output value and it will get worse as you go from the bottom of the range to the top. The calibration process assures that the ratio from one step to the next is equal. Your values indicate that there may be other issues you did not properly account for. Good luck with getting it straightened out. BillWB6BNQ David C. Partridge wrote: >I've added resistors in series with R1008 and R1044 to bring the >resistance for those parts of the A divider to a value that it would >calibrate OK (two of the resistor sets in the oil bath very low in >value). I also had to remove five turns (about 1.3 Ohms) from R1051 as the >combined value of R313 >and R314 was too high to allow me to calibrate S2. I also replaced R1041 >as it was worn out (almost impossible to get a null). > >I've successfully calibrated the C decade as per the maintenance part >of the manual. > >Sadly the beast isn't giving me a nice linear voltage sequence when >using just the A decade. > >Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set >to zero except Decade A. > >Decade A Output V > 0.1 9.999,88 > 0.2 19.967,8 > 0.3 29.936,7 > 0.4 39.903,8 > 0.5 49.874,7 > 0.6 59.855,6 > 0.7 69.852,4 > 0.8 79.871,3 > 0.9 89.919,6 > >See also attached graph. > >Any suggestions on where to look for trouble will be most welcome ... > >All sections of the A decade attenuator now measure about 9.999,2 ohms. > >It was suggested on eevblog that the next decade (or a subsequent one) >was loading it, but with all the other decades set to 0 I don't quite "get" >how. > >Thanks >Dave > > > > -- > -- > >--- >- > >___ >volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to >https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts >and follow the instructions there. > ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
David: Ignore my previous post. In thinking about this while out shopping this morning I know the answer. The output resistance of the 720A is about 66k ohms. Your voltmeter has an input resistance of 10 megohms. You are simply loading the output of the 720A. The reason that the 0.1 ratio has about the expected voltage is because the 10 Volt range your voltmeter probably has a very high input resistance. You are on "autorange" and when the output is set for 0.2 ratio the voltmeter is now on the 20 volt range which is now 10 megohms. I get the same results as you do when measuring 0.9 ratio on my setup. 89.920 volts. Check your 720A by applying 10 volts and then do your ratio checks on the first decade. I just haven't used a 720A for a while and I didn't remember about the output resistance. When you get to become a "well aged citizen" it takes a while to recall things. Bill - Original Message - From: "David C. Partridge" To: "'Discussion of precise voltage measurement'" Sent: Monday, August 07, 2017 1:49 AM Subject: [volt-nuts] Update on 720A > I've added resistors in series with R1008 and R1044 to bring the resistance > for those parts of the A divider to a value that it would calibrate OK (two > of the resistor sets in the oil bath very low in value). I also had to > remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313 > and R314 was too high to allow me to calibrate S2. I also replaced R1041 > as it was worn out (almost impossible to get a null). > > I've successfully calibrated the C decade as per the maintenance part of the > manual. > > Sadly the beast isn't giving me a nice linear voltage sequence when using > just the A decade. > > Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to > zero except Decade A. > > Decade A Output V > 0.1 9.999,88 > 0.2 19.967,8 > 0.3 29.936,7 > 0.4 39.903,8 > 0.5 49.874,7 > 0.6 59.855,6 > 0.7 69.852,4 > 0.8 79.871,3 > 0.9 89.919,6 > > See also attached graph. > > Any suggestions on where to look for trouble will be most welcome ... > > All sections of the A decade attenuator now measure about 9.999,2 ohms. > > It was suggested on eevblog that the next decade (or a subsequent one) was > loading it, but with all the other decades set to 0 I don't quite "get" how. > > Thanks > Dave > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
David: First suggestion would be to see what the output measures, with the input still set to 100.0014 volts (same as before), when setting the dials to: 0.9X(equivalent to setting the "A" decade to "0.1") 1.9X(equivalent to setting the "A" decade to "0.2") 2.9X(equivalent to setting the "A" decade to "0.3") 3.9X(equivalent to setting the "A" decade to "0.4") and so on up to 8.9X(equivalent to setting the "A" decade to "0.9") What is the output when the dials are set to 9.9X? It should be 100.0014 (or very, very close) just like the input voltage. Also what do you mean by your statement "Sadly the beast isn't giving me a nice linear voltage sequence when using just the A decade." Do you mean that other settings when decade B, C, D and so on are not "0", produce ok results? Bill - Original Message - From: "David C. Partridge" To: "'Discussion of precise voltage measurement'" Sent: Monday, August 07, 2017 1:49 AM Subject: [volt-nuts] Update on 720A > I've added resistors in series with R1008 and R1044 to bring the resistance > for those parts of the A divider to a value that it would calibrate OK (two > of the resistor sets in the oil bath very low in value). I also had to > remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313 > and R314 was too high to allow me to calibrate S2. I also replaced R1041 > as it was worn out (almost impossible to get a null). > > I've successfully calibrated the C decade as per the maintenance part of the > manual. > > Sadly the beast isn't giving me a nice linear voltage sequence when using > just the A decade. > > Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to > zero except Decade A. > > Decade A Output V > 0.1 9.999,88 > 0.2 19.967,8 > 0.3 29.936,7 > 0.4 39.903,8 > 0.5 49.874,7 > 0.6 59.855,6 > 0.7 69.852,4 > 0.8 79.871,3 > 0.9 89.919,6 > > See also attached graph. > > Any suggestions on where to look for trouble will be most welcome ... > > All sections of the A decade attenuator now measure about 9.999,2 ohms. > > It was suggested on eevblog that the next decade (or a subsequent one) was > loading it, but with all the other decades set to 0 I don't quite "get" how. > > Thanks > Dave > > ___ > volt-nuts mailing list -- volt-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts > and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
Re: [volt-nuts] Update on 720A
Hi Dave, I think you need to review how to calibrate the 720A and Kelvin Varley dividers in general. The loading you need to worry about is the load you put on the output terminals, irrespective of the particular decade. I do not know how you are measuring the output, but it would appear, from your list of values, that your method of measuring the output is loading the divider, particularly as the value of error increases as you step up the "A" range. The divider was designed to look into an infinite impedance. I.E. a null detector driven from an equally stable and known DC source. Like, for example a 332 or 335 Fluke DC voltage standard. Any loading is going to drag down the output value and it will get worse as you go from the bottom of the range to the top. The calibration process assures that the ratio from one step to the next is equal. Your values indicate that there may be other issues you did not properly account for. Good luck with getting it straightened out. BillWB6BNQ David C. Partridge wrote: I've added resistors in series with R1008 and R1044 to bring the resistance for those parts of the A divider to a value that it would calibrate OK (two of the resistor sets in the oil bath very low in value). I also had to remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313 and R314 was too high to allow me to calibrate S2. I also replaced R1041 as it was worn out (almost impossible to get a null). I've successfully calibrated the C decade as per the maintenance part of the manual. Sadly the beast isn't giving me a nice linear voltage sequence when using just the A decade. Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to zero except Decade A. Decade A Output V 0.1 9.999,88 0.2 19.967,8 0.3 29.936,7 0.4 39.903,8 0.5 49.874,7 0.6 59.855,6 0.7 69.852,4 0.8 79.871,3 0.9 89.919,6 See also attached graph. Any suggestions on where to look for trouble will be most welcome ... All sections of the A decade attenuator now measure about 9.999,2 ohms. It was suggested on eevblog that the next decade (or a subsequent one) was loading it, but with all the other decades set to 0 I don't quite "get" how. Thanks Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
[volt-nuts] Update on 720A
I've added resistors in series with R1008 and R1044 to bring the resistance for those parts of the A divider to a value that it would calibrate OK (two of the resistor sets in the oil bath very low in value). I also had to remove five turns (about 1.3 Ohms) from R1051 as the combined value of R313 and R314 was too high to allow me to calibrate S2. I also replaced R1041 as it was worn out (almost impossible to get a null). I've successfully calibrated the C decade as per the maintenance part of the manual. Sadly the beast isn't giving me a nice linear voltage sequence when using just the A decade. Input voltage 100.001,4V fed to 0/1.0 Input terminals All Decades set to zero except Decade A. Decade A Output V 0.1 9.999,88 0.2 19.967,8 0.3 29.936,7 0.4 39.903,8 0.5 49.874,7 0.6 59.855,6 0.7 69.852,4 0.8 79.871,3 0.9 89.919,6 See also attached graph. Any suggestions on where to look for trouble will be most welcome ... All sections of the A decade attenuator now measure about 9.999,2 ohms. It was suggested on eevblog that the next decade (or a subsequent one) was loading it, but with all the other decades set to 0 I don't quite "get" how. Thanks Dave ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
