Welcome! Here are the website rules, as well as some tips for using this forum.

# Honeywell L91B - Modulating Pressuretrol Throttling Range

Member Posts: 57
All - For the life of me I cannot get an understanding of how the throttling range in this control works. Specifically, if I have the set point at 65 and put the throttling range at B what does that get me? Thanks for any and all input.

• Member Posts: 12,623
Look on page 2&3 of the instructions. For a 0-15 psi control the difference between A-B=1.8psi, B-C 1.8psi etc. so the diff between a-c would be 1.8 + 1.8=3.6psi

You want the widest differential (or throtteling range) you can live with. To low causes hunting of the controlled device, to wide causes wider pressure swings.

To low a diff will cause the wiper arm to burn the potentiometer

To answer your question if you are using a 150psi control(which I would think you have if your set point is 65) A-B= 7.8 psi

So set point 65 diff set on B. I would think A-B 7.8psi so 65 psi would be low fire and high fire would be about 73 psi.

Scales are not perfectly accurate
• Member Posts: 57
@EBEBRATT-Ed - Thanks for the feedback. I really appreciate it. So if I'm following you; if I set the throttling range to min I would get 0 psi control diff and A would get me 1.8. Also If I'm understanding things wouldn't high fire be at 65 and low fire at 7.8 psi above the set point in this example? Also on page two it shows a throttling range at midscale to be 1.5 to 12 on this control. This is where I get really confused. I don't see how this line of thinking equals the midscale statement. Thanks again for trying to make me smarter.
• Member Posts: 12,623
Depends, I don't know which control you have. You say set at 65. Do you have a 150 psi control?

If so, the differential id 7.8 psi for a change of a-b, b-c etc. Differential is addative to the set point I think.

I don't usually worry about the settings. Once I get the main set I adjust the differential to keep the tight control without letting the mod motor (or valve) whatever you are controlling hunt. These two adjustments are contrary smaller diff=tighter control but causes hunting. Wider diff ==looser contrrol reduces hunting so every adjustment is a compromise
• Member Posts: 57
I do have a 150 psi control but but was trying to keep us on the same page by talking about the info on the actual control you were talking about. "sorry for the confusion" I just don't understand the midscale statement and the psi difference statement aligning with each other. Obviously is does but I'm just ignorant enough to not understand/follow it.

Thanks again for trying to help me.
• Member Posts: 12,623
@Dale_3

The settings on the scale are somewhat meaningless. You have to adjust it to get what you want.

If you look in the instructions it says that the set point (that's the main pressure setting) is on the low end of the throtteling range (differential) (see page 2 in the notes where it says setpoint).

That means if you set the main setting at 65 and the and the diff at minimum your pressure will modulate between about 65 and 70 psi (because on page 2 it says throttling range is 5-23psi. So 5 would be minimum and 23 is max. When you adjust A B C D this adjusts the differential between 5-23 psi but 65psi remains at the low end if that is what the main is set for