Hi, the current temperature relationship should be nearly linear, so if 100mA is needed at 20 C ambient and it follows that at 80 C no heating is required, then the heating current will decrease at about 1.7mA/K. cheers, Neville Michie On 23/11/2007, at 9:33 AM, Didier Juges wrote: > Bruce, > > I have some 4 terminal current sense resistors at work, but they > are very > low values (0.1 ohm and lower), so what I may gain at the sensor I > may loose > in the amplifier. Right now, the 1 ohm resistor and OP-27 give a > voltage > that is way out of the noise, as the curve shows. If I can find more > temperature stable resistors for the instrument amplifier, then I > could use > one of those 4 terminal resistors. I will see what I can get to > improve this > setup. As it is, it may not be extremely accurate in absolute > terms, but it > is precise and probably sufficiently relatively accurate for what I > want to > do. > > What do you think of the 2mA/degree current sensitivity? Is it in the > ballpark? I need to run the test much longer, and over greater > temperature > variations, but I am still tinkering with it... The engineer's > curse :-) > > Didier > >> -----Original Message----- >> From: time-nuts-bounces@febo.com >> [mailto:time-nuts-bounces@febo.com] On Behalf Of Bruce Griffiths >> Sent: Thursday, November 22, 2007 3:28 PM >> To: Discussion of precise time and frequency measurement >> Subject: Re: [time-nuts] HP10811 current versus temperature >> >> Ideally you should use a 4 terminal current sensing resistor >> and a high input impedance instrumentation amplifier. >> A Thomas style 4 terminal resistor in a temperature >> controlled oil bath would be nice, but failing that a low >> tempco 4 terminal resistor will suffice. >> >> Bruce > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/ > time-nuts > and follow the instructions there.

Didier Juges wrote: > Bruce, > > I have some 4 terminal current sense resistors at work, but they are very > low values (0.1 ohm and lower), so what I may gain at the sensor I may loose > in the amplifier. Right now, the 1 ohm resistor and OP-27 give a voltage > that is way out of the noise, as the curve shows. If I can find more > temperature stable resistors for the instrument amplifier, then I could use > one of those 4 terminal resistors. I will see what I can get to improve this > setup. As it is, it may not be extremely accurate in absolute terms, but it > is precise and probably sufficiently relatively accurate for what I want to > do. > > What do you think of the 2mA/degree current sensitivity? Is it in the > ballpark? I need to run the test much longer, and over greater temperature > variations, but I am still tinkering with it... The engineer's curse :-) > > Didier > Didier Its hard to find temperature stable 4 terminal resistors with tempcos better than 50ppm/C or resistances greater than 0.1 ohm off the shelf. The only source of 1 ohm 4 terminal resistors would appear to be used L&N, JRL, etc 4 terminal (Thomas style) standards which can be expensive ($US2000 or so). However have found listing for NBS pattern standard resistors at $US165. Such standard resistors have very low tempcos but are intended for oil bath operation. The only current US manufacturer of such standard resistors (including ones rated for operation in air) appears to be Ohm labs: http://www.ohm-labs.com <http://www.ohmlabs.com> Leeds and Northrup and Julie Research labs once made such resistors. All that can be done to check that the temperature dependence of the oven current is in the ballpark is compare your results with those in the HP journal article on the 10811A >From figure 4 on page 21 of the March 1981 issue. OVEN POWER 5.13W @ -50C 0.26W @ +70C Or 40mW/C with a 24V oven supply this corresponds to about 1.6mA/C. So you would appear to be in the ball park. Note above figures are for moving air somewhat less in still air. Bruce