Development of low-pressure calibration system using a pressure balance

Measurement 45 (2012) 2479–2481

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Development of low-pressure calibration system using a pressure balance Momoko Kojima ⇑, Tokihiko Kobata National Metrology Institute of Japan (NMIJ), AIST, Tsukuba Central 3, 1-1-1 Umezono, Tsukuba, Ibaraki 305-5863, Japan

a r t i c l e

i n f o

Article history: Available online 15 November 2011 Keywords: Gas pressure standard Pressure balance Pressure transducer calibration

a b s t r a c t A new calibration system for low-pressure is now under development at NMIJ/AIST. The new system is designed to calibrate pressure transducers in the pressure range of less than 10 kPa in absolute mode. The pressure generation technique needs only a single pressure balance and the pressure in the bell-jar is used as a calibration pressure instead of the system pressure. In this paper, the outline of the calibration system and the measurement results are described. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction At the National Metrology Institute of Japan (NMIJ/AIST), the national pressure standards are maintained in the pressure range from 1 Pa to 1 GPa by using a mercury manometer and many pressure balances. A pressure balance is reliable apparatus to generate/determine a pressure precisely so that it is used as the primary pressure standard in many places. However, there is the lowest limit of the pressure generated by a pressure balance due to the weight of the piston itself. The minimum pressure generated with a pressure balance is usually several kPa and our present minimum pressure (5 kPa) also comes from this limit. The pressure standards lower than several kPa is realized usually by other devices such as liquid column manometers or static expansion systems. They are good devices used as primary standards, although quite sensitive and complicated devices to get great performance out. Therefore, there are needs to develop the other primary standard for the low pressure range with easy to handle. One of the solutions for this is to use a pressure balance. The calibration system for low pressures using a pressure balance is a promising approach because it is relatively easy to use and it can also be used as primary standard.

⇑ Corresponding author. Tel.: +81 29 861 4378; fax: +81 29 861 4379. E-mail address: [email protected] (M. Kojima). 0263-2241/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.measurement.2011.10.041

In this paper, the principle of the calibration method with a procedure is introduced first, and then the outline of the developed calibration system and the result of some test calibration are shown. Finally the uncertainty of the pressure generated is estimated. 2. Calibration method The basic idea of the calibration system is similar to the Grohmann and Lee [1]. The main point of the idea is that the pressure in the bell-jar, instead of the system pressure, is used as a calibration pressure. Therefore devices under test (DUT) are connected directly to the bell-jar. The outline of the calibration method is as follows: first, the system pressure generated by the mass of the piston and weights is kept constant to make a line pressure. Then a small weight is loaded onto the piston and generates a small pressure additional to the line pressure. After the small weight is removed, calibration gas is let into the bell-jar until it generates the same pressure as when the small weight is loaded. Then the pressure generated by the calibration gas can be compared with the pressure generated by the small weight. Figs. 1 and 2 show the schematic drawings and the photograph of the calibration system, respectively. In figures, reference pressure monitor is used to measure the system pressure and a mass flow controller (MFC) is used for controlling the pressure inside the bell-jar. The detailed procedure of the calibration is as follows: Step 0: The pressure inside the bell-jar of the pressure balance is sufficiently evacuated (checked by a vacuum

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Fig. 1. Schematic drawings of the calibration system for a low-pressure transducer. (a) A small pressure is additionally generated to the line pressure by a small weight on the piston. (b) The same small pressure is generated by calibration gas (N2) instead of the small weight. The pressure inside the bell jar is controlled by monitoring the reference pressure monitor.

Automatic weight loading machine

small weight

Vacuum gauge

Small weight Δm

pref

gas

(1)

pref

(2)

small weight pref(3)

IR

DUT

Δmg/A

Pressure controller

IDUT

MFC

0

pbj(2)=Δp pbj(1)

pbj(3)

Time Pressure balance RP

Step0 Step1

Reference pressure monitor

Step2

Step3

Fig. 3. Measurement procedure in the calibration sequence.

Fig. 2. Photograph of the calibration system.

gauge), and the pressure balance is operated stably generating a certain line pressure in absolute mode. The DUT measures the residual pressure inside the bell-jar pbj (it is expected to be almost 0 Pa.), while the reference monitor measures the sum of the line pressure pl and the residual pressure as the reference pressure pref. Then the indications of the reference pressure monitor and the DUT, IR and IDUT, are recorded at the same time. Fig. 3 shows the measurement procedure in the calibration sequence (steps 0–3). In step 1, the reference pressure and the bell-jar pressure can be expressed as ð0Þ

ð0Þ

P ref ¼ pl þ pbj ð0Þ; ð0Þ

P bj  0:

ð1Þ

ð1Þ

ð1Þ

ð1Þ

Pref ¼ pl þ pbj þ Dmg=A; ð1Þ

Pbj  0:

Step 2: The small weight is removed and then the calibration gas is inlet into the bell-jar using the MFC. The MFC is controlled with monitoring the indication of the reference monitor to reproduce almost the same pressure recorded in step 1. In this situation, the reference monitor will measure the sum of the line pressure and the additional pressure Dp generated by the calibration gas in the bell-jar. Both of the indications IR and IDUT are recorded during the measurement. ð2Þ

ð2Þ

ð2Þ

Pref ¼ pl þ pbj ; ð2Þ

Step 1: A small weight with a mass of Dm is loaded onto the piston. The small weight generates a small pressure of Dmg/A additional to the line pressure. Here, g is the acceleration due to the local gravity and A is the effective area of the piston–cylinder. Then the indications IR and IDUT are recorded. The indication IR will be the sum of the line pressure and the additional pressure generated by the small weight, while the DUT still measuring the residual pressure inside the bell-jar.

ð2Þ

Pbj ¼ Dp

ð3Þ

Step 3: The pressure inside bell-jar is evacuated and the small weight is loaded again to make the same state as in the step 1, which is performed to evaluate the drift in the indication of reference pressure monitor during the measurement. ð3Þ

ð3Þ

ð3Þ

Pref ¼ pl þ pbj þ Dmg=A; ð3Þ

Pbj  0

ð4Þ

M. Kojima, T. Kobata / Measurement 45 (2012) 2479–2481

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Step 4: Assuming the following conditions: (a) the line pressure pl does not change during step 1  step 3, (b) the pressure inside bell-jar pbj is sufficiently evacuated and is almost the same pressure in both step 1 and step 3 and (c) the reference pressure in step 2 is the same with the average of the reference pressures in steps 1 and 3, ð1Þ

pl

ð1Þ Pbj ð2Þ Pref

ð2Þ

¼ pl

ð3Þ

¼ pl

¼

ð3Þ pbj

¼

ð1Þ ðpref

¼ pl ;

¼ pbj ð 0Þ; þ

ð5Þ

ð3Þ pref Þ=2;

we can obtain the pressure Dp with the following equation:

Dp ¼ Dmg=A þ pbj :

ð6Þ

The DUT can be calibrated by comparing its indication change during steps 1–3 with Dp calculated from Eq. (6). 3. Apparatus In the calibration system, the small weights are loaded/ unloaded onto the piston by using a weight loading machine which is originally fabricated for generating differential pressure [2]. The weight loading machine is connected directly onto the bell-jar so that the small weights can be loaded/unloaded without breaking the vacuum inside the bell-jar. The condition inside the bell-jar is well stabilized with the machine. The effective area of the piston–cylinder of the pressure balance is about 20 cm2. This large effective area is an advantage when the uncertainty of the pressure generated by a small weight is evaluated. Using the weight loading machine and the pressure balance, we can generate a small pressure from 1 Pa to 10 kPa in absolute mode with relatively small uncertainty. The system for controlling the pressure inside the bell-jar has been newly developed. The bell-jar pressure is controlled by an accurate mass flow controller (MFC) with a vacuum pump. The flow rate of the MFC is controlled precisely by a feedback control with monitoring the reference pressure monitor. With this controlling system, the belljar pressure is stably maintained enough time for calibration of DUT (several minutes). Fig. 4 shows an example of the indications obtained in a calibration cycle. In this calibration test, a diaphragm gauge with a full-scale of 13.3 Pa was used as a DUT. The fluctuation of the DUT indication in step 3 is within 0.001 Pa at a calibration pressure of 10 Pa. 4. Uncertainty estimation Uncertainty of the pressure generated using the calibration system is estimated. The most of the uncertainty factors are basically the same as those listed for the differential pressure calibration because almost the same instruments and operating procedure are adopted here for generating a small pressure. In the system developed, the means for evaluating the bell-jar pressure is added. The major factors of the uncertainty are listed as follows: Factors due to the calibration pressure (Dmg/A + pbj). – Small weight Dm.

Fig. 4. Results obtained in a test calibration. The calibration pressure is 10 Pa.

– – – – – – –

Acceleration due to gravity g. Effective area A(t). Temperature of piston–cylinder. Effective area at reference temperature. Temperature coefficient. Pressure distortion coefficient. Stability of pressure inside the bell-jar pbj.

Factors due to the comparison. – Due to the head correction. – Long-term stability of the reference pressure monitor. – Resolution of the reference pressure monitor. Factors due to the DUT. Using the present system, we are currently estimating the combined standard uncertainty (k = 1) to about 0.1 Pa at a calibration pressure of 10 Pa and about 0.2 Pa at 10 kPa, respectively. These values are expected to be reduced in the future improvement for the gas controlling system. 5. Summary A calibration system for low pressure has been newly constructed and is now under testing. We observed enough stability for the pressure inside the bell-jar. The uncertainty of the pressure generated is estimated to be sufficiently small for calibrating pressure measuring devices. In the near future, we are planning to improve the pressure controlling system to achieve smaller uncertainty and more efficient operation. References [1] K. Grohmann, H.K. Lee, Extension of the application range of a piston pressure gauge to low pressures, J. Phys. E: Sci. Instrum. 20 (1987) 1169–1172. [2] M. Kojima, T. Kobata, K. Saitou, M. Hirata, Development of small differential pressure standard using double pressure balances, Metrologia 42 (2005) S227–S230.