The Automatic Cold Valve
The automatic cold valve is used with the Orange Cryostats when they are operated primarily in the pumped mode.

The automatic valve is designed to control the flow of cryogen to the heat exchanger. One of its prime functions is to detect the onset of blockages in the valve and take steps to open the valve to counteract the reduction in flow.

When in operation a pressure / vacuum transducer feeds a signal to a process controller. The process controller compares that signal with a set point valve and passes a correction signal to an I / P converter. The I / P converter in turn sends a correction pressure to the actuating cylinder which increases or decreases the valve loading. In the event of a power failure the valve will fail closed and the liquid helium will remain in the cryostat. Failure of the air supply must be avoided.

• Electrical supply: 230 V Single Phase
• Air Supply: 6 bar, filtered
• Size: Front panel 116 mm wide × 135 mm high.
• Depth 300 mm

The Automatic Cold Valve System
Many of our cryostats are required to operate at base temperatures (less than 1.5 K) for more than a week. During this period the cryostat is filled several times and it is not uncommon for the cold valve to begin to block. One solution is to fully open the valve, and then reset the flow to the original specification. However, if the onset of blocking and reduction in flow occurs overnight, the result is a completely blocked valve in the morning. The cold valve and capillary must then be heated to remove the blockage. Sometimes the complete warm up of the cryostat is required, resulting in the loss of expensive machine time and cryogens.

The automatic cold valve system detects the onset of blocking and takes immediate steps to remove the blockage.

The System Layout
The system is comprised of the following components:

• Standard cold valve fitted with a 1" air cylinder
• Vacuum / pressure transducer
• Process controller
• Suitable I / P converter

The assumption is made that in steady state operation at base temperature, the pumping speed is constant and the vacuum in the pump line is proportional to flow.The system functions on the basis that any tendency towards blocking will reduce flow and will improve the vacuum in the pump line. This will be detected by the pressure transducer and a signal will be sent to the process controller.

The process controller will regard this changed signal as an error function and will send a correction signal to the I / P. The I / P will modify the air pressure applied to the valve cylinder and the pressure will decrease. This will allow the valve to open more and maintain the set flow, and hence the correct vacuum reading, in the pumping line.The automatic cold valve is an automatic flow control valve and functions independently of temperature.