Positive Pressure Leak Calibration Device

There are two types of standard leak holes: one type, one end of the leak hole is vacuum and the other end is an atmospheric pressure leak, which is called a vacuum standard leak; the other type, one end of the leak is added to the other end of the leak. Leakage gas is often called a positive pressure standard leak. The former leak is mostly used to calibrate the helium mass spectrometer leak detector; the latter is mostly used to calibrate the positive pressure leak of the halogen leak detector.
At present, positive pressure leak detection is widely used in refrigeration, machinery, automobile manufacturing and other industries because of its advantages of convenient operation, portability and low cost of leak detection. Positive pressure leak hole is used for sensitivity calibration of positive pressure leak detector, and the uncertainty of leak rate calibration is directly related to the sensitivity of leak detector and the effectiveness of leak detection. At present, the calibration specifications of many national standard leaks at home and abroad include the calibration methods and corresponding devices for positive pressure leaks. The relevant units of China's aerospace department have also established a very complex positive pressure leak calibration system for the needs of positive pressure leak detection, which is difficult to promote because of its high cost. The improved leak calibration device (hereinafter referred to as the device) established in this paper has the advantages of low cost, small uncertainty, high automation and simple operation, and is easy to popularize.

1.1 device structure
This device is a volume compensation structure. The device adopts a differential pressure film gauge 120AD produced by MKS Company, with a measuring range of 1Torr, a measuring accuracy of 0.15 of the indicated value, a minimum pressure resolution of 0.01Pa, a non-repeatability of 0.01 FS(FS is full scale), a zero temperature coefficient of 30ppm.FS/℃, a full scale temperature coefficient of 0.01 FS/℃, and a measuring piston outer diameter of 2.50mm, the measurement error is not more than ± 5μm, the piston rod is pushed by a micrometer, the standard thermometer has a resolution of 0.1 ℃, and the atmospheric pressure is measured by a 1000Torr capacitance film vacuum gauge produced by Setra Company with an accuracy of 0.2%. The device has two operation methods of constant pressure and constant volume.

1.2. Constant pressure operation method and working principle
Q=P0·X·A·Td/T·Dt (1).
Where: Q-leakage rate of standard leak hole, Pa · m3/s;
P0 -- Absolute pressure in the calibration chamber at t1 (can be replaced by the local atmospheric pressure at t1), in Pa;
A- sectional area of piston rod, m2;
X-piston moving distance (L2-L1), unit m;
Tτ -- Reference temperature of standard leak hole, unit: K;
T -- Ambient temperature, unit: K;
Δt -- time interval between start and end of calibration (t2-t1), unit 1.
The standard leak rate calibrated by this method cannot be too small, otherwise the pressure fluctuation caused by the piston movement will have too much influence. Through the experiment, the leakage rate of more than 10-5Pa · m3/s can be conveniently used this method. This calibration method and its calibration uncertainty have been introduced in the article. For the leakage rate less than 10-5 Pa · m3/s, another method is needed for calibration. In addition, this method cannot realize instantaneous leakage rate measurement and computer data acquisition.

1.3. Fixed volume operation method and working principle
The constant volume method cannot be calculated by the formula of Q = △P · Vx/△t, because the thin film meter measures the pressure by the principle of elastic deformation of the thin film, so Vt and Vx have changed, and the correctness of the calibration data is seriously affected. Taking this device as an example will cause a negative deviation of about 20%. Through theoretical calculation and experimental verification, we found that the leakage gas volume and pressure value are approximately linear.
According to the equation of state of the ideal gas under constant temperature conditions:
P0Vt=Pτ(Vτ-67V)2
P0 · VX = px· (VX ❓v) q· T (3)
According to the above formula can be obtained:
Qt=Vx[P0·(Px/P0Vx Pτ/P0Vτ)·△P] (4)
When △P is less than 1KPa, Px/P0 and Pτ/P0 are approximately 1, then
Qt=Vx[P0·(1/Vx 1/Vτ)·△
According to the working principle of the differential pressure film gauge, △V is proportional to p, then Q t is proportional to p, which can be obtained.
Q = k dp/dt = k· D (P0 p) /dt = K dp/dt (6)
The data in Table 1 proves that the leakage gas volume of the device and the pressure indication value of the film gauge conform to the linear relationship, and the linear error is not more than 0.3 of the indication value, which can meet the requirements of calibration of positive pressure leak rate. The volume coefficient k value can be measured by the following methods: install the standard leak, adjust the micrometer to the position of 0mm, close valve 1 and valve 2, read the pressure indication p1 of the film gauge after the system is stable, push the micrometer to x mm, and read the pressure indication P2 of the film gauge. The value of k can be calculated as follows:
　　k=P0·x·A/(P2-P1)  (7)
Where: k is the volume coefficient, unit m3;A is the piston cross-sectional area, unit m2.
Repeat the above steps for at least three times, and the deviation of the calculated k value shall not exceed ± 1%, otherwise the system must be checked for air leakage. Then close valve 1 and valve 2 again after the system is emptied, and start timing when the pressure indication value of the film gauge rises steadily, and calculate the leakage rate value of the standard leak hole according to formula (6).

2, the device's measurement range and calibration uncertainty.
2.1 measurement range is 1 × 10 -4 Pa · m3/s ~ 1 × 10 -6 Pa · m3/s
2.2 calibration uncertainty: U = 5%(k = 2)

3. Installation conditions of the device
3.1.1 Power requirements: 220V is not less than 5A.
3.1.2 Ambient temperature is (23±5) °C, and the room temperature change during calibration shall not exceed ± 1°C.
3.1.3. The relative humidity is not more than 80%.
3.1.4 Nitrogen with a purity of not less than 99.9%.

4. Equipment operation
This equipment is a fully automatic calibration device.