Siemens LOGO! AM2 Converting R22 Refrigerant Pressure

Problem Overview

Reading refrigerant pressure is trivial on a Siemens LOGO! 8 with an AM2 analog input module: a 0–10 V transducer outputs a value, the AM2 digitizes it, and a scaling block converts the raw count into engineering units (bar). The hard part is the second step — turning a saturated R22 pressure measurement into a meaningful saturated R22 temperature in the suction or discharge line. Because the LOGO! does not ship with a refrigerant property database, you must build the relationship between pressure and temperature explicitly inside the program, either as a linear approximation (T = a·P + b) or as a multi-segment piecewise table look-up driven by analog comparators.

This guide covers the practical path: confirming the AM2 hardware, wiring the 0–10 V pressure transducer, scaling the input, deriving linear fit coefficients from the R22 saturation table, and implementing T = a·P + b using the LOGO! Math and Analog Math blocks. It also documents the limits of a single linear segment, when to use two segments (low-side and high-side), and how to validate the result against a reference thermometer.

Prerequisites

Before you start, gather the following:

• Siemens LOGO! 8 (6ED1052-1xBxx-0BA8 or newer — 0BA8 family). Earlier 0BA6/0BA7 controllers also expose MUL/ADD blocks but with different menu paths and a smaller block count.
• One LOGO! AM2 analog input module (6ED1055-1MA00-0BA0 or the -0BA2 variant), seated on the right side of the base unit. The AM2 provides two 0–10 V or 0/4–20 mA inputs, 10-bit resolution (0–1000 raw counts mapped to 0–10 V by default).
• LOGO! Soft Comfort V8.x or later for offline programming and simulation.
• R22 pressure transducer, 24 VDC powered, 0–10 VDC output, –1 to 29 bar gauge range (as confirmed in the project source).
• Reference instrumentation: a calibrated RTD or thermocouple thermometer for verification.
• Spreadsheet (Excel/LibreOffice Calc) for fitting T = a·P + b to two chosen chart points.

Hardware: LOGO! AM2 Module Specifications

The AM2 is the cheapest Siemens-blessed way to add two analog inputs to a LOGO! base unit. The relevant electrical limits for a 0–10 V pressure transducer are:

You can confirm AM2 parameters against the official manual on the Siemens LOGO! System Manual (chapter 4.2 — "Expansion modules"). For the dedicated AM2 data sheet see the 6ED1055-1MA00-0BA2 product page.

Pressure Transducer Specifications

The transducer in the project is generic and inexpensive; the key ratings are:

Wiring the Transducer to the AM2

The transducer uses three wires. Connect as follows:

1. Brown (V+) → LOGO! base unit 24 V sensor-supply terminal (or an external 24 VDC PSU).
2. Blue (GND / 0 V) → LOGO! base unit M terminal. On the AM2, the negative input of each channel is also labelled M; bond them together with a short jumper.
3. Black / white (signal 0–10 V) → AM2 terminal AI1 (positive input) on the chosen channel. M of the channel goes to the same 0 V referenced in step 2.

R22 Pressure–Temperature Saturation Table

R22 saturation pressure is a strong non-linear function of temperature. The table below shows gauge pressure (absolute − 1 bar) for selected saturation temperatures, derived from the ASHRAE Handbook (Refrigerant 22 — chlorodifluoromethane). Use this as the reference for fitting coefficients.

The transducer spans –1 to 29 bar, so 0 V = –1 bar, 10 V = 29 bar. The conversion from raw count (0–1000) to gauge pressure is therefore:

P_gauge [bar] = (raw_count * 30 / 1000) - 1
             = (raw_count * 0.030) - 1

For example, raw count 264 → 264 × 0.030 − 1 = 6.92 bar gauge ≈ 7.92 bar abs ≈ 15 °C saturation.

Deriving the Linear Coefficients

A single linear fit T = a·P + b works only across a narrow saturation band. Pick two chart points that bound your operating range and solve:

a = (T2 - T1) / (P2 - P1)
b = T1 - a * P1

Worked example (suction line, chiller evaporator 4 °C to 20 °C, gauge):

• Point 1: P₁ = 5.81 bar gauge → T₁ = 10 °C
• Point 2: P₂ = 14.34 bar gauge → T₂ = 40 °C
• a = (40 − 10) / (14.34 − 5.81) = 30 / 8.53 ≈ 3.517 °C/bar
• b = 10 − 3.517 × 5.81 ≈ −10.43 °C

So the simplified equation is T ≈ 3.517 · P − 10.43 for that band. The discussion thread used a slightly different pair (10 bar → 4 °C and 20 bar → 20 °C) and produced a = 1.6, b = −12, which is correct for that band but gives wildly wrong results (T = 1.6 × 5.81 − 12 = −2.7 °C instead of the true 10 °C) because 10 bar gauge is well outside the 10–20 bar range. Always pick your fit window to enclose the real operating pressure.

LOGO! Block Implementation

LOGO! Soft Comfort supports the equation in Function Block Diagram (FBD) form. The minimum useful chain is:

1. AI1 Analog Input block — pre-scaled, range 0 to 1000 (raw counts).
2. Analog Amplifier / Scaling block (or a Math Instruction with Gain and Offset) to convert raw counts to bar. Use parameters Gain = 0.03, Offset = –1.0, Output Range 0 to 30.
3. Math Instruction (gain) with Gain = 3.517 (the value of "a").
4. Math Instruction (offset) with Offset = –10.43 (the value of "b").
5. Wire the result to a function key display or a message text for monitoring.

LOGO! 0BA8 has 200 function blocks; this chain consumes 4, leaving plenty of room for alarms, hysteresis, and a two-segment refinement.

Two-Segment Refinement

If the LOGO! must cover the entire 0–55 °C suction/discharge range, a single fit is too coarse at the extremes. Use a two-segment approach:

IF P_gauge < 7.0  THEN T = 2.4 * P - 6.2    (suction band)
IF P_gauge >= 7.0 THEN T = 3.7 * P - 19.4   (discharge band)

Implement with:

1. One Analog Threshold Trigger on the scaled pressure (P < 7.0 bar) feeding the suction-coefficient branch.
2. An AND / NAND combination to select the discharge branch when the threshold is exceeded.
3. Two parallel Math Instruction blocks producing both candidate temperatures.
4. An Analog Multiplexer (LOGO! 8) or two parallel paths gated by Q-outputs of the threshold trigger to pass the correct branch to the final display.

This pattern is documented in the LOGO! System Manual, section "Analog multiplexer / Analog arithmetic functions".

Step-by-Step Configuration in LOGO! Soft Comfort

1. Open the project in LOGO! Soft Comfort and confirm the AM2 is detected in the "Network Project" view (right-click → Properties → I/O Assignment).
2. Add an AI block (function group "Analog"). Set the input source to AM2/AI1, type "AI", range 0–1000.
3. Insert an Analog Math block (block number B001 in default). Set:
     • Gain = 0.03
     • Offset = –1.0
     • Result range = 0 to 30 (bar gauge).
   The output of this block is now the engineering pressure value.
4. Insert a second Analog Math block (B002). Connect its input to the output of B001. Configure:
     • Gain = 3.517
     • Offset = –10.43
     • Result range = –20 to 80 (°C).
5. Wire B002 output to a display element: right-click the result line, choose "Message text". Add the variable, format it as a number with one decimal (T = ###.# °C).
6. Add a high/low alarm using two Analog Threshold Triggers — for example, alarm on T < 0 °C (frost) and T > 55 °C (compressor overload). Drive a relay or a digital output to indicate the alarm.
7. Save (Ctrl+S) and simulate (F3). Move the AI1 slider in the simulator; the displayed temperature should track the chart values within ±0.5 °C across the selected band.

Verification and Calibration

Field verification proves the math and the wiring are aligned. Use the following protocol:

1. Power up the LOGO! with the transducer connected to a known reference pressure (a dead-weight tester is ideal; a calibrated hand pump to 0.5 % accuracy is acceptable).
2. Apply 0 bar gauge (vent the transducer to atmosphere). The AM2 should read ≈ 0.33 V (raw count 33). If it reads 0.00 V, the signal ground is floating; re-bond the M terminal.
3. Apply 5 bar gauge. The display should read P = 5.0 bar ± 0.1 bar and T = 8.7 °C ± 0.4 °C for the 4–20 °C fit window.
4. Apply 10 bar gauge. The display should read P = 10.0 bar ± 0.1 bar and T = 24.5 °C ± 0.5 °C.
5. Cross-check against a calibrated Pt100 inserted in the suction line at the same point. Allow at least 10 minutes of stable operation for the line to reach saturation. Deviations greater than 1.5 °C indicate either a fit error, a non-saturation condition (two-phase flow with significant superheat), or a transducer zero/span error.
6. Log the result to the LOGO! data log (LOG! option: micro SD card in the LOGO! TDE or base unit) for trend verification across one duty cycle.

Quick Diagnostic Matrix

Alternative: Why Temperature Sensors Are Usually Better

The pressure-to-temperature conversion is an inferred property of the refrigerant. It assumes the fluid at the transducer port is two-phase (saturated) — i.e., liquid and vapor in equilibrium. In real HVAC systems, the suction line is almost always superheated (vapor above saturation, typically 5–15 K of superheat) and the discharge line is superheated by 30–60 K above the saturation temperature corresponding to the discharge pressure. A temperature sensor (Pt1000 on the AM2 RTD, or an NTC wired to a digital input via an analog front end) measures the actual fluid temperature directly, which is usually the parameter you need for control decisions (thermostatic expansion valve superheat, oil cooling, capacity modulation).

Use the pressure-derived temperature only when:

• The measurement point is guaranteed two-phase (e.g., a vessel dip tube, an economizer).
• No temperature wiring can be run, and the application is monitoring-only.
• You need the saturation temperature for an enthalpy calculation, not the actual fluid temperature.

Advanced: Piecewise Table Look-Up in LOGO!

For users who need higher accuracy across the full range, you can implement a piecewise linear interpolation with five segments. Use five Analog Threshold Triggers to select a region, five corresponding Math blocks producing (a_i · P + b_i), and an Analog Multiplexer (LOGO! 0BA8 native block) to output the active segment. The LOGO! System Manual section on "Analog multiplexer" covers the wiring. A typical 5-segment fit across 0–50 °C produces ±0.3 °C residual error.

Pre-computed 5-segment coefficients (gauge pressure, °C/bar and °C)

These coefficients reproduce the chart values above with a maximum error of 0.27 °C — usually better than the transducer's own ±1.5 % of full scale.

FAQ