|I thought about something but I wonder if it works|
Message #8 Posted by Vieira, Luiz C. (Brazil) on 9 July 2003, 11:01 a.m.,
in response to message #7 by James
I had something in mind that I proposed to Renato through an e-mail. It would involve a voltage reference and I'll change it a bit in this post. Please, if there is any lack of reasoning, I'd be glad to know. I do not want this to be a pattern, I want to know if it works. I did not test the procedure, it's a first proposal. Analog-related guys (Norm? Ellis? Others?), support me, please!
Let's take a new, high-capacity fully charged NiCad or NiMH battery and make sure it is in a 0ºC environment, what can be granted by putting the battery inside an insulated plastic bag immersed in melting ice. With the use of a thermometer, the temperature inside the bag may be confirmed, and tracking it's voltage till it does not change for about half an hour will make sure it is stable. Under these circumstances, this battery is measured with the use of a precision instrument and reading value is written down. Then this same battery is sent to someone who wants to calibrate another instrument. The "receiver" will charge it again (let's consider days between sending and delivering) and measure it under the same circumstances: 0ºC (melting ice) environment, so the reading value is adjusted to match the one read with first precision instrument.
Three facts must be taken into account to avoid battery voltage drifting:
- atmospheric pressure; if the sender and receiver locations have considerable highness difference, then it must be checked if battery voltage will vary as well; I think three-digit precision instruments would not be affected, but I'm not sure if five- or six-digit instruments will not.
- recharging method; electronic "smart" chargers of the same type and brand would be strictly necessary in both places OR sending the recharging equipment with the battery so the receiver will charge it in the same way;
- maximum voltage drop after being recharged; I think the first charges will not cause maximum voltage to drop, but after many recharging cycles, battery voltage must be checked again in a precision instrument. If the battery has already been charged many times, then fully charging it, measuring under these circumstances and sending it for a one-shot calibration must be safe as for reference, but if it is recharged again will probably give a lower voltage in its terminals.
I'd like to be able to test these procedures at home. Even if the 3421A is not calibrated it will serve to confirm my proposal: fully charge a NiCad or NiMH, measure its voltage under these circumstances, wait for a few days to simulate mail postage (average in local mail is about two days, and long-distance posting usually goes for two weeks), fully charge it again and measure its voltage. In this case, if there is a difference when reading with the same 3421A unit, this is the difference that must be taken into account when sending the battery to someone else.
In my first proposal, I asked Renato to use a regular, fresh new alkaline battery if the mail time is small (two days), but I thought about a way to use rechargeable batteries and keep precicion, and I think the approaches I mention above may be of use. The 0ºC temperature reference is necessary to reduce the number of external "variables".
Any other ideas to increase precision?
Hope this is of any use.
Edited: 9 July 2003, 11:07 a.m.