Transducers
Troubleshooting the Transducers
The Math
There are a few ways to check the transducer for accuracy. If we are talking about transducers found in the outdoor unit, or an air handler, you may simply go to the EcoNet control center to check it. Take caution in reading it though. In the 600 Series thermostats, it is likely displayed as PSIA. As technicians we use PSIG (Gauge Pressure). You can similarly convert PSIA to PISG by subtracting 14.7 from the PSIA value. In the 700 Series smart thermostats, it will be displayed as PSIG. Just be careful reading this.
Also, the low pressure transducer (Part Number 47-103454-05) , may not read if the pressure is higher than the range of the transducer. Consider an air handler for instance. In heat mode, it will be subject to high side pressures as it is between the discharge and liquid line at that point. The low pressure transducers measure between 14.7 PSIA and 314.7 PSIA (That’s 0 and 300 PSIG to you and me). If the pressure is over 300, it will not show a pressure. This does NOT mean the transducer is defective.
If you are dealing with a cased furnace coil or if you have a legacy wired system, you can only check the transducers with voltage. It’s not complex math, but it will generally require the use of a calculator. There is a calculator with the formula set up at the bottom of this page. If you are using Internet Explorer, it may not load properly. Suggest Firefox, Chrome, Safari, or mobile device.
The Formula for the suction transducer is:
PSIA = 375 x (Vout/Vin)-22.8
PSIG = PSIA – 14.7
Scroll to the bottom of this page for a link to an online calculator but its important to know the math too.
For some of us, this can be intimidating. Basically what it says is that you need to measure the voltage in the back of the 3 pin connector from the transducer where it plugs into the board. Its best to use smaller test probes and you will be measuring voltage in DC volts. From Red to Black, you should see something like 5 volts DC. It can be higher or lower, but often close to 5.0 or 5.1 volts DC. This measurement is your voltage in. (That 5 vdc goes into the transducer.) let’s call it 5.0 for argument’s sake. Then measure from Green to Black. This is the variable voltage out of the transducer. Today we will use 2.1vdc as an example.
Getting back to the formula… PSIA = 375 (2.1 / 5.0) – 22.8
You will need to do the math in the parentheses first. (Google: PEMDAS). So… 2.1 divided by 5.0 which equals 0.42. Now multiply that times the 375. 375 x 0.42 = 157.5. Now subtract 22.8. 157.5 – 22.8 = 134.7. This is your PSIA. You may now subtract 14.7 from your 134.7 resulting in a PISG of 120. You may then compare this to your gauges. This is assuming your gauges are remotely close to being accurate. For what it’s worth, zeroing your gauges is NOT calibrating them. I recommend regular proper calibration of your gauges. You’ll have to search the internet if you are not sure how that is done. More often than not, the transducer is going to be more accurate than analog gauges that have been tossed in the back of a service truck for years.
Liquid Line Transducer
In the (-)P20**B models, a liquid line transducer is also being used. The range for this transducer is 14.7 PSIA to 664.7 PSIA. Again for you and me, that’s 0 to 650 PSIG. Because its range is different, the formula is also different. The part number for the Liquid Transducer is: 47-103454-01. You will notice the 3 wire harness connecting it is Red, Purple and Black. Red to Black is your Vin, and Purple to Black is your Vout.
The formula for the liquid transducer is:
PSIA = 812.5 x (Vout/Vin) – 66.55
PSIA = PSIG – 14.7
