On Oct 8, 2011, at 3:10 PM, Jed Rothwell wrote:
Horace Heffner <hheff...@mtaonline.net> wrote:
On Oct 8, 2011, at 1:51 PM, Jed Rothwell wrote:
This was shown in the video on the table. Lewan says this was a
Termometro a 4 canali TM-947 SD.
4 canali evidently means you can attach up to 4 thermocouples.
http://www.bergamomisure.it/parametri-ambientali/termometri/
termometro-datalogger-4-canali-tm-947-con-scheda-sd.html
- Jed
Not very accurate for this purpose.
"Risoluzione: 1°C , 1°F / 0.1°C,0.1°F"
I take it this means 1°C absolute, and 0.1°C relative.
This means for a 5°C delta t there could be 2°C error, or 40%.
No it does not mean that. I have used many handheld meters and K-
type thermocouples in other configurations. At a normal temperature
range for a 5°C Delta T, the error is less than 0.1°C. That is to
say, every single meter or thermocouple you use will report the
same temperature difference to the limits of the display. They may
start off at different absolute temperatures, but they will all
show the same change.
Even the cheapest red liquid thermometer Omega offers, the GT
736590, does not have a 2°C error in precision. I have 3 of them,
and they are all accurate to within a degree, and precise to 10 or
20 degrees. (That is, when 1 goes up 18 deg C, so do the other 2,
and so do the thermocouples.)
A thermocouple or thermometer with a 2 deg C error in the normal
range of use, 0 to 100 deg C, would be absurd. It would be useless.
This particular one goes much higher, and you might see a 2 deg C
error at higher temperatures, but not around room temperature.
Trust me, a 250 Euro instrument would never do that. I have seen
and used dozens of them.
- Jed
Two thermocouples were used to measure the delta T. If one is off 1
degree hot absolute and the other is off 1 degree cold absolute,
then delta T is off by 2°C, systematically. It looked to me the
actual error was on the order of 0.8°C, but who knows without
calibration.
It is possible, even easy, to make a thermocouple (pair) specifically
for measuring delta T. One means to greatly simplify and improve the
delta T measurement is to use paired type T thermocouples at the Tin
and Tout measuring points. (See "The Temperature Handbook" by Omega
Corp. p z-31.) Copper leads are used from the data aquisition device
to and from the Tin and Tout measuring points, but a constantin lead
connects the Tin and Tout points. Therefore, there is no junction
compensating voltage involved in interfacing with the thermocouple,
provided only copper leads are used. Delta T = alpha*(Tout - Tin)
is then purely a product of a thermocouple constant alpha times the
thermocouple voltage. There are no zeroing problems or non-linear
functions.
I don't know why this method was not applied for CF experiments
where delta T was small.
Of course this will not help the situation at all if one of the
measuring points is under the influence of the primary circuit hot
water supply, i.e. connected to that nearby heat source by a thick
high thermal conductivity material like brass or copper.
Best regards,
Horace Heffner
http://www.mtaonline.net/~hheffner/
