Simulating Small Pressures; My Inch Water Column Simulator

I suspect that just about everyone who does what I do and has been doing it for a while has found themselves standing out in the field trying to set up or check calibration on a static pressure switch or transmitter by trying to blow gently and steadily into the input tube while watching the reference instrument and turning a calibration screw.  This is usually done on top of a ladder with poor lighting and no place to set anything down.

After having gone through that one time, I decided there must be a better, more elegant way that didn’t cost a fortune (you can find a number of elegant ways if you poke around on line but they are kind of “spendy”).  Thus evolved the design of my inch water column simulator pictured below.

image

It operates on the principle that flow through a pipe generates a pressure drop.  So by varying the flow, I can vary the pressure that exists at the end of the circuit and use that pressure as a way to simulate a low pressure signal.

I was sharing this with Bill Pottinger at the Pacific Energy Center the other day while setting up a lab with him, and he thought it was a good enough idea that I should put up a blog post about it.  So here it is.  I know for a fact that I am not the only person who came up with something like this.  For instance, Ron Simens, whom I have mentioned in the past, came up with a similar arrangement at one point.

The heart of the system is small aquarium pump and the general idea is to use a combination of tube run, fixed leaks (the tees) and variable leaks (the valves) to vary the flow from the pump through the tubing.  High flow rates generate high pressure drops and vice versa. 

So, by connecting the instrument you are going to set up (along with the reference instrument) into the circuit near the valve assembly, you can simulate a wide range of pressures by manipulating the valves and adding or removing tees, as illustrated in these pictures. of my machine in action.

image

image

image

image

image

Having a pretty good length of tubing in your coil is critical in terms of getting to the lower pressures.  I could get lower than the .01 in.w.c. value by adding more tubing, but for my purposes, that usually has been low enough.

The needle valves give you quite a bit of adjustability, meaning you can fine to the signal quite a bit by manipulating them.  Mine is pretty non-linear so I usually just play with it by adding or removing tees with everything wide open until I get in the lower end of the  range I need.  Then I fine tune it to the exact signal I need by playing with the valves.

Depending on what is lying around in your shop and the quality of the stuff you buy, you can probably put one together for $30 or less, maybe  nothing if you happen have had an aquarium at one time that is no longer in use since the little gang valves you buy at the pet shop to manage where  the bubbles go can take the place  of the needle and ball valves.

 David-Signature1_thumb1_thumb_thumb_

David Sellers
Senior Engineer – Facility Dynamics Engineering
PowerPoint-Generated-White_thumb1_th[2]

Click here for a recent index to previous posts

This entry was posted in Controls. Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s