Calibrating a VC99 - Page 1
Forum section: electronics / testgear

author: groston
author_rank: Newbie
timestamp: April 09, 2014, 09:52:13 pm
content: All,I just purchased a Vichy VC99. Though I have a Fluke 75 (which 30 years old and still going strong), the VC99 can measure temperature, capacitance, etc., things which the Fluke cannot do. Since I use a meter infrequently, spending big dollars did not make sense.The meter arrived today as did a thermocouple probe. To test the meter, I made some ice water and put both thermocouples in it. Both indicated 36 F, not 32 F as expected. I then stuck my instant read food thermometer in the glass and it did read 32 F.Question: Is there some way to field calibrate the meter? An error of 4F is horrifically bad and it is not clear if it is within spec. They claim +/-0.75%, but from what reference point? If they are referring to absolute zero, then this meter is right on the ragged edge of being within spec.

author: pickle9000
author_rank: Super Contributor
timestamp: April 09, 2014, 10:27:46 pm
content: Normally when using water to check for 32 deg make sure you have more ice than water, an insulated or plastic cup, stir at least 3 minutes to let the temp to settle down, the probe should be moving while the measurement is taking place, and not touch the outer edge of the cup or rest on an ice cube.I can't say I know the particulars on calibrating the vc99 but check like this first.

author: groston
author_rank: Newbie
timestamp: April 09, 2014, 11:47:11 pm
content: pickle9000 - Thanks for the reply, but this is not my first rodeo. Please note that I used a completely independent device to check the experimental validity.

author: sleemanj
author_rank: Super Contributor
timestamp: April 10, 2014, 01:13:36 am
content: Ice in a glass and comparing with some other device, which may or may not be more accurate that the DUT is no way to go through life (neither is measuring in F, but that's another story).You want a triple point cell.http://webcache.googleusercontent.com/search?q=cache:GA93z0BKNKUJ:www.scientificamerican.com/article/tackling-the-triple-point/+&cd=1&hl=en&ct=clnk&gl=nz

author: Rick Law
author_rank: Super Contributor
timestamp: April 10, 2014, 05:48:47 am
content: Ice in water means that you are likely measuring a temperature above freezing.Water is >=0C, ice is <= 0c. At 0C, ice aborb energy but remain ice until it aborb enough energy to turn into water.Water is most dense at about 4C, so those at 4C will sink. Warmer or colder water will rise to where your ice is. Only at the very narrow interface of water and ice (that few mm if that much) would it be close to zero.Also, freezing temperature depends on your location: how much above sea level and loads of other factors - particularly important is impurities in your water. So ice in water is really not a good "standard" for calibration.

author: HKJ
author_rank: Super Contributor
timestamp: April 10, 2014, 06:10:09 am
content: Thermocouplers always measures temperature differences. With DMM's the difference is between the terminals on the DMM and the probe point. The DMM then has an internal temperature sensor, it uses to compensate for the temperature of the terminals.To get the best temperature measurement with the DMM you need to ensure that the terminals has the same temperature as the internal sensor, i.e. the meter has to stay at a stable temperature for some time and you must not touch the connector, before you measure.You can always check the internal temperature sensor value, just short the input with a wire and read the temperature on the DMM.

author: nihilism
author_rank: Regular Contributor
timestamp: April 10, 2014, 11:54:16 am
content: VC99 Calibration Tool.

author: rdl
author_rank: Super Contributor
timestamp: April 10, 2014, 01:35:25 pm
content: The freezing point of water is not affected very much by air pressure. Impurities however, can make a big difference.

author: Smokey
author_rank: Super Contributor
timestamp: April 10, 2014, 09:15:20 pm
content: All discussion about the temp of ice water aside... The idea of "calibrating" a VC99 is a little silly. And this is coming from a guy that made a wireless bluetooth VC99 data logger so I'm not all anti-Vichy in general.Quoting the spec and expecting it to be accurate is also silly. You already did your cross reference (with something) so subtract the 4C. It is always going to be 4C? Who knows. Is that 4C offset constant through the whole range? Who knows.If you want more confidence, you may need to spend more than 25USD.