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| Keysight 34461A Fan Noise |
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| Messtechniker:
--- Quote from: Gandalf_Sr on December 21, 2019, 12:22:25 pm --- If the 34461A has a temperature-controlled voltage reference (does it?) then, as long as the new fan is made to match the same air flow rate (perhaps using a resistor) then is a recalibration really needed? --- End quote --- Yes, I am thinking about a speed dropping resistor. Planning to make a quick and dirty air flow indicator based on a suspended piece of thick paper to determine the resistor value which will cause the Noctua to produce the same air flow as the stock fan. But first I will be doing some internal temperature logging for both fans. It would be nice to know how the temperature is measured, i.e. sensor type and location on the pcb. and within the circuitry. Pity no schematic is available as in the old days. Sigh. What I do know is that the temperature is measured in increments of 0.1 deg C. So a bit of averaging will be need to smooth things out a bit. |
| iMo:
I made measurements on my 34401A (with the temperature sensor mounted inside). The voltage reference is the same as yours (LM399). The TC of the meter at 10V range (Tdmm=38..43C) is aprox 5uV/C. Yours will be similar, imho, unless 34461A+ does a TC correction. I do TC correction externally (stm32). The internal temperature Tdmm is always aprox Tdelta=17.5C higher than ambient Tamb (no fan). With the fan the delta will be lower, but still Tdmm=Tamb+Tdelta. |
| nctnico:
--- Quote from: Messtechniker on December 21, 2019, 08:12:44 am ---See graphs of the tolerance measurements on a 1 k resistor using the stock fan and the Noctua replacement fan. --- End quote --- What kind of resistor? If this is a standard one then you are likely seeing the temperature dependance of the resistor. It makes no sense for the DMM to be sensitive to the amount of airflow because this is likely to vary anyway by the environment of the DMM, variations between fans, built up of dust and aging of the fan. BTW I just ordered the Noctua fan with a low-noise-kit (adapter cables to reduce the speed of the fan). |
| Messtechniker:
--- Quote from: nctnico on December 21, 2019, 11:10:24 pm ---What kind of resistor? If this is a standard one then you are likely seeing the temperature dependance of the resistor. It makes no sense for the DMM to be sensitive to the amount of airflow because this is likely to vary anyway by the environment of the DMM, variations between fans, built up of dust and aging of the fan. BTW I just ordered the Noctua fan with a low-noise-kit (adapter cables to reduce the speed of the fan). --- End quote --- Got me! It was an ordinary 1k 0.05% 35 year old metal film resistor. Probably with a TC of 50 ppm/K. It was simply a stopgap measure, because at that time I did not know how to measure the internal temperature of the DMM. :palm: Even so, the logging graphs above indicate that the higher airflow of the Noctua fan seems to stabilize the DMM readings more rapidly. |
| Performa01:
--- Quote from: Messtechniker on December 22, 2019, 12:00:32 am ---It was an ordinary 1k 0.05% 35 year old metal film resistor. Probably with a TC of 50 ppm/K. --- End quote --- If a resistor is specified as 0.05% (500ppm) tolerance, it certainly has to have a better TC than 50ppm. I just had a look at my old stock of precision components and most 0.05% resistors have a temperature coefficient of 4-5ppm; the worst one I could find was 10ppm. Nevertheless the original objection is valid. You should not use Resistance measurements to verify the calibration of a DMM in the first place, even if you have a good resistor reference with TC<1ppm available. A DMM has its best accuracy and stability when measuring DCV. Furthermore you should use the base DCV range with direct connection (without internal attenuator), which is usually the 10/20VDC range. Quite obviously you need a stable (preferably 10V) reference voltage for this. |
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