Here at CPS we’re all about making continual improvements to the services we provide to our customers. This often means investment in new and better test equipment to enable us to provide more accurate calibrations with lower uncertainties. Our temperature calibration lab in New Plymouth is one area we’ve recently made such improvements with the additional of a Fluke 7321 liquid calibration bath. A good liquid bath offers far greater spatial uniformity than a dry block calibrator and therefore a lower uncertainty contribution to temperature calibrations. The Fluke 7321 bath operates between -20 °C to 150 °C and has greatly reduced our reported uncertainties within this range.
So how do we work out exactly what uncertainty the 7321 bath does contribute to our calibrations? In order to arrive at a figure we must do a spatial survey of the bath to determine how uniform the temperature is throughout the bath. The manufacturer has specified what the uniformity should be, but we need to validate their claims. The manufacturers specs are also obtained with different fluids at different temperatures and it’s just not practical for us to be constantly changing 16 litres of fluid during calibrations.
The fluid we selected to use in our bath is a silicone oil with a viscosity of 10 centistokes (cs) at 25 °C. The viscosity changes with temperature, it will be more viscous (or thick) at lower temperatures and will become less viscous as the temperature increases. This has an effect on the stability and uniformity of the bath. The uniformity will also tend to get worse the further you move away from the ambient room temperature, so we needed to do the spatial survey at either end of the usable range. Our survey looked for three factors. The axial variances (up & down), the radial variances (side to side), and the stability of the bath. We placed a stationary probe into the centre of the bath at a mid depth of around 200 mm. Our second probe was our moving probe which was moved around the bath while measurements were taken from both probes. The moving probe had a 180 deg bend at the end where the sensor is located. This is to eliminate or at least minimise immersion errors when the moving probe nears the top of the bath.
To survey the axial variances we first placed the moving probe in one corner of the bath at the very bottom. With the 180 deg bend this meant that the sensing element was approx 35 mm up from the bottom at a depth of 390 mm and 35 mm from the outside edges of the bath. 15 readings were taken from both probes and then the moving probe was moved up 40 mm and left to stabilise for 5 minutes. This process was repeated with the last set of measurements taken at 10 mm below the surface of the bath liquid. This whole process was again repeated in each corner of the bath. I was amazed at how little axial variance there was even up to only 10 mm below the surface!!
To survey the radial variances the two probes were placed at the same depth in opposite corners of the bath approx 35 mm in from the outside edges. A series of approx 40 readings were taken before the probes positions were swapped and another set of readings taken. This process was repeated at three different depths.
To survey the stability of the bath was a simple matter of taking a series of readings from a stationary probe in the middle of the bath over a period of time.
The variances observed were then combined to give us an overall uncertainty figure for the bath at -20 °C with the results shown here:
Value Type Divisor Std Uncertainty Deg of Freedom
Uaxial 0.0032 Std 1.00 0.0032 706
Uradial 0.0019 Std 1.00 0.0019 276
Ustab 0.0037 Std 1.00 0.0037 707
Uc 0.0053
Eff Deg of Freedom 1648
K 1.96
Expanded Uncertainty 0.010
The steps in this survey were then repeated at the top end of the range at 150 °C with the results shown here:
Value Type Divisor Std Uncertainty Deg of Freedom
Uaxial 0.0064 Std 1.00 0.0064 706
Uradial 0.0010 Std 1.00 0.0010 276
Ustab 0.0022 Std 1.00 0.0022 707
Uc 0.0068
Eff Deg of Freedom 907
K 1.96
Expanded Uncertainty 0.013
As you can see from the results the expanded uncertainty ends up being very similar. The axial variances were a lot higher at 150 °C however the bath was more stable as you would expect. These are some pretty good numbers when you consider the immersion depth required is only 10 mm and we’ve used the same fluid right throughout its range.
To complete this survey took 4 days with a total of 3,388 measurements made!!
Overall we are very happy with the performance of our 7321 bath, far better than what Fluke specify. The joy for us is our uncertainties within its working range are now 5x better than what we had previously. It also gives us the ability to handle larger and odd shaped probes as well as multiple devices simultaneously which saves us time and money. A great investment for us and it means you our customers can have even greater confidence in the accuracy of our measurements in your thermometer calibrations.
Paul Martin
Laboratory Manager
CPS (NZ) Ltd – New Plymouth, NZ