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Why do I care about slew rate?

Posted by Jennifer Collins & William Lewental on Jul 23, 2020 2:00:00 PM

In a critical process it may be necessary to slowly bring the vacuum level up or down at a specified rate so as to not disturb process material. This can be referred to as the slew rate, or ramp rate, of the vacuum level. A new feature for the Stratavac Touch is the ability to set a slew rate along with a setpoint for a valve and the controller handles the details.

 
In Vacuum, what we mean by slew rate is a specific or defined change in pressure over a defined timeframe.
 
Some people may also refer to this as Ramp Rate. ΔP/Δt Change in pressure/change in time.
 
Okay that makes sense but how do I actually utilize this feature?

 

A catch-22 in solvent recovery is trying to achieve a high vacuum level as quickly as possible. The faster you arrive at the vapor pressure level for the specific solvent, the faster you can recover said solvent. However, evacuate the chamber too quickly and a user can end up bringing the solution along with the solvent into the condensing column (bumping), wasting time and material. You can even end up with solvent in your pump.

Bumping and foaming are problems that commonly arise during evaporation and can lead to sample loss and contamination.

Top 4 Reasons Bumping or Foaming Occur

  1. Heat/Temperature is too high in the water bath
  2. Rotational Speed of RotoVap is too high
  3. Vacuum pressure is too high
  4. RotoVap Flask is more than ½ full

Foaming may result from surfactants that may be present in natural extracts as well as artificially made products. While foaming is slower to occur it is also harder to get rid of. Foaming is hard to avoid, but can be minimized if you slow the rate of vacuum on the system.

Automated vacuum controllers like the StrataVac Touch can help you minimize or quickly resolve bumping or foaming in your process.

Tips for Using Automated Vacuum Control to Avoid Common RotoVap Mistakes

A user may want to arrive at 250 Torr as quickly as possible. But to avoid the common rotovap mistakes, you want to avoid going full throttle on your vacuum pressure. Think of it like the difference of jogging down a slope or shooting down the slope on a skateboard and possibly wiping out some of your desired product along the way.

Or you can utilize a Slew Rate (Ramp) to Set-point.

Initial RAMP: SP 250 Torr | Slew 0 Torr/min

Set a setpoint of 250 Torr with a Slew Rate of 0 Torr per minute, and the controller will regulate as quickly as your vacuum pump can evacuate the chamber.

Secondary RAMP: SP 180 Torr | Slew 60 Torr/min

Once at 250 Torr, let's say the user wants to go to 180 Torr. The user can enter the setpoint as 180 Torr and set a slew rate of 70 Torr/ Minute. The controller will now maintain a vacuum level that decreases by about 1 Torr per second until the vacuum level reaches 180 Torr, which the controller will maintain indefinitely. A more controlled vacuum pressure rate of increase will help to avoid bumping.

Changing the temperature in the water bath is a very slow process. Once bumping is occurring, changing the temperature will not help you save precious solutions. Modulating vacuum, however, can stop bumping the moment it is noticed with the help of a vacuum controller.

The moment you see any sign of bumping, a user can press “CLOSE” and the valves close allowing the pressure to rise. Press the “setpoint” button once bumping has stopped and now the valve will begin climbing towards the specified setpoint once again.

DigiVac offers 2 Product Options that offer onboard RAMP control

CONCERTO

Vacuum & Pressure Controller | Multiple Rotary Evaporator Controller (Up to 4). It will control a combination of up to four RotoVaps, Vacuum Chambers or Vacuum Ovens with a Single Vacuum Pump. See product page here

Concerto is Another innovation from Across International and DigiVac

See a video of the Feature Set of the Concerto Vacuum Regulator here

See a video on RAMP vacuum control here. On the Concerto we call it Recipe Control. And how to customize recipes for RAMP control here.

Key Features

  • Fine control of target vapor pressure for more precise compound separation through distillation
  • Enables using reliable and long lasting rotary vane oil pumps & dry scroll pumps in applications where diaphragm
    pumps were previously required
  • Keeps vacuum pumps near their base pressure resulting in greater longevity
  • Ease of use through precise automated control with programmable recipes to support RAMP vacuum control and graphing of vacuum levels
  • DigiVac Proprietary Valve Design provides Innovative Valve Control | performs both proportional throttle and bleed vacuum control (vent to atmosphere). The integrated bleed design allows you to quickly vent to atmosphere to stop bumping during processing

StrataVac Touch with the DINAMO valve

Solvent Recovery Control Bundle | Get More Flow with Dinamo

See product page here

Control Rotary Evaporator or Vacuum Chamber with this Dual-Valve design for both proportional and VENT control. This StrataVac Touch bundle includes the proprietary valve by DigiVac that has integrated bleed and high flow throttle control in one.

Key Features

  • Measurement range: 1 Torr to 775 Torr
  • Control Range: 2 Torr to 770 Torr
  • Recipe Control: Program RAMP and HOLD vacuum at different duration (Time) and vacuum levels (Torr)
  • Ideal for Falling Film Applications: The Dinamo valve delivers precise vacuum control with the benefit of being able to pump down large vacuum chambers very quickly
  • Regulates Chambers from 1 to over 100 Liters
  • Check out a video on how fast the Dinamo Valve can pump down a 50 Liter Rotary Evaporator from atmospheric pressure here
  • Check out the DigiVac software platform DV Cup that provides and overview of available commands here

Just another way we are innovating, as the vacuum control experts.

Topics: StrataVac 2018, Vacuum Control, Multi-sensor Vacuum Controller, Vacuum Technology