What the boss showed you is the way I learned as well. The output should be at the mid-point of the valve spring range at setpoint. I am assuming that your valves had a spring range of 3 - 9 psi. In order to find this point you must set the setpoint at the current temp in the space.

 

that doesnt sound right. I would set thermostat setpoint to actual room temp then set pressure to 9 psi for a d.a.& n.o. valve. this is the point to which the valve should be closed and not calling for heating. I may be wrong but if you set it to 6 psi you would still be heating when the stat is sattisfied. you can find your spring range if it is not labeled by putting a inline gauge on the branch tubing feeding the valve. you could also use a squeeze bulb and manually do it.

 

follow dappers advice, it is correct. Find the spring range, get an accurate reading of the room air temp, set the dial on the stat to that temp and adjust the air pressure to the mid point of the springs. You are doing it correctly.

 

JOHNSON's calibration info is set it for 9 PSI when setpoint is = to room temp as they have ALWAYS used the midpoint of the 3 - 15 PSI range as the correct calibration.

I have been doing PNEUMATICS for 32 years and the info from back in the 50's - 60's right up to today all state the 9 PSI.

 

Trust Control Man,,, He's right..... At HW we were taught to calibrate to 8# or midway between 3-13# control range. Only the PP904 at 9# which I was told is the JCI number.

 

and what if we are contolling a heat valve with a 3-6 spring range. You still going to set that stat up at 9 pounds at room temp? Dont think so... I was taught at the same school that Johnson had several of it own employees at. You calibrate at the mid range of the actual springs that you are dealing with, not just some wag of 9. You set your RC's up to 9 or do you actually set them to the spring ranges of what your controlling?

 

I have to agree with mtnwrench on this. There does seem to be some confusion however. The original post only mentioned reheat coils, therefor the tstat should be calibrated to the spring range of the valve.

If this was a VAV system on the other hand things would be different. Then you would calibrate to the midpoint of the total range. The VAV controller would be used to set the low and high limits of the damper position. When the output of the stat is in the 3 - 8 psi range the valve will modulate with the damper at minimum. From 8 - 13 psi the valve will be closed and the damper will modulate open. In this case you would use the low point of the valve spring range and the high point of the damper spring range and set to the midpoint of these. You could also use the midpoint of the deadband between the two ranges.

 

General calibration for BARBER COLMAN , ROBERTSHAW , HONEYWELL , POWERS / SIEMENS and KMC thermostats all use the 9 PSI calibration.


As for the 3-6 spring range , as the average TSTAT changes the output 2.5 PSI per 1'F sensed value change you would actually get better control using 9 PSI the valve would be closed at setpoint and would start to open at a 1'F temperature drop., the midpoint method would only have the valve closed 50% and would overshoot the setpoint but then we are talking + or - 2'F accuracy.

But as noted on many posts by many posters everyone has their way of doing things , you do it your way I'll keep doing mine my way and as I have had no complaints after all these years

 

controlman where are you getting your 3-15 range? I'm not following!

[Edited by svc on 08-30-2006 at 08:51 PM]

 

But as noted on many posts by many posters everyone has their way of doing things , you do it your way I'll keep doing mine my way and as I have had no complaints after all these years

On this, I will agree with you, as long as your customers are happy and comfortable, how you get here really doesnt matter.

 

3 to 6 PSI =heat 9 to 13= cool. So at 6psi on a DA stat the heating valve will start to open and will be fully open at 3psi and below. At 9 psi the cooling valve starts to open and will be fully open at 13 psi and above. Therefore there is a 3psi deadband from 6to 9 and the calibration setpoint is 8. There are different spring ranges and reverse acting stats out there so be carefull. Ken

 

I dont think you will see a huge diference between setting the calibration @ any of the later listed post. keep in mind there is a easily accesible calibration point integrated in all thermostats! The thermotat! just bump it and it will change

 

T-4002 and T-4100 (See Fig. 1)
1. Insert the test gage.
2. Furnish supply air to the
instrument (restricted supply
for T-4100).
3. Note the ambient
temperature in the space.
4. Turn the set point dial to that
temperature.
5. Turn the calibration screw
until the output pressure is at
the mid spring range of the
controlled device.
6. Turn the set point dial to the
desired set point.

From the JCI website.

 

I have always done them with a 0-7 for one band and then 9 to 15 for the other that way there is a deadband in between where you wont have anything running. I have found this to be useful with a fan-powered box that utilizes heat strips. Fan will energize at 9 psi, 1st stage of heat at 12 psi and then the 2nd stage of heat at 15 psi.

I am by no means a pneumatics supertech but do operate a whole lot on common sense and this seems to fall into that category.......

 

Setting the calibration for 9 PSI instead of 6 PSI is = to a 1.2'F variance , not that critical for your BASIC heating control as most thermostats have an accuracy of + or - 2'F.

 

Calibrating a Pneumatic Thermostat

1. One thing to remember is that there's a difference between the stated spring range and actual spring range. The spring range includes all the output devices the thermostat is controlling. This may include reheat valves, vav boxes, mixing boxes, fan coil units, cooling coils, etc... It may take a little longer, but I would verify the actual spring range before calibrating. You can use a pump up bulb with an accurate pressure gauge.

2. After gathering the information from the output devices I would verify the thermostats sensitivity. What is sensitivity? It's the pressure change per degree of temperature change. Most thermostats come with a 2.5# factory calibrated sensitivity. That means if the temperature changes 1 degree then the output pressure changes 2.5 PSI, 2 degrees then would change the output 5 PSI.

3. The last step would be to set the thermostat to the room temperature and calibrate to mid-spring range. There are times when you may have to calibrate at a different pressure depending on the system..

 

1. One thing to remember is that there's a difference between the stated spring range and actual spring range. The spring range includes all the output devices the thermostat is controlling. This may include reheat valves, vav boxes, mixing boxes, fan coil units, cooling coils, etc... It may take a little longer, but I would verify the actual spring range before calibrating. You can use a pump up bulb with an accurate pressure gauge.

2. After gathering the information from the output devices I would verify the thermostats sensitivity. What is sensitivity? It's the pressure change per degree of temperature change. Most thermostats come with a 2.5# factory calibrated sensitivity. That means if the temperature changes 1 degree then the output pressure changes 2.5 PSI, 2 degrees then would change the output 5 PSI.

3. The last step would be to set the thermostat to the room temperature and calibrate to mid-spring range. There are times when you may have to calibrate at a different pressure depending on the system..

Hey thanks just trying to get a grasp on this...
I have a Johnson Controls T-4002 DA thermostat that controls 1 VAV box damper actuator ( normally closed with 5 - 10 psi spring range) as well as a HW valve (normally open with a 3-7 spring range). The VAV box controller is a KMC controls CSC-3011-10 controller. Given that information it would seem at 6 psi we would be cooling and heating.... It doesn't seem right but then again I'm still learning... Would I still be right to calibrate the thermostat to 8 psi?

 

Calibrating a pneumatic thermostat

Here is a link to a really good video showing how to calibrate a Honeywell pneumatic thermostat, the thing is if you know how to calibrate on brand of pneumatic thermostat you know how to calibrate every brand:Removed Link

 

Here is a link to a really good video showing how to calibrate a Honeywell pneumatic thermostat, the thing is if you know how to calibrate on brand of pneumatic thermostat you know how to calibrate every brand: Removed Link

Although the intent is there, that is not really a good control calibration video simply because he is holding the thermostat he is calibrating and assuming the air temp is at 74 degrees. His heat from his hand offsets the true calibration process. I'm surprised to see this on a training video.

And, the only way to properly calibrate any control system is to first understand that individual control system and sequence. Calibrating any control according to the mid range of one control device does not take into consideration the intent of the entire control sequence.

 

Pneumatic Theory

Send me your email address and I will email you the Johnson Controls Pneumatic Control Manual

 

You calibrate for the midrange of the application. Yours is midrange between cooling and heating under the system you describe.

 

I just drew it out I understand it much better. thank you for responses.

 

This may help, I recently did a cabinet unit heater with a 5 to 10 PSi normally open hotwater valve and a pneumatic to electric switch that brings on the fan. I left the sensitivity at 2.5 PSi change per degree. I decided to set the fan to come on at 5 PSi and calibrated the tstat at 7.5 PSI. What happens is as the temperature lowers from setpoint the heat valve comes on and starts to warm the area. Normally the fan doesn't come on until the tempeture drops a little more than a degree drom setpoint.

 

I'm trying to calibrate a reverse acting Johnson Controls T-4002 stat. Would the calibration set point still be 8psi?

 

Real world check here. Doesn't really matter if you calibrate to 5,6,7,8,or 9# air pressure at the stat. Just always do it the same. I always use 9# because the midrange of the JCI stat in the question here and the stat I like is 9#. (15#-3#)/2+3#=9# midpoint. Now to qualify "always", I might cheat a stat a little to get rid of complaints. Set all stats at 72°F, but if a particular person needs their room warmer to not complain cheat the Cal a degree to be able to set the stat at 72 and still have a happy occupant. Now I say this while I have not calibrated a Pneu stat in years. All computers now. Just don't let the smoke out and most of these new electronic things work pretty good!:yes::yes::yes:

 

Thanks Bill.

 

Back in the day we could also use a chart recorder monitoring temperature and see how the thermostat reacts through the day. Then we could recalibrate as necessary to achieve our setpoint. Then once we got a good setting we would mark them with nail polish to make sure they didn't get tweaked.