redrabbit said:
I have built his J-Can for measuring noise.
Redrabbit -- is that a "trinacria" on your flag?
Geller seems to do a nice job, especially considering there's nothing else around near his price. According to his web site, the $10 calibration is only for his products. It's a phenomenal deal. He has the Fluke 732B standard, which is IMO, the holy grail of voltage standards. Way out of my price range. With that and his hi-res meter, accuracy isn't much of a question.
You could build standards much more cheaply, but the effort and equipment necessary to prove them out is way more than Geller's price.
I do things more the old fashioned way. I maintain three Fluke 731 standards, the previous model. They get inter-compared with an 845 null meter on a regular basis, and one gets certified on an irregular basis. Only by maintaining three or more standards can one be absolutely sure of what's going on. The issue of everything drifting the same amount in the same direction is minor, but it could potentially happen. My solution for that is inter-comparison with other standard of completely different design (Weston cells and zener references of my own design). The odds of different designs all drifting in the same fashion are pretty remote!
For another source of parts, be sure to check out Thaler-
http://www.thaler.com/thchtml/SelectorGuides/selector_refs.html
Note that they also have a 26 bit A/D converter. They used to have an AC reference, but I don't see it listed anymore.
You could build standards much more cheaply, but the effort and equipment necessary to prove them out is way more than Geller's price.
I do things more the old fashioned way. I maintain three Fluke 731 standards, the previous model. They get inter-compared with an 845 null meter on a regular basis, and one gets certified on an irregular basis. Only by maintaining three or more standards can one be absolutely sure of what's going on. The issue of everything drifting the same amount in the same direction is minor, but it could potentially happen. My solution for that is inter-comparison with other standard of completely different design (Weston cells and zener references of my own design). The odds of different designs all drifting in the same fashion are pretty remote!
For another source of parts, be sure to check out Thaler-
http://www.thaler.com/thchtml/SelectorGuides/selector_refs.html
Note that they also have a 26 bit A/D converter. They used to have an AC reference, but I don't see it listed anymore.
I just came across some viewable (video) seminars from Fluke....
http://us.fluke.com/usen/apps/PM/RefMM03.htm
=RR=
http://us.fluke.com/usen/apps/PM/RefMM03.htm
=RR=
Good stuff, but modern techniques are beyond my budget. Today's 6 1/2 to 8 1/2 digit meters are so good, that's all you really need. The internal references will be better than most stand-alone references you could buy or build. Software calibration of modern equipment keeps the cost down, as opposed to my surplus equipment that would cost an arm and a leg to do a yearly calibration on. Fortunately, with a good reference, KVD plus a few other things, I can do it myself just as well. All it costs is time.
I purchased a KVD and HV PS on EBay, then got tired of looking at it so relisted it -- the KVD went to a guy at Cal Tech, I kept the PS and the rack cabinet the thing came in. The KVD is all he wanted.
Most of what I have been doing is solving big problems, not little ones.
Most of what I have been doing is solving big problems, not little ones.
DC Null Meter
Hi Guys,
Could someone kindly let me know the function of a DC Null Detector. I will greatly appreciate your answers.
Regards
Haroon
Hi Guys,
Could someone kindly let me know the function of a DC Null Detector. I will greatly appreciate your answers.
Regards
Haroon
Google "wheatstone bridge".
You'll see a bridge consisting of 4 leg resistances, a driving voltage across two nodes and a voltage detector across the other two nodes.
It does not require complex maths to see that if the RATIO of the resistances are equal, there will be zero voltage across the null detector, without depending on an accurate driving voltage.
So the null detector just needs to indicate zero, or plus or minus away from zero as the adjustable "leg" is varied.
This basic circuit is used in many variations to measure impedance, with servos altering the variable leg until a null is achieved, thus automating the measurement.
Wheatstone was a smart guy!
You'll see a bridge consisting of 4 leg resistances, a driving voltage across two nodes and a voltage detector across the other two nodes.
It does not require complex maths to see that if the RATIO of the resistances are equal, there will be zero voltage across the null detector, without depending on an accurate driving voltage.
So the null detector just needs to indicate zero, or plus or minus away from zero as the adjustable "leg" is varied.
This basic circuit is used in many variations to measure impedance, with servos altering the variable leg until a null is achieved, thus automating the measurement.
Wheatstone was a smart guy!
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