Absolute capacitance diaphragm gauge measures pressure in the range 1- 1000 mbar with resolution less than 1 mbar. This device is a direct-type of heated gauge where measurement is independent of the gas type thereby ruling out the variation in its sensitivity to different gases.

This gauge works on a principle of change in capacitance as a function of pressure. In particular, this device consists of two regions namely 'measurement volume' and 'reference volume' separated by a thin diaphragm of 0.3 mm thickness made up of Inconel-625. The measurement volume is made up of Inconel-625 and reference volume made up of SS304. The reference volume is permanently sealed at vacuum 1 x 10^-8 mbar using NEG (Non-evaporable Getter) pumps. In reference volume an electrode is mounted in close proximity to the diaphragm which together forms a capacitor.

The variation of pressures in measurement volume leads to variations in gap between the diaphragm and reference electrode resulting in variation of capacitance. Capacitance of gauge is measured using principle of 'Discharge Time Measurement'.

The signal is transmitted through the industry standard 4-20 mA signal and MODBUS/RTU on RS-485 network.

1. Measures pressure in the range 1 – 1000 mbar with resolution of < 1 mbar
2. It is a direct-type of gauge whose performance is independent of gas type.
3. Wetted material is Inconel-625 and wet volume is maintained at 343 K
4. Response Time: 100 ms
5. Warm-up time: 3 h
6. Over pressure limit without damage: 3 bar
7. Industry standard 4-20 mA and MODBUS-RTU communication on RS485
8. Calibration : In-situ 10-points
9. Works with input power of +24 VDC/16 W
10. Tested shelf life > 4 Years

The Absolute CDG along with signal processing circuit and heater circuit is an instrument developed for measuring the pressure in the range 1-1000 mbar with accuracy less than 1mbar. This device is a direct type of gauge which is independent of the type of the test gas. This device consists of two parts:

1. Mechanical Part: The diaphragm of 0.3 mm welded between the measurement volume of Inconel-625 material and reference volume of SS304 material. The reference volume has reference electrode is firmly placed close to the diaphragm and permanently sealed at vacuum 1 x 10^-8 mbar by activating NEG pill pumps. This diaphragm and reference electrode together forms a capacitor.
2. Electronic Part: The mechanical movement of diaphragm leading to variation in capacitance is measured by measurement circuit. And there is another circuit for heating the measurement volume and keeping it at 350 K.

The device is fixed in an aluminum/SS304 container with mil-grade 6 pin connectors. The capacitance signal is processed and transmitted through industry standard 4-20 mA connector and USB connector for continuously monitor the data.

Technical Specifications

Sr. No.	Specification	Value
1	Pressure Range	1-1000 mbar
2	Resolution	0.025% of Full scale
3	Fittings	KF-1/4" Or CF-16
4	Leak integrity	< 10-9 mbar l/s
5	Ambient operating temperature	25 to 50º C
6	Input power	+24 VDC, 16W
7	Output Signal	4-20 mA (up to 500 m)
8	Communication	MODBUS-RTU on RS485 (up to 1 km)
9	Electronics	Analog at front end and digital thereafter
10	Warm-up time	3 hours
11	Connector	6-Pin mil-grade connectors
12	Wetted volume	13.7 cc
13	Overpressure limit	3 bar
14	Wetted material	Inconel-625
15	Life span	> 5 Years
16	Dimension	150 mm x 80 mm (Diameter)

1. R&D laboratories.
2. Oil industries (Refineries)
3. Various experimental stations involving high corrosive gases.
4. Storage tanks of reactive gases.

Facilities required for commercial production	Welding Machine Hydraulic based pinching tool RF heater facility for localized heating for activation of NEG pill pump PVD System: Rotary + Turbo molecular pumps Helium Leak Detector Vacuum Gauges: One standard reference gauge (Absolute CDG: 1000-1 mbar) Full range gauge (Atmosphere-10-9 mbar) Temperature measurement system
Raw materials and their sources of availability	Inconel 625, SS304, Gold, and alumina are easily available in local market.
Manpower	Science Graduate: 1No Electronics Engineer: 1No Technician: 1 No